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The islands, the lakes and the caves.

Every seven or eight months, a ship would arrive run by an illiterate captain (the less erudition, the more they push). He would be half-smuggler, half-pirate. He would have crossed the Pacific in a capricious and arbitrary manner. He would unload some crumpled dirty letters, left at the bottom of a drawer, mixed up with tobacco dust and a little spray salt. Tahiti would be once again a small island lost in the immensity of the Pacific, tranquil, patriarchal, heavenly.

Aurora Bertrana

Ocean paradises (1930)

1 Islands of water, islands of land

1. From isolation to insularity

1.1 Isolated systems

The basic concept of an island is simple and intuitive: a piece of land totally surrounded by water. However, the concept has long been applied, equally intuitively, to many other realities that share the fact of being separated from their equivalents by a different surrounding reality that is in some way their opposite. Thus, it is possible to speak of islands of houses surrounded by streets (private space as opposed to public space), pedestrian islands (spaces removed from the roadtraffic circulation system) or green islands (spaces with trees surrounded by croplands or urban spaces). Recall the phrase, "no man is an island" which expresses the idea that none of us can avoid the influence of others when living in society.

The concept of insularity

Ecologists and biogeographers use the term island, taking the concept of insularity for granted, and thus any portion of the biosphere surrounded by a frontier or an interface through which exchanges are minimal and mainly passive, could be considered an island. An island would thus be a space separated from its immediate surroundings by a frontier, a space where phenomena or events occurring on either side are virtually unconnected and hardly influence each other. When considered in this way, the phenomenon of insularity is widespread in nature and present in many different environments. Insularity is obviously present in true islands but is also seen in by lakes and pools and other environments as different as mountain peaks, the tepuis of Venezuela, underground cavities, patches of forest within other ecosystems, calcareous areas in the middle of areas with different substrates, submarine trenches, the surroundings of abyssal vents, habitat islands, and so on. In many of these cases the interface with the surrounding environment is usually not as clear as in the typical examples of insularity, true islands and lakes; rather, the frontiers limiting these ecological islands are asymmetric and permeable and allow a flow of materials and energy with the surrounding environments that are not exclusively based on the detritus pathway.

The geographic space of the isolated systems

True islands, that is, islands with a solid substrate separated from the nearest land by water, are very numerous. There are about 500,000, although it is impossible to give an exact figure. Their total area represents approximately 5% of the world's land surface.

The islands of land are intrinsically attractive areas, truly different worlds where life has evolved along special paths, different from those on the continents and often different from those on other islands. Islands are isolated worlds where unusual organisms interact in unusual ecological networks and where evolution often seems fantastic rather than merely inventive.

Lakes, that is, liquid islands, represent roughly 1.8% of the world's total area of dry land and may occur within large continental landmasses or on islands. Lakes, though less important than islands, can be seen as very much their equivalents in terms of spatial organization. The frontiers separating terrestrial and aquatic environments are clearly asymmetric and show great contrast, and any material exchanged is usually through the detritus pathway.

To the contrary, caves (hypogeal islands) are restricted to certain geological substrates that are unequally distributed around the world. The 'islands' at the top of many mountains, and the others mentioned above, also show very irregular distributions.

1.2 Isolation and biological evolution

The biosphere can be considered as being crossed by countless pathways and full of relative frontiers. The pathways encourage genetic flow, while the frontiers make it more difficult, and all this also depends on the organisms involved. These roads and borders are the weft and warp of life's rich tapestry and are continually creating new situations.

The fascinating behavior of isolated systems

The study of island populations has always been stimulating. Charles Darwin made his breakthrough when considering the differences between the species apparently of common origin that lived in distinct environments of the different islands of the Galapagos Archipelago. The study of the isolation of populations as an agent of speciation still arouses the interest of evolutionary biologists. Giant tortoises on the Pacific islands, fish in different lakes and even in different spaces within a single lake, land snails on islands or aquatic snails in lakes, and a wide variety of cave-dwelling organisms are notable for their high level of specialization on a relatively small scale. The smallness of the populations in many cases has encouraged rapid divergence, as in the lizards in the Mediterranean islands. Of course, evolution does not only occur on real islands or inverted islands (i.e., lakes), but they provide excellent examples, especially when there are additional factors, such as almost impermeable frontiers and a surface area so small that it completely excludes large predators.

In groups of islands, lakes, and caves, and also in tepuis and other comparable environments, segregated and isolated parallel worlds can be recognized that are relatively closed, in each of which the evolution of a few shared immigrants has followed separate paths that have to some extent diverged. Evolutionary divergence reflects external conditions that have remained different or relatively unchanging or seem to be the result of chance. Many isolated species are now in danger of extinction, especially if the surrounding conditions become more ruderal (natural vegetation disturbed by man) or suburban as this makes the invasion of foreign species easier. Many inhabitants of island habitats seem to be especially fragile; the example normally given is the dodo on the island of Mauritius, which was given the specific name ineptus in early descriptions, reflecting contempt for a bird lacking features that it did not need for survival until the arrival of human predators.

The increase in diversity and the slowing down of dynamics

The establishment of the concept of isolation or of insularity shows some awareness by naturalists of the operation of parallel or convergent evolutionary processes, which occur in diverse conditions and which give rise to relatively small populations consisting of individuals with long life spans and a low metabolism in relatively unchanging conditions, often dependent on a reliable but not very generous food source. All these conditions slow down the dynamics of the ecological system in question. The main consequence of this slowdown is that it favors genetic divergence and leads to an increase in diversity.

The arthropods that live in caves are usually larger than the norm in their group. They are often colorless, with long life spans and slow metabolism, and produce only a few large eggs. It is noteworthy that they share these characteristics with the animals of the deep ocean trenches, whose surroundings, though uniform over large areas, are comparable with the cave environment because of the low density of food. In both cases, the low density of food is compensated for by its reliability; in the case of the ocean floor, the food input is transported from the sunlit surface by a long process providing a small but reliable flow. For similar reasons, the food supply in caves may be low, but it is relatively guaranteed.

The isolation of life on the ocean bottom also leads to a situation with a slow dynamic and great diversity. Against this background, another type of island stands out, inverted islands, namely the hydrothermal oases that form around water vents that allow the growth of some chemosynthetic organisms (see volume 10, page 206). These ecosystems are typically speeded up, showing low diversity and consisting of fast-growing organisms, a revealing contrast with other situations of isolation. The concentration of life around springs of water on dry land may be comparable.

Evolutionary dead ends and fluctuating environments

Isolation understood in the traditional way, with very effective boundaries, can be related to evolutionary scenarios in which the limits are virtual. In spaces characterized by more fluctuating conditions, isolation acts to generate new taxonomic forms, at the same time as the older forms diversify and occupy peripheral positions, where great biodiversity accumulates. This is one possible way of interpreting the fluctuations in temperate zones, which would thus have contributed to even further enriching the biodiversity of the adjacent areas with more constant climates.

In lake basins there is greater subdivision and potential enrichment of the peripheral environments. The subdivision of a large lake into smaller lakes during repeated episodes of falls in water level may be the cause of considerable speciation in the lake basin as a whole, and the results of this process can be seen in the different races of fish and molluscs in the lakes of Africa and the Philippines. Endemism, that is to say the existence of species that are characteristic of and restricted to a particular locality, is relatively low among the species living in temporary lakes but never completely absent. The most characteristic species produce large numbers of resistant forms that are transported by the wind or by birds and maintain an intense genetic flow that successfully opposes (although not always with the same degree of success) the differentiation of local forms. Although the species of the plankton may be more ancient than those of the benthos, the geographic distribution of the benthic species appears to be older, because the plankton is continually redistributed by the movements of the ocean waters. The areas that underwent glaciation were stripped of true cave-dwelling organisms (troglobionts), but those that persisted in peripheral areas seem to be older than the recent epigeal (non cave-dwelling) fauna in the same regions. This is why there are troglobionts that are more similar to the presumed descendants of the former inhabitants of the same area than to the more recent arrivals, whose ancestors were displaced upwards or downwards. The underground waters are connected, to a variable extent, by aquifers and local karstic systems that may mix and homogenize the populations, depending on the fluctuations of their volume of flow. Routes under riverbeds, which are both underground and interstitial, are sites of exchanges between halophile species of coastal marine origin and freshwater species.

In new environments, colonization by alien species is easy. This has happened in reservoirs in Spain stocked with forms from more northerly countries, which are now abundant in all sorts of lakes. Everything changes, and species that were formerly endemic may become invaders with a tendency to become more or less cosmopolitan. This is true of two molluscs: Potamopyrgus antipodarum, a gastropod from New Zealand that appeared for the first time in England in 1859 and has since spread throughout western Europe (where it was known as P. jenkinsi) and Dreissena polymorpha, a bivalve originally from the Ponto-Caspian region that has spread throughout the inland waters of almost the whole of North America.

In excessively uniform environments, it unlikely that the complementary conditions needed for complex lifecycles, such as those of amphibians, insects, and many parasites, will occur in close proximity. Species with needs that vary little throughout their life cycle are at an advantage. Cave-dwelling amphibians that retain their gills as adults, such as the olm (Proteus), have achieved this. Many cave insects produce very large eggs and are born at an advanced larval stage, often almost as pupae. The same strategy is also obligatory for benthic animals living at great depths that have had to abandon their free-living, plankton-eating larvae, because there is no plankton, and are also born at an advanced stage of development. Fewer eggs are produced, but they are larger. There is also the alternative of spending the entire life cycle, or most of it, in a phase similar to the larval (known as neoteny), like the olm, but attaining sexual maturity.

These and other regularities should be assessed very critically. The increase in the size of the eggs may help to suppress the long larval stages that are also too different from the adult ones, in an environment that offers little differentiation. But it is not easy to find a convincing explanation for changes in allometry, that is to say, in proportions of body parts, such as when a moderate increase in body mass is accompanied by an exaggerated lengthening of the limbs. In both caves and the deep layers of the soil, as well as at great depths in the seas and some lakes, vision is not affective and animals often lose their sight.

In some ways, parasitic organisms, confined within their hosts, are also victims of isolation in terms of the conditions in which they evolve. In some senses this is undeniably true, but this isolation is broken when the parasite has free-living larvae or ones with complicated lifecycles with changes from one host to another. Yet their fidelity to their hosts continues to show some degree of isolation.

The MacArthur and Wilson model

The search for a quantitative or statistical expression of the richness of island biotas led to MacArthur and Wilson's approach (1967), which arose from study of the bird fauna of islands but is of more general application. The authors perhaps centered on the number of birds because the probability of immigration and extinction is biased with respect to the situation on the mainland. They proposed a general expression in which the value of S, the number of species present on an island, is equal to the number of immigrations minus the number of extinctions (S = i-e) and tends to stabilize at a number such that dS/dt = aSi-bSe. In practice, it seems that S is regulated so that immigrations depend little on this figure (i tends to 0), while extinctions depend more on the number of species already accumulated (e tends to 1).

This model does not allow very accurate predictions when applied to islands, but it helps give an idea of the natural regulatory processes, and not only in relation to islands. It shows the great importance of the study of island populations, as they are such clear and spectacular examples of some aspects of laws governing the differentiation of the entire biosphere. There are different kinds of evolution: the biosphere is continually subject to changes, or pulsations, that sometimes accelerate the flow of energy and sometimes slow it down, or that maintain slow energy flows in certain situations and specific sites (such as ocean bottoms, soils, caves, and islands).

1.3 Evolutionary phenomena

To understand the phenomena linked to insularity, it is necessary to consider some evolutionary processes specific to isolated systems. Evolutionary radiation and adaptive radiation play an especially important role.

Evolutionary and adaptive radiation

When speciation processes are so great that many new species evolve from a single ancestral species, evolutionary radiation is said to have occurred. Radiations may occur within a mega-archipelago, an archipelago or even within an island (or a lake). In these cases, the species that have originated may have evolved sympatrically (within the same area) or allopatrically (in different areas), though allopatric evolution is more common. This is true for species with similar habits and ecology, such as the snails of genus Hemicycla, which are restricted to a single gorge or a single island in Canary Archipelago; the giant tortoise of the Galapagos (Geochelone elephantopus); the lizard Podarcis on the islands of the Mediterranean; and the small frogs of the pictosissimus group of the genus Eleutherodactylus on Hispaniola.

When the processes of speciation produce a series of closely related species with different ecological requirements, many of them occurring sympatrically, this is called adaptive radiation, a term applied to the divergence into different ecological niches of species with a common ancestor. Adaptive radiation gives rise to groups of species that often live in the same area and have distinguishing ecological and morphological features.

Examples of adaptive radiation include the finches of the Galapagos Islands, the honeycreepers of Hawaii; mice (Muridae) in the Philippines; the lemurs, tenrecs, and viverrids in Madagascar, the ancient conifers of New Guinea; and the cichlid fish in the lakes of eastern Africa. Water snails are another zoological group that has given rise to major adaptive radiation in ancient lakes, with dozens or hundreds of endemic species in lakes Tanganyika and Malawi (but, surprisingly, not in Lake Victoria) and in other lakes outside Africa: Lake Ohrid, on the frontier between Macedonia and Albania; Lake Baikal in Siberia; Lake Biwa on Honshu Island (Japan); and Lake Titicaca in the Andes. The shells of many of these species closely resemble those of sea snails, and it was thought that these lakes might once have been arms of the sea. Anatomical studies have shown that they are not related to the marine snails, and that their similarity is an example of convergent evolution under similar selection pressures. Lake Baikal has one third of all the world's known freshwater and marine amphipods, which have undergone an extraordinary radiation.

The Hawaiian honeycreepers

Perhaps one of the most spectacular examples of adaptive radiation is shown by the Hawaiian honeycreepers (Drepanididae), a monophyletic group of passerine birds that appear to be phylogenetically related to the goldfinches. Until recently, it was considered that the diversity of the Hawaiian subfamily reached 23 species in 11 different genera, differing in form of their beak and tongue and feeding habits. Since 1991, it became known that 14 species and 4 genera more were exterminated by the first inhabitants of the islands. The current diversity of beaks is extraordinary but was even greater before humans arrived.

Among the forms that have long been known, one species, the o'u (Psittirostra psittacea), has a bill similar to parrotbills that it uses to cut flowers and fruit. Another species, the Maui parrotbill (Pseudonestor xanthophrys), which was thought to be extinct until it was rediscovered in 1967 in an inaccessible valley in Maui, has an even stouter beak that it uses to cut branches when searching for the wood beetles it feeds on. Dysmorodrepanis munroi, a species that went extinct before 1913, had a strange bill, with the premaxilla (the upper part of the bill) curved downwards, and the mandible (the lower part) curved upwards, so that it could never have completely closed its mouth; it is supposed to have used them as tweezers. Other species, such as the insectivorous Oahu creeper (Paroreomyza maculata), have a long narrow bill and elongated tongue. The amakihi (Viridonia [=Loxops]) are also insectivorous but take nectar as well. The greater amakihi (V. sagittirostris), a species that went extinct in the early twentieth century, had a mandible similar to that of the crossbill, with the point of the premaxilla crossing the mandible. The larger genus Hemignathus, which has a longer, more curved bill, contains nectarivorous and insectivorous species. The insectivorous species include one remarkable species, the akipola'au (H. wilsoni), which uses its short, straight mandible to hammer the wood of tree trunks, as woodpeckers use their beak. The longer, curved premaxilla is used to remove the insects from the holes it makes with the mandible. This combination of a short, straight, hammer-function mandible with a long, curved premaxilla is unique. The most nectarivorous genera have a very long, curved beak.

The more interesting extinct honeycreepers include Chloridops regiskongi, the species with the thickest beak (similar to the hawfinch). There were species with bills like finches and others with an insectivorous-type beak. The species Hemignathus upupirostris had a beak similar to the hoopoes. The two known species of the genus Vangulifer had an unusual beak like nothing known among living birds.

The cichlid fish of the African lakes

Another spectacular case of adaptive radiation is the cichlid fish in the large lakes of eastern Africa. Lake Victoria has more than 300 endemic species of cichlid, Lake Tanganyika more than 230, and Lake Malawi between 500 and 1,000. These figures are even more spectacular if it is borne in mind that the African continent only has 3,000 known species of freshwater fish. The main selection pressure has been the class of food available, as morphological differences (apart from their highly variable coloration) are generally in their mouthparts. Thus, species that graze on the layer of algae growing on the stones at the bottom (Pseudotropheus) have rounded heads and brush-like teeth, whereas those that eat algae growing on submerged plants have pointed heads and smaller teeth. Those that eat macrophytes have large mouths with strong teeth, whereas detritivorous species have elongated mouths, and pelagic phytoplankton-eaters have thin bodies.

Others can greatly elongate their mouths to catch small zooplankton animals, after following them on their daily vertical migrations. Some species prey on molluscs and so have buccal teeth or pharyngeal mandibles that have atrophied and become grinding tools. Insectivorous species have thick, often prominent, lips, whereas those that hunt shrimps have an elongated beaklike premaxilla and crab-hunting species have strong curved teeth and a long lower mandible. There are also fish-eating species (Haplochromis, Boulengerochromis), each with its own hunting technique but all with large, well-equipped jaws. This diversity even includes cleaner fish that feed on the parasites living on other fish, and even others that pose as cleaner fish, approaching their victims and then snapping at their scales or eyes. This specialization even extends to species that eat the recently laid eggs before the females can collect them in their mouths, and this is surpassed by specialists in "kissing" the females with their soft lips to suck out the eggs or larvae in their mouths, such as the gourami (Helostoma). Other species have equally specialized but as yet unknown habits. The most diverse communities are in rocky habitats, while the open water communities have fewer species.

Convergent evolution

Convergent species often evolve on far distant islands that may even belong to different biogeographical regions. This is the result of several factors. In the first place, the colonizing species are not a random sample of the mainland species as a whole, and so even in far distant sites, the new arrivals usually come from the same major groups. The fact that new arrivals are members of the same groups of species means that the ecological interactions between the different species are not very different on far distant islands. Empty ecological niches on far distant islands are often occupied by species derived from related mainland stocks.

Adaptive ecomorphological responses imposed by evolution in island environments (nesoevolution) are similar, and sometimes different endemic species showing convergence, after evolving on far distant islands and never having been in contact, were thought to be a single species. Many similar convergence phenomena also occur in lakes. Many pairs of species of cichlids in Lake Malawi and Lake Tanganyika, despite not being closely related, are very similar. In Lake Titicaca, Lake Tanganyika, and Lake Baikal there are pelagic flightless trichopterans that have adopted the habits of the gyrinid coleopterans, which are very different.

Dwarf elephants provide a very surprising example of convergent evolution on islands. They are all now extinct, but there were island forms of elephants on several Mediterranean islands, on the Channel Islands in California, on Wrangel Island in northeast Siberia, and on several islands in Insulindia (Timor, Flores, Sulawesi, etc.). Some were mammoths and others were true elephants and others were stegodontids, but all of them were similar in that they measured at most 6.6 ft (2 m) at the withers. The smallest (adult males less than 31.5 in [80 cm] at the withers) lived on the Mediterranean islands of Sicilia and Telos (in the Dodecanese). Some of these species became extinct before humans arrived, but the others were victims of the first people to settle the islands where they lived.

Giant tortoises

One notable example of convergence is provided by the giant tortoises. There are many examples of the evolution of giant tortoises on islands: These large species, such as the Galapagos tortoises (Geochelone elephantopus), have evolved from smaller mainland ancestors. Others, however, may come from equally large, or larger, mainland species. This may have been the case of the extinct tortoises of Timor and Sulawesi, which were perhaps related to the enormous fossil tortoise Indotestudo atlas.

Whatever their origin, the result is more or less the same: On islands uninhabited by mammals, tortoise species are all large. Very similar large tortoises have evolved on far distant islands with no past or present relationship (the Galapagos, Aldabra, Seychelles, Mascarene Islands, some of the islands of the Antilles, Timor, Sulawesi, the Canary Islands). Some of these species became extinct before humans arrived, but many others have become extinct since humans arrived, and thus only a few species survive.

Rats and mice

The murids (rats and mice) show some interesting cases of convergent evolution on islands. One especially instructive case is the giant Galapagos rat (Megaoryzomys curioi), a species that was first identified in 1964 from ancient owl regurgitated stomach pellets found on the island of Santa Cruz. It was first classified as a species of the genus Megalomys, the giant rats (body length of 11.8-13.8 in [30-35 cm], with an equally long tail) from the lesser Antilles. The remains of only three species of giant rat are known, and two of them became extinct on Barbuda and Saint Lucia in the 19th century (M. audreyae and M. luciae, respectively), while the third (M. desmarestii on Martinique) appears to have become extinct as a result of the eruption of Mount Pelee in 1902. It has now been decided that the species from Santa Cruz should be classified as a new genus endemic to the Galapagos Islands, a genus that does not even belong to the same tribe as the Megalomys of the Antilles. Further remains found on Isabela belong to another, as yet undescribed, extinct species of the genus Megaoryzomys.

The malpais rats (Malpaisomys insularis) were recently described (1988) from subfossil material derived from malpais volcanic deposits, the recent lava fields of the eastern Canaries (Fuerteventura, Lanzarote, and La Graciosa) between 5,000 and 1,000 years old. They are clearly convergent with the rats of the lava fields of the genus Nesoryzomys of the Galapagos, five species of which have recently become extinct, while three still survive. These long-legged rats are adapted to moving between the cracks in the rocks, but even this ability has not allowed most species to resist the influence of the rats (Rattus) that arrived with human beings or the predatory action of cats and dogs.

The Ryukyu Islands spiny rats (Tokudaia osimensis and T. muenninki) have dentition very similar to that of Rhagamys orthodon, a rat that disappeared from Corsica and Sardinia after the arrival of humans. Both these endemic genera are derived from related mainland species. The same thing happens with a series of rats on islands close to the Sonda Strait: Rattus palmarum and R. burrus, from the Nicobar Islands; R. simalurensis, from the island of Simeulue, northwest of Sumatra; R. lugens, from the Mentawai Islands, southeast of Simeulue; R. adustus, from Enggano Island, southwest of Sumatra; R. hoffmanni, from Sulawesi; and R. tawitawiensis, from Tawitawi Island, in the Sulu Archipelago to the southwest of the Philippines. Although they lived in distant islands that were never connected to each other or to the mainland, all these species have more in common with each other than with the species on the mainland.

2. Isolated land: islands

2.1 The planetary archipelago

Strictly speaking, all the dry land's on the planet, even the continents, are islands. All the land masses, the Old World, the New World, Antarctica, Australia, and so on, are surrounded by water. But below an area of 3,860,000 [mi.sup.2] (10 million [km.sup.2]), doubts start. Is Australia, with an area of 2,895,000 [mi.sup.2] (7.5 million [km.sup.2]), an island or a continent? It is generally accepted that Australia is a continent, but its plant and animal population has many insular features, and it is not uncommon for it to be called an island-continent. Greenland, with an area of more than 772,201.2 [mi.sup.2](2 million [km.sup.2]), is generally considered to be the largest island, and because of its ice cap can be considered as Antarctica's smaller equivalent; it is often treated as a part of, but differentiated from, the Americas. New Guinea, with an area of less than 308,800 [mi.sup.2] (800,000 [km.sup.2]), is the second largest island, and the largest undeniable island. There are more than 500,000 islands, and including Greenland, they represent a total of about 5% of the world's land surface.

Continental islands and oceanic islands

Continental islands are those that have, over the course of their history, been joined to the nearby mainland, and their faunas share many features in common with that of the mainland, regardless of their geological origin.

Most islands are continental islands, but they are not so clearly islands in the strictly biological sense, as their flora and fauna resemble that of the neighboring mainland and contain few endemic species.

Oceanic islands have never been in contact with the mainland, or if they were, it was many millions of years ago. These islands show the most unusual island biotas, characterized precisely by their taxonomic poverty, their disharmony (they do not show the same basic adaptive types) with respect to the continental biotas they are derived from, and their high levels of endemism.

A third category, paraoceanic islands, are the islands that have been in contact with the mainland but still show a biota comparable with that of an oceanic island (with a taxonomically very poor biota, great disharmony and many endemics). Islands such as those in the Mediterranean, the Falkland Islands, Wrangel Island, and the Mentawai Islands are paraoceanic. Although they have been in contact with the continent, the composition of their fauna has been influenced by the action of extremely ecologically effective filters. In the Mediterranean, some of the islands were colonized in the Messinian 5.5 million years ago, when the sea dried out after the Strait of Gibraltar was closed. The colonizing fauna had to cross genuine saline deserts to reach the islands, and these deserts filtered the colonists even more effectively than the sea. In the case of islands like Wrangel and the Falklands, the filter must have been imposed by conditions of extreme cold. The Mentawai Islands, west of Sumatra, may have been joined to the continental platform during the glaciations but by a very narrow isthmus that also acted as a filter to their colonization.

As a whole, the oceanic and paraoceanic islands, which account for twothirds of the planet's islands, should be considered as a biodiversity hot spot, especially in terms of certain animal groups. These islands represent about 1.5% of the world's land surface but contain (or contained until humans arrived) about 15% of the modern species of mammals, more than 20% of the modern species of bird and an even higher proportion of reptiles.

The biological colonization process

Islands, like living organisms, have a birth, a life, and a death. Some are very young and some are very old. Some are ephemeral, while other are long-lasting. When they are born, islands provide naturalists with an opportunity to study the phenomenon of colonization. Yet the birth of an island is a rare occurrence. Some recently born islands include Surtsey, in Iceland, which was born in 1963, and Anak Krakatau, which emerged in 1930 in the Sonda Strait, between Java and Sumatra, in the submerged crater left by the explosion of Krakatoa Volcano on August 27,1883. It has also been possible to study the colonization of islands after the total destruction of their living population, either naturally (normally devastation by a volcanic explosion or a typhoon) or artificially (in the 1960s the fauna of some Caribbean keys was eliminated with biocides to study their recolonization).

Transoceanic dispersal

Terrestrial organisms arrive on oceanic islands at random, and usually passively. The ability to survive a long ocean crossing varies greatly from one taxon to another, so the floristic and faunistic composition of the resulting communities is anomalous in comparison with those of the continents. Some families are dominant, while many others are absent. For example, slugs, amphibians, and nonflying mammals are virtually absent from islands. One of the most notable features of islands is the absence of mammals that prey on vertebrates, partly because it is difficult for them to reach islands but mainly because the resources available in a small territory cannot support a viable population of carnivores. Populations of land snails, however, can survive for long periods on tiny sites, which explains their enormous diversification on almost all islands.

The wind may blow the fruits of wind-dispersed plants as well as fern spores and tiny orchid seeds to islands. It may also transport very small animals, flying insects, and bats. The wind is thought to have been responsible for the colonization of some remote Pacific islands by land snails. Endodontid snails (almost all of them very small) are represented by a multitude of species in Micronesia: more than a hundred endemic species have been described from the small island of Rapa alone.

Sea currents are responsible for the arrival of small floating islands, originating in the mouths of large rivers, and consisting of the remains of riverside vegetation mixed with floating plants. These masses of plantlife contain many organisms, many of them able to resist a transoceanic voyage. But all organisms dispersed by the sea have to be able to resist the external salinity. It is not surprising that the vegetation of the coastal strip of the remote islands in the tropical regions of the Pacific and Indian oceans consists of species with a very wide distribution, whose seeds are still viable after drifting for several months in seawater. Ocean islands may also be colonized by marine organisms that adapt to unoccupied environments and form part of the land and freshwater communities.

Pioneer species

In the first phase of an island's life, it is colonized by pioneer species, those that are successful colonists (several birds, flying insects, etc.). The populations of these species normally maintain a gene flow with other populations of the same species, so that they do not, at least to begin with, develop into island endemic species. The efficient colonizers obviously include the "globe-trotter" species that have opted for dispersal as a way of life: They are present in spectacularly high densities on island available for them to colonize. These species are excellent at crossing stretches of open sea and have a high reproductive capacity. They usually live on islands that are too small to support stable populations of other species or on ones devastated by typhoons, hurricanes, or volcanic explosions. They are displaced by other species that arrive later but can use the island's resources more effectively and for longer.

Islands are not equally important for all species. Thus, islands that may represent an very special particular territory for a large vertebrate may be indistinguishable from a continent for a small animal or a moss. This does not mean that these organisms do not give rise to new species but that they may do so following patterns similar to those on the mainland. Even so, many islands also impose their own model on evolution. One of many examples is the wetas, a group of flightless nocturnal orthopterans that are the largest insects in New Zealand (up to 2 mm long and weighing up to 71 g in the case of Deinacrida heteracantha, the wetapunga). Further examples include the giant earwig of Saint Helena (Labidura herculeana) and several giant spiders, such as those on the islands of Deserta Grande (Madeira), Sao Tome, Vanuatu, and the Fijis.

Relicts and novelties

Endemic species on islands may be ancient species derived from lineages that were formerly widespread on the mainland, or they may be species (or higher taxa) that have arisen on the island as a result of island evolution.

For endemic species to arise on islands, genetic flow with the mainland population has to be broken and sufficient time has to elapse for evolutionary changes to occur. Endemic species arising as a result of island evolution are found on a relatively large scale when the island is so far from the regions of origin that the arrival of immigrants is random, and it is in practice impossible to maintain contact with the original populations. The species that colonize islands by this random method fall into a geographical and evolutionary trap, as they have almost no chance of leaving these islands. Often, species descended from animals that reached the island by swimming eventually lose ability to swim, such as for example, the extinct dwarf hippopotamus of Cyprus (Phanourius minor) or the New Caledonia land crocodile (Mekosuchus inexpectatus). Likewise, the descendants of many species that arrived by flying eventually lose their ability to fly, such as the now-extinct moas (Dinornis) and the kiwis (Apteryx) of New Zealand and the elephant bird (Aepyornis) of Madagascar.

The absence of the normal mainland predators, the new combination of species in the islands ecological networks, and the supply of available resources in island environments they have colonized largely condition the evolution of these strange inhabitants. On these islands, historic processes are of major importance, because the order of arrival often conditions the different forms of speciation.

The biological population and interrelations

The exchange of matter and energy between genuine islands and the surrounding waters is basically through the detritus pathway. There are, however, some animal species that make use of both media, such as seals, many sea birds, sea otters, turtles, and marine iguanas. These species relate the coastal environment, especially islands, with the surrounding waters, and these relations mean the ecological importance of islands is greater (and may be far greater) than their geographical size implies.

Saint Helena (47 [mi.sup.2] [122 [km.sup.2]]) and Ascension Island (37 [mi.sup.2] [97 [km.sup.2]]) are very small, but they are the only patches of dry land in an area of 5,790,000 [mi.sup.2] (15 million [km.sup.2]) (3% of the world's surface). The marine birds that nest on them in fact live over a very large area of the Atlantic Ocean, so that, through these species that live between the islands (where they reproduce) and the surrounding waters (where they feed), islands influence the surrounding sea. Since the arrival of humans on these islands, the populations of seabirds have declined from tens of millions to a few thousand. Thus, minor interventions by human beings of very small areas of dry land have modified the ecological functioning of large areas of ocean.

The unusual features of island evolution

The result of island evolution is always surprising and often gives rise to different phenotypes "banned" in larger environments with greater diversity.

A good example is the giant earwig of Saint Helena (Labidura herculeana), 3.1 in (8 cm) long, that is by far the largest earwig known. Oceanic islands are thus natural laboratories in which evolution may give rise the most unexpected forms.

The most frequent paradoxes

Small populations, very intense selection, and the relaxation of any morphological and behavioral limitations present in more diverse ecosystems all mean that adaptive evolution of populations on oceanic islands is fast and is accompanied by a series of unpredictable changes, a consequence of genetic drift. This genetic drift is due to the low number of initial colonizers, the moderate size of the population, and its large oscillations in numbers. In many cases, sexual selection also acts, as this factor is not as limited by adaptive factors as it is on the continents.

Evolution in isolation is responsible for the existence of plants so strange that a new family has had to be created to include them, such as Lactoris fernandeziana, the only member of the Lactoridaceae, which is endemic to the Juan Fernandez Islands, and Degeneria vitiensis, which is endemic to the Fiji Islands and is the only member of the Degeneriaceae.

Other plants are members of families well represented on the continents but having an unusual morphology, such as the arborescent composites of Saint Helena, Rarotonga, and Juan Fernandez. The Asteraceae disperse better than most trees from tropical and temperate zones, and so they have been able to colonize those islands and evolve into arborescent forms before conventional trees arrived, taking advantage of this unique ecological opportunity.

Birds that migrate long distances may stop over on the most isolated islands and even settle on them as residents. Studies performed on island birds have shown that a species isolated on a remote island may eventually acquire highly specialized feeding habits, without any other selection pressure than the optimal use of the circumstances in a restricted environment with a limited range of resources. This is of great importance in ecological theory, because it contradicts the theories that explain divergence between closely related species as the consequence of interspecific competition.

In this sense, consider the case of the Fernando de Noronha vireo (Vireo gracilirostris), a small insectivorous bird endemic to Fernando de Noronha, an island off the southern tip of South America. It is a divergent population of a migratory bird common on the South American mainland. On the island, it rapidly deviated towards a morphology and a behavior that allows it to catch small insects among the foliage, especially on the underside of leaves; as a result, it has a reduced body size, rounded wings, and longer legs and has developed a longer, thinner beak than its ancestor as well as different and more conspicuous markings. The white-eyes of Reunion (Zosterops borbonicus and Z. olivaceus) are comparable; the first is more generalist and closely resembles other members of the genus, whereas the second has differentiated more and is nectarivorous. It thus seems reasonable to suppose that the specialization of Z. olivaceus preceded that of Z. borbonicus and that it reached the Mascarenes independently.

Pelagic birds

Most pelagic birds nest on very isolated islands, where they form enormous colonies. These birds have long wings and short legs and are perfectly adapted to flying enormous distances by riding the wind at the surface of the sea. They would be easy prey for many predators, if they ever landed on the mainland coast. This is why they congregate to breed on remote islands, where they have no terrestrial enemies. Each nesting pair produces a single egg, incubates it for a long period, and then cares for the chick, which grows slowly and accumulates a lot of fat before it can fend for itself. The food for the chick normally represents 14 to 18% of the weight of the adult bearing it. In these conditions, these birds have developed a reproductive strategy that allows them to maximize the net available energy and minimize the number of trips by the adults, which both participate in incubating the egg and feeding the chick. Procellariiform birds (albatrosses, petrels, etc.) have acquired a further adaptation, favored by their fat-rich diet: Their stomach has a special chamber where the food is partially digested to increase its density.

Pelagic birds, bearing in mind the population densities they attain and the time they spend on the islands, are a fundamental element of the ecology of the most remote islands. The nesting areas receive enormous deposits of excrement, which accumulates and forms guano. In addition to establishing a link between a very small area of land and a usually much larger area of sea, they may also transport small animals, propagule, and seeds, adhering to their plumage or within their digestive tract. This would appear to be the only plausible explanation for the distribution of Peperomia berteroana (Piperaceae), present only on two far distant islands in different oceans, Juan Fernandez and Tristan de Cunha. This might also be true of the composite genus Rhamphogyne, which only has two species, one in the Mascarene Islands and the other in New Guinea.

Nonflying birds

In birds, the loss of the ability to fly has occurred repeatedly on many remote islands, giving rise to many endemic species. In the absence of enemies, natural selection favored the ability to store fats and minimize metabolic expenditure associated with useless activities, such as flying when there are no terrestrial predators to flee. The family with the most examples of the loss of flight is the rail family (Rallidae), which appear to have been very abundant on many islands. Very few survive, as they were easy prey for sailors and the rats accompanying them. An estimated 2,000 island species of nonflying rail have become extinct since humans arrived on the islands where they lived, such as the Hawaiian rail, which is only known from archeological remains. The last known extinction in this group was Gallirallus wakensis, which was exterminated at the end of the Second World War by soldiers who were abandoned on the island of Wake, in the middle of the Pacific. The takahe (Notornis mantelli) of the South Island of New Zealand was considered to be extinct in the early twentieth century but was rediscovered in 1948.

Probably, the best-known inhabitant of these remote islands is another nonflying bird, the dodo (Raphus cucullatus), a bird famous for its unusual shape and apparent stupidity. It was endemic to Mauritius and was both admired and ridiculed by the European sailors who reached this uninhabited island. Who had ever seen such a fat bird, with short wings and a strange head, that did not flee from human beings? Everyone knows that dodoes became extinct within a few years of their discovery, as they were slaughtered by sailors and the animals they introduced. All that remains are some 17th century descriptions, fragments of stuffed animals and the few bones found in archaeological digs. Another species, the Rodrigues solitaire (Pezophaps solitarius), endemic to Rodrigues Island, also became extinct in the 17th century.

The level of endemism

The generally high proportion and number of endemic species varies greatly in the many islands scattered throughout the oceans. This is basically due to differences in geographical location, size, climate, relief, and geological history. A large, mountainous, tropical island will have many more species and a higher proportion of endemic species than a small, low island outside the tropics. This contrast is shown by the fact that Iceland's snail fauna consists of just 20 species that are widely distributed throughout the boreal regions of Europe, far fewer than the equivalent communities of the high islands of Polynesia. For example, the genus Partula on the island of Moorea has diversified into at least 11 species, 2 of which gave rise to clearly distinct subspecies. The hot, mild climate of this steep island covered in thick forests allowed a process of allopatric evolution comparable to that with other snails on Hawaii and in the Canary Islands.

The most typical oceanic islands (except for the largest ones, Madagascar and New Zealand) are geologically very young. The few that might be considered old have a maximum age of about 20 million years, and few are more than 10 million years old. The oldest islands have sheltered the most diversified groups, including the rarest species, those that have had the longest time to evolve in isolation. Thus 80% of the flora of the Mascarene Islands consists of endemic species, showing their antiquity.

The geological history of each island decisively affects the evolution of its inhabitants. Many oceanic islands of the Arctic and Antarctic regions have poor biota lacking endemic species owing to glaciations. The repeated submersion of Aldabra and other atolls during the interglacial periods caused the Quaternary faunas of these islands to have been the result of separate episodes of colonization, diversification, and extinction. In the Bermudas, where there was always dry land during the Pleistocene, the situation is more complex. The snails of the genus Poecilozonites have left an excellent fossil record showing a pattern of repeated diversification caused by the fragmentation of the islands and adaptation to environmental changes.

The phenomenon of giantism

The dodo (Raphus cucullatus) is a good example of evolution on oceanic islands. It was a giant pigeon that evolved without predators for millions of years. The dodo occurred on Mauritius, but on Rodrigues Island, another of the Mascarene Islands, there was another nonflying giant pigeon, known as the Rodrigues solitaire (Pezophaps solitarius), that was exterminated in the same way. It is interesting that on both of these remote islands in the Indian Ocean pigeons independently evolved in parallel from flying forms capable of colonization into these celebrated flightless species. It seems that the time available to undergo these modifications is decisive, as the Mascarene Islands are some of the oldest oceanic islands. An endemic genus of giant tortoises, Cylindrapsis, which showed greater differentiation between islands than in the Galapagos, shared the same fate as the dodo and the solitaire.

There were also giant tortoises on the coral islands to the north of Madagascar. In this case, they were probably transported from Madagascar by marine currents, as they were related to the tortoises known as fossils from this large island. Only a single species survived, the Aldabra giant tortoise (Aldabrachelys gigantea), which had a shell 41 in (105 cm) long and weighed 265 lb (120 kg), with the distinguishing feature of a sort of lip on the front nasal cavity that allowed it to drink with its nose and thus make use of the small, shallow pools that are the only supply of fresh water.

Giant tortoises and birds were surely of great importance in the ecosystems of the most isolated islands. They were the main herbivores and frugivores, and all the contemporary accounts say they were very numerous. Yet there have been few studies to clarify the role of these giants in the evolution of the local flora. It is known that the only survivor of these giant tortoises, the Aldabra giant tortoise, has devastating effects on the natural vegetation.

As it needs the shade of the plants to withstand the midday heat, it often digs a hole to make itself more comfortable, and this causes the death of many trees and prevents the regrowth of seedlings. The accumulation of their feces at the base of and around trees is a food source for many terrestrial hermit crabs. The voracity of these tortoises was without doubt a decisive selection factor acting on the plants that form "tortoise turf"; these dwarf varieties rarely exceed 0.8 in (2 cm) in height and are adapted to intense pressure from herbivores. These tortoises have an ambiguous relationship with the nonflying rail Dryolimnas cuvieri aldabranus, which eats young tortoises but eliminates external parasites from the adults. There are now an estimated 150,000 giant tortoises on Aldabra, and they are without a doubt an essential factor in the atoll's terrestrial ecosystem.

One well-known case involving an extinct giant species is that on Mauritius between the dodo (Raphus cucullatus) and the bois de fer (ironwood), also called tambalacoc (Sideroxylon grandiflorum, Sapotaceae), an endemic tree in danger of extinction, which is now represented almost exclusively by trees that germinated before the extinction of the dodo. The seeds of the tambalacoc require a gentle scarification to germinate, and it has been shown that this happens when the seeds pass through the digestive system of a duck. There are no records of the dodo's behavior, but it is known to have swallowed small stones to grind the fruit it ate. All the evidence suggests a relationship of mutualism between the tree that produced the fruits and the bird that dispersed and activated the seeds. Anyway, there is still debate as to whether this mutualism, which developed in a community consisting of few species, was obligate or not.

Extreme cases of isolation

The floristic composition of the Mascarene Islands allows one to appreciate the importance of dispersal by the ocean in oceanic islands that do not form part of large archipelagoes. The terrestrial species living on them are the species that managed to arrive (sporadically and always in low numbers) and then successfully colonized the new territory and managed to maintain demographically viable populations. The Mascarenes, located in the southwest Indian Ocean, consist of three distant volcanic platforms. Reunion has an active volcano, while Mauritius and Rodrigues Island are now highly eroded. Despite the large number of endemic species discovered on each of them, their geographical location means the three islands' biotas share many species of plant and animal. This is natural, if it is borne in mind that 36% of the genera in their flora come from Madagascar, and another 44% of genera present are common to east Africa and Madagascar. Thus 80% of the islands' flora arrived from the west, borne on the dominant marine currents. The remaining 20% of the genera arrived with the Equatorial current from much further away (12% from the Indo-Pacific tropical region, and only 8% from the oriental region).

The terrestrial isopods of Ascension Island are another good example of the history of the colonization of remote islands. Thirteen species of amphipod have been found, nine of them introduced by trade from regions as far apart as southeast Asia, the Mediterranean, South Africa, and South America. The tenth species has a pantropical distribution but undoubtedly arrived without human aid, because it is a halophile and can be found on floating wood. The three remaining species are endemic to the island but are related to African groups, which is to be expected, bearing in mind that Ascension Island is located in the center of the route of the Atlantic South Equatorial current. This current comes from the coasts of the Gulf of Guinea, where there are large rivers that might have transported the ancestors of these endemic species across the sea on vegetation rafts.

2.2 Large isolated islands

The emergent parts of some drifting fragments of continental plates, or quite large microplates, may form isolated worlds, large islands, in some cases almost comparable with micro-continents. These lands have long been separated from the continental landmasses they were joined to, and since then they have wandered through the seas. This is exemplified by Madagascar, New Zealand, New Caledonia, and the Cyrno-Sardian massif that forms what are now the islands of Corsica and Sardinia.

Madagascar: lemurs and chameleons

Madagascar covers 227,801 [mi.sup.2] (590,000 [km.sup.2]) and is 994 mi (1,600 km) long and has a maximum width of 580 km, making it the largest oceanic island in the world (the fourth in absolute size, if the continental islands are included as three of them are larger--Greenland, New Guinea, and Baffin Land). Madagascar has been separated from the African mainland for 165 million years by what is now the Mozambique Channel (between 217 and 747 mi [350 km and 1,200 km] wide), and lies almost entirely in the tropical area, between 12[degrees]S and 26[degrees]S. The climate is not, however, uniform, as there is a very wet eastern strip occupied by a wide variety of tropical rainforest, while the western parts are much drier and the southern tip is almost a subdesert. This southern tip has the highest percentage of endemic plants on the entire island, 95% (it is 85% for the island as a whole). In the arid areas in the south and southeast, there are unusual communities of succulent plants, including members of the Didieraceae, a family endemic to the island whose morphology recalls that of the cacti but is totally unrelated to them.

Madagascar's flora and fauna are derived from the small number of lineages that colonized the island in the past. The only mammals that have formed part of the native flora are bats, insectivores (with 1 endemic family, the Tenrecidae, with 21 species), rodents (with a single endemic subfamily including 10 living species and 2 or 3 extinct ones), primates, carnivores, and hippopotamuses (with 2 dwarf species that are now extinct). There are very few native carnivores on the island. The most important carnivore on Madagascar, its top predator, is the fossa (Cryptoprocta ferox), a large viverrid convergent with the large cats of the Americas, especially the puma. There used to be a larger closely related species, the cave fossa (C. spelaea), which must have preyed on the large prosimians that are now also extinct.

The lemurs are a group of primates, endemic to Madagascar, that were very numerous in the past. Apparently, when humans reached the island there were about 50 species, from the size of a rat, such as the mouse lemur (Microcebus murinus, weighing about 2 oz [50 g]), the smallest of all the primates), to the size of a gorilla (such as the extinct Archaeoindris fontoymonti, with an estimated weight of 375 lb [170 kg]). The lemurs occupied a large range of habitats, from tropical rainforest to open formations, like the higher primates in Africa, and they were represented by many ecological types. Some species (Palaeopropithecus ingens, Babakotia radofilai) showed convergence with the South American tree sloths. Others (Megaladapis) showed convergence with Australian koalas but were bigger. There were also forms (Archaeoprolemur majori and A. edwardsi) that showed convergence with the African baboons, while the aye-ayes (Daubentonia madagascariensis and the extinct D. robusta) occupy (and occupied) the ecological niche of the woodpeckers. After the arrival of human beings, at least 15 species of lemur became extinct, including all the large ones with specialized habits.

Some of the 21 species of tenrecs, such as the Madagascar hedgehog (Setifer setosus) and the tambotriky (Echinops telfairi) are convergent with the hedgehogs. Others, such as many species of the genus Microgale, converge with shrews, whereas others such as the voalavonarabo (Oryzorictes talpoides and other species of the same genus) converge with moles. Plesiorycteropus madagascarensis, a mammal that probably went extinct in the eighth or nineth century and that for many years was considered to be related to the anteater and later as an insectivore related to the tenrecs, has but recently (June, 1994) been separated into a special order, the Bibimalagasea, of which it was the only member.

In Madagascar there are more than a hundred endemic species of birds, including the vanga-shrikes that are endemic to the island and show a spectacular adaptive radiation comparable with the Darwin finches in the Galapagos Islands and the honeycreepers in the Hawaiian archipelago; each species has a different beak, depending on its feeding habits. Madagascar's extinct birds include the remarkable endemic family the Aepyornidae, the giant flightless elephant birds (Aepyornis), which ran rather than flew and were similar to the Australian emus. The largest species, A. maximus, was 9.8 ft (3 m) tall, weighed about 1,102.3 lb (500 kg) and laid eggs with a volume of 3.2 gal (12 l).

Madagascar's reptiles included 56 species of chameleon (two thirds of the world's species of chameleon): 34 of them are typical chameleons (Chamaeleo), locally known as tanala, sangorita, or tarondo, and 22 are dwarf chameleons or anjava (Brookesia). There is also great diversity of girdle-tailed lizards (cordylids) (12 endemic species), skinks (46 species), and geckoes (more than 60 species), with many conspicuous diurnal species of the genus Phelsuma. Madagascar also has 7 endemic species of iguanas, a few snakes (3 boas, 8 blind snakes [typhlopids] and 50 colubrid snakes) and some tortoises. Subrecent remains of giant tortoises have been found. The presence of boas and iguanas is surprising, as both groups are absent from the African mainland. Many groups of reptiles that are well represented in Africa are absent from the island (worm lizards, agamid lizards, monitor lizards, "true" lizards, cobras [elapids] and vipers). The amphibians include 150 species of frog. Unlike most oceanic islands, Madagascar has many freshwater fish (28 endemic species as well as 10 endemic euryhaline species). Most are declining sharply because of habitat destruction and the introduction of exotic fish.

New Zealand: kiwis and giant worms

The two islands of New Zealand have been isolated for 80 million years in the South Pacific. Joined and separated several times over the course of history, they form a geographical unity, the largest "world apart" after Madagascar. The flora and fauna have many elements that originated on the ancient continent of Gondwana, and in several groups the level of endemism is 100%.

New Zealand differs from Madagascar, not only because it is smaller and farther from any continent or other island but also because it has a temperate and occasionally sub-Antarctic climate, quite unlike the warmer climate of Madagascar, with its tropical and semidesert areas. It differs from other oceanic islands that contain unusual faunas and from other islands it is often compared with (mainly the Galapagos Islands, Hawaii, and the Fiji Islands) because of its large size (104,247.6 [mi.sup.2] [270,000 [km.sup.2]]), its age, and its climate and because it suffered the effects of the glaciations. Perhaps its temperate and cold climate has prevented major evolutionary radiations like those that occurred on Hawaii.

There are no native terrestrial mammals in New Zealand, except for an endemic family of bats, the Mystacinidae, which contains two species, Mystacina tuberculata and the apparently extinct M. robusta, whose ancestors presumably colonized the islands about 35 million years ago from South America. They can use their wings as rear legs, and can thus "walk" with some facility in trees and on rocks. Their walking habits are said to be convergent with those of some types of mouse. These two species, together with the bat Chalinolobus tuberculatus, are New Zealand's only native nonmarine mammals. Mysticina tuberculata pollinates the flower of Dactylanthus taylorii (Balanophoraceae), a nonphotosynthetic root parasite of trees and shrubs, that is the only plant in the world pollinated by a walking bat.

Unlike mammals, birds and reptiles show relatively high diversity. These classes of vertebrates contain members with a high ability to colonize islands by migration overseas. Two of the 62 species of reptiles, the tuaturas (Sphenodon punctatus and S. guntheri), form a family (Sphenodontidae) and order (Rhyncocephalia) that are endemic to New Zealand, while the other species are members of just two families, the skinks and the geckoes. The tuaturas now survive on only 24 small islands in Hauraki Gulf and the Bay of Plenty, in the north of North Island, and in Stephen Island, North Brother, and Trios, in the Cook Strait. The tuataras were thought to be a single species until 1990, when it was discovered that the population on the North Brother Island is very genetically distinct and belongs to a different species (S. guntheri), now in danger of extinction. Tuataras thrive on islands that have not been colonized by introduced rats and where seabirds breed. Although they can dig their own burrows, they use those made by some petrels (such as Pachypytila turtur). They are nocturnal and may live for more than 70 years. Many other New Zealand reptiles are very rare. The geckoes arrived during the Miocene, and the skinks in the Plio-Pleistocene, and both underwent major evolutionary radiation. The only known specimen of the gecko Haplodactylus delcourtii measures 15 in (37 cm) from snout to cloaca and is one of the largest members of the gecko family.

Until humans arrived, there were no fewer than 89 species of terrestrial bird, of which 36 (40%) are now extinct. Perhaps the most extraordinary and characteristic were the moas (Dinornis), 11 species of bird varying in size from medium to gigantic (the largest species, D. giganteus, was 10 ft [3 m] tall and weighed about 592 lb [270 kg]), which disappeared with the arrival of the Maoris, as did the their predator, the moa-hunting eagle, or Haast's eagle (Harpagornis moorei). Other notable birds include the kiwis (Apteryx), which are not related to the moas, although they were until recently thought to be. Kiwis are running birds about the size of a chicken, with nocturnal habits, the only birds with a beak that really functions as a nose (using sensitive tactile bristles at the base of the bill). They are declining on the two main islands. They live for more than 30 years, and the females lay a single 14 oz (400 g) egg (proportionally enormous at more than 25% of the mother's body weight) that they incubate for almost three months. It has been suggested that they might be the descendants of flying birds that remained in New Zealand and evolved into flightless forms. The kakapo (Strigops hapbroptilus) is the largest of all parrots (the adult males weigh 5.5 lb [2.5 kg]) and the only nonflying species in its order; it is nocturnal and fewer than a hundred individuals remain. New Zealand's other unusual birds include the weka (Capellirallus australis), the kea (Nestor notabilis), and the takahe (Porphyrio mantelli). New Zealand, like Madagascar, has endemic insular amphibians that are usually missing from oceanic islands: the four living species and the three apparently extinct species of the genus (Leiopelma).

It is not only New Zealand's vertebrates that are extraordinary. Its invertebrates also include many endemic species, many of them very unusual. In addition to the weytas mentioned before (Deinacrida), there are also giant earthworms such as Spenceriella gigantea, which is almost 4.9 ft (1.5 m) long and 0.4 in (1 cm) in diameter, and giant snails (15 species of the genera Powelliphanta and Paryphanta). Entire groups have shown evolutionary radiation into a large number of endemic species (snails and slugs, earthworms, planarians, butterflies, beetles, etc).

The endemic nature of the New Zealand flora and fauna, together with the different levels of taxonomic disharmony that occur in each group, means that the ecological relationships between the different species are also often unusual. The reproductive biology of many New Zealand plants shows many unusual features: small, simple, inconspicuous flowers and fleshy fruits or nuts. Among the plants with archaic origins are the Winteraceae, which show the most primitive reproductive and vegetative structures of any angiosperm, indicating their proximity to their primitive, and extinct, ancestors. Apart from the unusual case of the pollination of Dactylanthus taylorii by a walking bat, Mystacina tuberculata, some New Zealand flowers are pollinated by saurians; this is unknown elsewhere in the world, except for the island of Cabrera in the Balearic Islands, almost exactly on the other side of the world from New Zealand, where the endemic lizard Podarcis lilfordi pollinates some plants.

New Caledonia: araucarias and kagus

The island of New Caledonia is one of the most extraordinary of these large isolated island worlds. It is 1,119 mi (1,800 km) northwest of New Zealand, covers 6,467 [mi.sup.2] (16,750 [km.sup.2]) and has a flora and fauna that is almost entirely endemic and of ancient origin. One of its characteristic features is the great diversity and abundance of gymnosperms (43 endemic species and 1 species found throughout the shores of the Indo-Pacific): it has the world's greatest concentration of conifer species. Of the 19 known species of araucaria (Araucaria, or monkey-puzzle), all of them restricted to the southern hemisphere, 13 are from New Caledonia. Araucaria columnaris, the New Caledonia pine, is abundant in some bays in New Caledonia and on the neighboring Loyalty Islands, reaching a height of more than 164 ft (50 m); it is the traditional tree of the Melanesians. The podocarp Parasitaxus rustus is the only known parasitic gymnosperm. Of the 17 genera of palms that live on the island, 16 are endemic, with about 30 species, some of them in danger of extinction. There are also tree ferns, such as those of the genus Cyathea, with 7 endemic species, 2 of them reaching heights of more than 82 ft (25 m). In total there are 1,575 species of plants, 89% of them endemic. The fauna is as unusual as the flora and does not include a single terrestrial mammal, although there are six endemic species of bats. The highly endemic bird fauna has also suffered a wave of extinctions since the arrival of humans, 40% of the species having disappeared. The island's most unusual animal, Sylviornis neocaledoniae, was exterminated by the first Polynesian settlers. Remains have only recently been discovered of this nonflying bird 5.2 ft (1.6 m) long and weighing about 66 lb (30 kg). It was first considered to be a running bird; later it was suggested that it was a highly modified galliform, a giant megapode, or a scrubfowl, thanks to the discovery of large earth tumuli (megapodes built large earth mounds to breed), up to 131 ft (40 m) in diameter, that were first studied by archaeologists unsuccessfully seeking human burials. One of the most interesting of the island's surviving birds is the kagu (Rhynochaetos jubatus), which together with the now-extinct lowland kagu (R. orarius) forms the endemic family, the gruiformes. The kagu, the national bird of New Caledonia, is a flightless bird that nests on the ground and lays a single egg; its current population is estimated at a few hundred individuals. The reptile fauna was severely affected by the impact of human arrival: a horned giant tortoise (Meiolania mackayi), a terrestrial crocodile (Mekosuchus inexpectatus), a monitor lizard 4.9 ft (1.5 m) long, and some geckoes and skinks, of the 33 species originally present, have all become extinct.

The invertebrates also show an extraordinarily high level of endemism. The snails (more than 300 species) belong to two endemic families, one of them with carnivorous species. The primitive butterflies of the genus Sabatinca have radiated into more than 30 species.

The Seychelles: a small, ancient isolated world

The Seychelles Archipelago lies in the middle of the western Indian Ocean, almost on the equator, and is biogeographically unusual. It consists of about 40 islands, with a total area of about 154 [mi.sup.2] (400 [km.sup.2], scattered over the Seychelles Bank, a platform of more than 16,602 [mi.sup.2] (43,000 [km.sup.2]), at a depth of less than 328 ft (100 m), surrounded by depths of more than 13,123 ft (4,000 m). The islands consist of granites formed in the Pre-Cambrian in the continental crust, and to a lesser extent, of other igneous rocks of more recent origin and with more oceanic affinities. This microcontinent's origins lie in the breakup of Gondwana, when India drifted northwards and Madagascar separated from Africa. Halfway between India and Madagascar, the Seychelles have been isolated since the early Tertiary, about 50 million years ago, and the biota has continued evolving without any further contacts with other dry land. From then until its discovery by Arabic, Chinese, and European navigators, the only arrivals were occasional birds and drifting organisms, as on authentic oceanic islands.

The Seychelles flora and fauna is surprisingly diverse, despite the disturbances suffered in the last three centuries of human occupation. The lowlands were covered by forests of bois-de-fer (Vateriopsis [=Vateria] seychellarum, Dipterocarpaceae), a large tree, of which only about 50 specimens remain on Mahe Island, where there used to be abundant giant tortoises (Dipsochelys) related to the Aldabra giant tortoise. The rivers contained large crocodiles, but all these reptiles were hunted to extinction.

All that remains of the original vegetation is on the peaks and high slopes of the mountains, which reach an altitude of 2,999 ft (914 m). Of the 766 recorded species of flowering plant, only 222 are native, including 69 endemic species that still survive. The archipelago's great age is shown by the abundance of endemic genera, with 10 genera of flowering plant, including six palms. One especially interesting case is Medusagyne oppositifolia, the only member of the family Medusaceae, a very rare small tree of which a dozen wild specimens are now known, although it is easy to raise in cultivation. The plant was thought to be extinct between 1903 and 1970, when a population was found on the island of Mahe at an altitude of 820 ft (250 m), and this is thought to be the last wild population of the species. The double coconut, or coco de mer (Lodoicea maldivica), has been known since antiquity for its fruits, which weigh up to 44 lb (20 kg). This species is a good example of insular specialization, producing few descendants, taking two years to germinate, 25 years to produce its first fruits, and seven years to pass from fertilisation to the ripening of the fruit. The vegetation of the highest levels of the Seychelles is also unusual, consisting largely of meadows of carnivorous plants of the genus Nepenthes, the only plants that can grow directly on the granite dome.

Terrestrial snails are highly diversified, but they are almost all limited to their original ecosystems and are thus in danger of extinction or are already extinct. Most of the species on the archipelago belong to endemic genera and thus show weak taxonomic relations with those of other regions, either Madagascar or India. More than half of the species of insect, which are still little-known, are endemic. Most native species of insect are closely associated with native plants, and so they too are only found in the steepest areas.

One of the most surprising features of the Seychelles is the abundance of amphibians, a group that is absent from all other remote islands. Discoveries include four species of caecilians, a little known group of underground legless amphibians that recall earthworms and are of ancient origin and unknown affinities. The Sooglossidae is another endemic family, consisting of three species of small tree frogs (Nesomantis thomassetti, Sooglossus gardineri, and S. seychellensis). Another endemic tree frog (Tachycnemis seychellensis) is a very primitive representative of the family Hyperoliidae, which is common in Madagascar and Africa. The amphibians also include a true frog introduced in historic times.

Apart from the crocodiles and the giant tortoises, the reptiles also include endemic skinks (especially those of the genus Mabuya) that have diverged greatly from their mainland relatives. The only species showing African affinities are one species of chameleon and the freshwater turtles. Fifteen species of endemic bird are known, three of them (and surely many more) already extinct and most of the rest are among the most endangered species in the world. Surprisingly, a sunbird has adapted to humanized environments, where there are many gardens full of flowers. The native mammals are restricted to two endemic species of bats, one insectivorous and the other frugivorous.

2.3 The intercontinental megaarchipelagoes

At the zones of friction between the large continental plates, large groups of archipelagoes have arisen with both continental and volcanic rocks. Their flora and fauna are strange mixtures of components from the continents that surround them. The most notable examples are the Mediterranean islands, the Antilles (Lesser Antilles, Greater Antilles and Bahamas), and the eastern oceanic islands of Insulindia (the Philippines, except for Palawan and the Calamian Group [which are a continental prolongation of Borneo] Sulawesi, the Moluccas, Timor, Flores, etc.).

The Mediterranean islands' great diversity of lizards

The Mediterranean is like a miniature ocean, with marine currents, areas of high productivity, and both continental and paraoceanic islands. The Mediter-ranean's paraoceanic islands cover a total area of more than 30,888 [mi.sup.2] (80,000 [km.sup.2]). Despite having been in contact with the surrounding lands, their flora and fauna was as extraordinary as that of oceanic islands, at least until human colonization. These islands ,with an arid and highly seasonal climate are rich in endemics but do not show such high biodiversity as the Antilles or the Philippines. In the Pleistocene, all the native land mammals, about 45 species, were endemic. There were dwarf elephants (Elephas falconeri) only 3 ft (1 m) tall at the withers, dwarf hippopotamuses (Phanourios) the size of a pig, dwarf deer (Praemegaceros) that measured 20 in (50 cm) at the withers, a dwarf goral (Myotragus) with frontal eyes and rodentlike incisors, and giant dormice (Leithia, Hypnomys). The Mediterranean islands have not been sufficiently isolated for the birds to evolve. Few groups have speciated into endemic island forms, and the most remarkable ones are now extinct. There was a dwarf owl (Bubo insularis) that lived on Corsica and Sicily, and a giant flightless swan (Cygnus falconeri) in Malta. The few Mediterranean island endemics that still survive are small passerine birds, such as Marmora's warbler (Sylvia sarda) on Corsica, Sardinia, Mallorca, and Ibiza; the Cyprus warbler (S. melanothorax) and the Corsican nuthatch (Sitta whiteheadi) from Corsica; and Kruper's nuthatch (S. krueperi) from several Greek islands. These birds have coevolved with the Mediterranean vegetation and disperse the seeds of the fleshy fruit of several plant species, such as mastic trees (Pistacia), laurels (Laurus), mock privets (Phillyrea), and wild olives (Olea). On Mediterranean islands there has been some diversification of the lizards of the genus Podarcis. A dozen different species differentiated into about a hundred subspecies, with forms that are in color melanic, cyan, green, brown and that vary in shape--stout and slender, short-legged and long-legged, and so on--distributed throughout the islands of the Mediterranean Basin. They reflect well the variety of adaptations to different island environments, which are more heterogenous than they appear at first glance. There are also some relict Miocene species, such as the Bedriaga lizard (Archaeolacerta bedriagae) or the dwarf lizard (Algyroides fitzingeri), both from Corsica and Sardinia. There are no native snakes on the Mediterranean islands. Yet Corsica and Sardinia are among the few islands where there are urodeles, cave-dwelling salamanders to be precise (four species of the genus Speleomantes [=Hydromantes] that live in different karstic massifs), and newts (two species of the genus Euproctus). These species show that the Cyrno-Sardian microplate was formerly connected to the mainland, and they are not the result of colonization from across the sea. There are also some anuran amphibians on the Mediterranean islands: some discoglossid toads, such as the small frogs Alytes muletensis from Mallorca; the extinct A. talaoticus from Menorca; and the bell-toads (Discoglossus sardus and D. montalentii) from Corsica and Sardinia, which reached the islands when the Mediterranean dried up and the Sardinian tree frog (Hyla sarda), an amphibian whose ancestors reached Sardinia by migrating from over the sea.

The Antilles: lizards, hutias, and alamiquis

As they are a bridge between North America and South America, the Antilles have a complex geological history, one that is not yet entirely clear. The Antilles consist of three main archipelagoes, the Greater Antilles, the Lesser Antilles, and the Bahamas, with a total surface area of about 92,664 [mi.sup.2] (240,000 [km.sup.2]). They are located in the tropics with high and relatively uniform temperatures throughout the year. The Greater Antilles are very old, although there is debate about whether they were ever joined to the continents or not; anyway, connections to the mainland need not be postulated to understand their biotas, so unusual are they. The Lesser Antilles do not appear to be so ancient and do have active volcanoes. The Bahamas are flat coral islands located in a large area of shallow water (the Bahamas Shelf) and have suffered large changes in their area from the end of the last ice age to the present day, having declined from 49,807-4,247 [mi.sup.2] (129,000-11,000 [km.sup.2]).

The fauna of the Antilles is largely endemic. Before the arrival of humans, there were about 90 species of nonflying terrestrial mammals, all of them endemic (more than 80% of them now extinct), as well as 45 species of bats (with 1 endemic family). There were 12 species of insectivores, 18 of sloth (both terrestrial and arboreal), and between four and seven simians, and more than 55 rodents, mainly hutias (Capromys and related genera). The insectivores belonged to two very primitive families (solenodontids and nesophontidas), the only survivors of which are two solenodons (alamiquis) (Solenodon cubanus, the Cuban and the Hispaniola S. paradoxus). There were even sloths, ranging from small arboreal species to relatively large terrestrial sloths, such as the Cuban Megalocnus rodens, with a highly specialized dentition; all are now extinct. There were simians on Cuba, Hispaniola, and Jamaica, the home of the Jamaica monkey (Xenothrix mcgregori), of unknown affinities; all these monkeys are now extinct. The rodents underwent adaptive radiation and evolved into arboreal species that occupied niches similar to those of monkeys and squirrels, more terrestrial species that occupied niches similar to those of porcupines, and other terrestrial species that were much larger. The chewing apparatus of these rodents were related to their adaptive radiation, and different models of mastication have been reported. Only a dozen species of hutias survive, most of them on Cuba.

The birds of the Antilles included one endemic family with five species, the todies (Todidae), whose relationships are unknown. There are 160 species of endemic bird in the West Indies, and at least 40 species have become extinct since humans arrived, including the long-legged giant owls (up to 4.9 ft [1.5 m] tall), large barn owls, flightless rails, carrion hawks, giant falcons, finches, and martins. The current diversity of birds in the Antilles is a shadow of the former diversity.

The herpetological fauna of the Antilles is extraordinarily diverse, much more diverse than that of any other archipelago in the world. There are more than 480 species of reptiles and amphibians. There are six families of saurians, mainly the geckoes and iguanas, and there are two major genera: Anolis, a genus of iguanas that has undergone a noteworthy evolutionary radiation into 110 species, and Sphaerodactylus, a genus of geckoes with about 80 species. There are also 41 species of colubrid snakes, Antilles boas (Epicrates), and a few crocodiles. The anurans include some armored toads (Peltophryne) and more than 110 species of Eleutherodactylus.

Wallacea: rats, mice, and tarsiers

Between the Australian and Oriental biogeographical regions there are several oceanic archipelagoes that are not exactly on the Sonda Platform nor the Sahul Platform. The islands located outside these platforms have floras and faunas with many endemic species, and which cannot biogeographically be included within the oriental region or the Indo-Malaysian region (to which the islands of the Sonda Strait belong) or the Australian region (to which New Guinea and the other islands of the Sahul continental platform belong).

This truly intercontinental mega-archipelago is known as Wallacea after the first naturalist to recognize the region's particular character. The north part consists of the Philippines (except for Palawan and Calamian, which are northward extensions of Borneo), the central part is the Moluccas and neighboring islands, and the southern part is the chain formed by the islands of Lombok, Flores, Arum, and Timor. Radiations of species have never occurred on these islands, strange endemic species have arisen, and entire taxonomic groups never reached them. As in other mega-archipelagoes, the origin of their fauna is mixed, with species proceeding from both the Australian and the eastern regions.

The many species of rats and mice clearly show the mouse family underwent adaptive radiation in Wallacea (more than 100 endemic species, including at least 46 in the Philippines and 36 on Sulawesi). There are very large species, tiny ones, some that are arboreal, others that are insectivorous, as well as frugivorous and granivorous species, and others with broad diets.

The pig family (Suidae) colonized several islands, giving rise to several endemic forms: the Celebes pig (Sus celebensis) and the extinct Celebochoerus heekeneri, in Sulawesi; Sus philippensis and S. cebifrons, which were respectively thought to be subspecies of the wild boar (S. scrofa) and the bearded pig (S. barbatus), in the Philippines; and S. heureni and S. timorensis, also sometimes considered subspecies of the wild boar, on Flores and Timor. As on some Mediterranean islands, the Philippines and Sulawesi have dwarf species of cattle, the anoa (Bubalus depressicornis) and the mountain anoa (B. quarlesi) in Sulawesi and the tamarau (B. mindorensis) in the Philippines. Some islands (Timor, Flores, Sulawesi) were colonized by stegodonts, which evolved into dwarf island forms that became extinct long ago.

In the Philippines and on Sulawesi there are four endemic species of tarsier (Tarsius), primates whose phylogenetic relationships are still unclear. There are also other modern primates and seven species of Macaca in Sulawesi alone. The Philippines has endemic species of dermopterans (flying lemurs), one of the favored preys of the endemic monkey- eating eagle (Pithecophaga jefferyi).

2.4 The large oceanic archipelagoes

Archipelagoes of volcanic islands have often gradually formed above hot spots on the earth's crust over millions of years. They are very different, because of their origin and materials, from those formed by fragmentation of ancient crust. These archipelagoes include islands of different ages, sometimes arranged in order by age, such as the Hawaii Archipelago, the Galapagos Islands, the Canary Islands, the Cape Verde Islands, and many other isolated archipelagoes.

The Canary Islands: malpais rats and laurel-forest pigeons

The Canary Islands are a volcanic archipelago covering 2,895 [mi.sup.2] (7,500 [km.sup.2]) that lie about 62 mi (100 km) off the African coastline. The seven large islands have been inhabited for about 2,000 years, and there are several smaller islands. The oldest islands, Fuerteventura and Lanzarote, are closest to the mainland and are more than 25 million years old, while the most recent ones are Hierro and La Palma, both less than two million years old. The Canary Island flora and fauna are very rich in endemic species. The four islands closest to the continent were colonized by terrestrial mammals. There were at least two endemic species of giant rats (Canariomys), one malpais rat (Malpaisomys insularis), and two shrews (Crocidura), but only one survives.

The endemic birds include the blue Teide finch (Fringilla teydea); the laurel-forest pigeons (Columba junoniae); the "paloma rabiche" of Tenerife, La Palma, and La Gomera, and C. bollii, the turquoise pigeon of the same islands; the Fuerteventura stonechat (Saxicola dacotiae); and the recently extinct oystercatcher (Haematopus meadewaldoi). There are two further species whose classification is uncertain; a small barn owl (Tyto alba gracilirostris) and a kinglet (Regulus regulus teneriffae). Two other endemic species of land bird (a long-legged quail and a goldfinch) and two extinct terns are known. As in the Galapagos, there is also an endemic hawk (although in this case it is a subspecies).

Some of the most extraordinary animals of the vertebrate fauna are the lizards of the endemic genus Gallotia (probably derived from the long-tailed lizards of the genus Psammodromus). The largest of the four surviving species, G. simonyi, now survives on a few cliffs on the islands of Hierro but is thought to have been widespread throughout the western islands of the archipelago. The extinct G. goliath (probably conspecific with G. simonyi) was the largest and robust lacertid lizard in the world (more than 4.6 ft [1.40 m] long). It was apparently greatly appreciated as a foodstuff by the Guanches, the first humans to settle the islands. There have been other adaptive radiations of species of reptiles in the Canaries (geckoes of the genus Tarentola and skinks of the genus Chalcides).

The Hawaii Archipelago: finches, fruit flies and snails

Hawaii is the world's most isolated oceanic archipelago and consists of eight main islands and 124 smaller islets, covering a total of 6,471 [mi.sup.2] (16,759 [km.sup.2]), and continuing under the Pacific as the Emperor Seamounts. The Hawaii Islands form a line nearly 1,678 mi (2,700 km) long that almost follows the parallels. The largest and newest islands are farthest to the east, and farther to the west the islands are older and generally smaller. The nearest mainland, the coasts of California are 1,864 mi (3,000 km) from Hawaii (the Big Island). The archipelago is relatively recent: the main islands, the eastern ones, are less than five million years old (the Big Island of Hawaii is less than one million years old!). The small western islands, almost all of them lacking a surface rock mantle, are much older. With a subtropical climate, the islands have the most unbalanced fauna in the world.

The Hawaii Archipelago had no terrestrial mammals, reptiles, or amphibians. There was only a single species of bat that was native but not endemic, Lasiurus cinereus, an arrival from North America. It was, however, a paradise for birds: The original terrestrial bird fauna, derived from about twenty colonizations from different continents and from other Polynesian islands, was entirely endemic. The adaptive radiation of the honeycreepers has already been mentioned (see page 333), but they were not the only birds to have undergone a comparable radiation. On the islands of Molokai and Maui, for example, there were flightless ibis, Apteribis that were convergent with the kiwis of New Zealand. There are four known extinct species of moa-nalo, large nonflying ducks, with unusual beaks. The most unusual of the moa-nalo was Chelychelynechen quassus, which had a beak like that of some tortoises. There were also at least a dozen species of flightless rails, but only two survived into the historic past, and they are now extinct. The others went extinct when the first humans, the Polynesians, arrived together with their associated fauna. The predators were also birds: two species of hawk (Buteo), a harrier (Circus dossenus) that was convergent with the sparrow hawk (Accipiter), and like it ornithophagous; the fearsome eagle (Haliaeetus); and four species of owl of the endemic genus Gallistrix. The only one that still survives is the Hawaiian hawk (Buteo solitarius).

Hawaii's invertebrates fauna is also very unusual. An estimated 99% of the insects (more than 10,000 known species) are endemic and appear to have derived from about 400 immigrant species. Many groups have radiated greatly. One genus of butterflies, Hyposmocoma, has more than 220 endemic species. There are more than 700 species of fruit fly (Drosophila), the product of two separate colonizations. There are more than 1,200 species of terrestrial mollusc, which may have derived from about 20 colonizing species. There are also some invertebrates whose evolution has led them to adopt lifestyles otherwise unknown in their group. Thus, the larvae of one species of damselfly, Megalagrion oahuense, are the only damselfly larvae in the world that are not aquatic but terrestrial and live in the soils of the wet forests in the mountains. A cricket of the genus Caconemobius has ceased to be terrestrial and adopted a partly marine lifestyle. The caterpillars of the moth Eupithecia have ceased to be herbivorous and evolved into predators.

The Galapagos Islands: tortoises, iguanas, and finches

The Galapagos Islands are 1,000 west of South America. There are nine main islands and 10 smaller ones, with a total surface area of 3,089 [mi.sup.2] (8,000 [km.sup.2]). They are the best conserved of the oceanic archipelagoes and were apparently never colonized by South American indians or Polynesians. The first known reference to their discovery was by Fray Tomas de Berlanga, Bishop of Panama, who visited them in 1535. They were not immediately colonized, and there were no significant human settlements until 1832, and this helped to conserve their wildlife. Even now the population is less than 10,000. Some of the large islands are uninhabited and there are no introduced mammals on them, something that is now exceptional on oceanic islands.

The archipelago is recent in origin. The oldest islands, to the east, are estimated to be between three and five million years old, although some datings suggest that there was an island about 9 million years ago. The large western islands (Isabela and Fernandina) are probably the most recent and seem to have emerged less than a million years ago. The Galapagos Islands were settled entirely by an immigrant fauna that had crossed the ocean, and some, such as the finches that Darwin made famous, underwent adaptive radiation.

In the Galapagos Islands, there are 625 species of plant, more than 200 of them endemic. The vegetation is dominated by prickly pears (Opuntia, Cactaceae) that have diversified greatly and include some giant forms (in several cases up to 13 ft [4 m] tall, with trunks over 3 ft [1 m] in diameter). Their fruit ("prickly pears") are eaten by some of the island's vertebrates, such as the giant tortoises and terrestrial iguanas. It is also worth highlighting some tree species of Scalesia (Asteraceae), an endemic genus that has undergone as great an adaptive radiation (14 species on the islands) as the finches, although less spectacular and well known. Interestingly, the Galapagos tomato (Lycopersicon cheesmanii) can be watered with seawater. The Galapagos Islands were invaded three times by rats and mice before the last invasion accompanying human settlement. One of these colonizations gave rise to a large genus of rats (Megaoryzomys), that is now extinct. Another invasion gave rise to a group of smaller rats typical of lava fields, with some species living on the islands of Santa Cruz (Nesoryzomys darwini), Fernandina (N. fernandinae, N. indefessus), and Santiago (N. swarthi) and with others that recently went extinct on the islands of Santa Cruz, Isabela, Baltra, and Rabida. A further invasion is represented by an endemic species of rice rat (Oryzomys galapagoensis).

Unlike most oceanic archipelagoes, human arrival on the Galapagos does not seem to have led to the extinction of a single species of bird. One of the most notable, unlike any other living bird, is the flightless cormorant (Compsohalieus [=Nannopterum] harrisi), twice as large as flying cormorants and a much better diver than them. (It can reach a depth of 148 ft [45 m]). It has very small wings and is the only living species that can give us an idea of what the famous Cretaceous genus Hesperornis must have been like. In addition to the famous finches, 10 other native species of land birds breed on the Galapagos, including the two main predators of the small vertebrates, the Galapagos hawk (Buteo galapagoensis) and the small Galapagos barn owl (Tyto punctatissimia). There is an endemic turtledove (Zenaida galapagoensis); four endemic species of mockingbirds (Mimidae); the Galapagos Island mockingbird (Nesomimus); a rail, the pachay (Laterallus spinolotus); a muscicapid, the Galapagos flycatcher (Myiarchus magnirostris); and a hirundine, the Galapagos martin (Progne modesta).

The Galapagos Islands are a genuine paradise for reptiles. It was in fact from their reptiles, the giant tortoises, that they gained their name (galapago means freshwater tortoise in Spanish) and became so celebrated. In the 17th and 18th centuries they were visited by pirates and corsairs who knew they would find freshwater and tortoise meat. On almost all the islands there were large populations of giant tortoises (Geochelone elephantopus), although they are now mainly abundant on Isabela Island, especially on Alcedo Volcano. Other notable reptiles include the marine and terrestrial iguanas. The marine iguanas (Amblyrhynchus cristatus), which reach a length of 3.9 ft (1.20 m), are among the few saurians that have been able to colonize coastal waters. They are vegetarians, like the terrestrial iguanas, and eat seaweeds. They can drink seawater thanks to special glands that excrete the excess salt. The land iguanas (Conolophus subcristatus and C. pallidus) are much less numerous. There are only two intact populations, the one on Fernandina Island (despite the devastating effects of a recent volcanic explosion) and those at Plaza Sur on the island of Santa Cruz. The other populations appear to be declining and some have already disappeared.

3. Islands of water: lakes

3.1 Miniature seas

For an observer, a lake focuses and beautifies any landscape. It contrasts the water with the land, and as a lake's size is usually moderate, it is seen as a symbol of the calmness not found in the sea. The naturalist sees it as a miniature sea, summing up the rules and the problems of aquatic ecology in general. Lakes are often of economic importance as fisheries or sources of other products, as water reserves, and even as tourist attractions. Equally positively, they make the local climate milder.

When talking of epicontinental waters (see volume 10, page 325), more importance is given to the means of transport (rivers) than to their diversions, extensions, or associated systems, broadly referred to as lakes. This is because this circulation system integrates the biosphere. But here and there the transport system is slowed down in "containers," where its circulation slows down or ceases: lakes. In lakes, differentiation along the vertical axis, called stratification, is more important than horizontal transport, a major difference between lakes and rivers. Reservoirs, human built diversions that intercept a river's flow, may be considered as hybrids between lakes and rivers.

Glacial erosion lakes

In the high mountain of glacial countries, lakes are usually the result of erosion by ice that has excavated flat places and depressions in valleys, which are in turn signs of cirques that collected snow or of the erosion caused by ancient glaciers. These mountain lakes occupying depressions in relatively insoluble rocks and lacking enough sediment to buffer the water, are more sensitive to rain, especially acid rain, and thus function as antennae to detect acid rain.

All the nordic and alpine lakes are postglacial, meaning that their populations are less than 10,000 years old, not long enough for major evolutionary change to have occurred. Most occupy valleys that were excavated by former glaciers and are now closed by moraines. The lakes house, as a result of their relative youth, a relatively uniform and poor, but not insignificant flora and fauna, as they include organisms of great beauty with thought- provoking adaptations. There are many lakes in the frozen regions of the northern hemisphere as well as in the southern hemisphere, for example, in South America and in New Zealand.

Volcanic lakes

The lakes occupying former volcanic craters may have waters that are very chemically different from the waters of most freshwater lakes. For example, they often contain disproportionate levels of some metals and other elements, and there is sometimes underwater input of gases, which accumulate in the deeper layers, where they dissolve, together with the gases produced by living organisms, as a result of hydrostatic pressure (Lake Kivu and others). The retention of gases may eventually lead to spontaneous and sudden release of the gas, as happens when a bottle of sparkling wine is opened. This release of gas occurs if there is a sudden movement in the lake's water, which forms chimneys through which gas escapes. This occurred in two small lakes in Cameroon in 1984 (Lake Monoun) and in 1986 (Lake Nyos): the explosive release of gases meant the air at ground level was rich in carbon dioxide and this caused the death of many people and animals in the area. This gives us an idea of what the terminal greenhouse effect would be like.

Tectonic lakes

Although there are tectonic lakes in direct contact with rivers (former abandoned meanders, delta lakes) or dependent on karstic circulation (such as Banyoles Lake in Catalonia) (see pages 372-375), most tectonic lakes have origins in tectonic structures that far predate and were totally unconnected to their current role as a deposit for water. The most spectacular depressions are perhaps those related to the edges of the plates of crusts in which there are active sutures. Iceland has emerged at a site of divergence, the Mid-Atlantic Ridge, and has many recent lakes, postglacial (less than 10,000 years old) and populated by recently arrivals from both Europe and the Americas.

Even more interesting are the famous lakes of the Rift Valley system in tropical eastern Africa, another site of major volcanic action that rose in the Miocene and the Pliocene, about 12 million years ago, in an area where there were already lakes. This is why there has been the continuity necessary for the flora, and especially the fauna, to diversify, but unfortunately its survival is now increasingly threatened. Lake Ukerewe (Victoria), in the center, is the largest (26,641 [mi.sup.2] [69,000 [km.sup.2] ]) and is relatively shallow (260 ft [79 m]). In a line stretching along the meridians are Lake Malawi (Lake Nyasa), Lake Tanganyika (the deepest, reaching 5 ft [1,470 m]), Lake Kivu, Lake Rutanzige (Lake Edward), and Lake Albert, among others. Between the last two, at the base of Mount Ruwenzori, lies Lake George, with water whose chemical composition is very different from the others. The lakes of the eastern parallel rift, such as Lake Turkana, Lake Rudolph, and some other smaller ones, have more alkaline waters with a higher mineral content. European explorers named these lakes after royal personages from their own countries, and now at least some have been renamed after leaders of states created after decolonization (such as Lake Albert in the Democratic Republic of Congo, renamed Lake Mobutu Sese Seko). The vernacular names have rarely been used, as there were often many local names, because each population saw 'its' lake from a different point of view and had a different name for it.

There are tectonic lakes that lie on a fault or occupy a tectonic basin. When the bottom of the lake is subsiding, and sinking at the same rate as the sediments accumulate, the lake will last for a long time; examples include Lake Biwa in Japan and Lake Baikal in southern Siberia. The histories of these lakes goes back more than a million years and can be interpreted by using the remains conserved in the sediments, such as the remains of aquatic organisms or pollen, or the variable and characteristic ratios in each level of the different major isotopes ([sup.12]C, [sup.13]C, [sup.14]C, [sup.16]O, [sup.18]O, and other elements). The very old lakes have been the setting for major evolutionary processes, contain many endemic species, and may have been the site of origin of other species that later dispersed.

The Great Lakes of North America have relatively recent populations owing to the effect of the glaciations, and their biota are not as ancient as those of the African lakes. The five Great Lakes on the St. Lawrence River (Lakes Superior, Huron, Michigan, Erie, and Ontario, with their minor extensions) contain the largest mass of freshwater on the earth's surface (5,907 [mi.sup.3] [24,620 [km.sup.3]]). These very old tectonic lakes were totally reshaped by the glaciations, which must also have eliminated any trace of previous life, as their current population is clearly postglacial and relatively recent.

Endorheic lakes

There are also, paradoxically, lakes in arid regions. The largest of all is the Caspian Sea, with a salinity of 10-20 g/l, representing a volume of salts equivalent to all the other continental bodies of water put together. Apart from this sea, which has many unusual organisms of its own, there are many other inland basins subject to excessive evaporation, or that at least evaporate most of the water they receive. They are the final destination of rivers that do not flow into the sea, although these watercourses generally only flow in one season of the year.

The washing of their basins--called endorrheic because they flow inland and not to the sea--means that the water in these lakes is often very salty and the lake may be below sea level. One of the best known examples is the Dead Sea, which is 1,286 ft (392 m) below sea level and has a salt concentration that may reach 226 g/l, slightly greater than that of the Great Salt Lake (203 g/l). On the island of Hispaniola, a small dead sea is forming: Lake Enriquillo (Lake Xaragua), which is about 164 ft (50 m) below sea level and is twice as saline as the sea.

3.2 The ecological functioning of lakes

Although several aspects of lake ecosystems can be considered as parts of the biomes in which they are located, it is worth taking an overview of lakes as a whole. They may vary greatly, and they have generated a specialized nomenclature, not always consistent with everyday usage, that distinguishes between lakes on the basis of their depth (pools are shallower than lakes), salinity, persistence, and other characteristics.

The chemical composition of the waters

Chemically, lakes vary greatly. All the world's oceanic waters, surprisingly, tend to have a constant composition, but the waters on dry land, particularly lakes, which are not always freshwater, vary much more in terms of their proportions of mineral ions. Waters whose composition varies to a greater or a lesser extent from what generally occurs in its region have been exploited as medicinal mineral waters.

Most lakes have waters with low salinity, but some are very salty. The ratios of the different mineral ions vary much more between lakes than in seawater. Both these differences and the relative isolation of the continental water bodies mean that many groups of organisms (e.g., chlorophytes, euglenophytes, cladocerans) show greater species diversification in continental waters than in seas. Freshwater acted as the cradle for the development of the true fish. Unlike seawater, freshwater on land has been invaded by many terrestrial organisms (mosses, ferns, flowering plants, pulmonate molluscs, arachnids, insects, amphibians) that, with only a few exceptions, are absent from oceans. Many groups of marine organisms are poorly represented or absent on continental waters. It seems that the change from one environment to the other, at least in the case of several crustaceans, may have taken place through the continuously connected phreatic water, filling the interstices of the sediments of beaches and riverbeds. The Baikal seal (Phoca sibirica) is an unusual case, as its ancestors arrived from the Arctic Ocean over the glaciers covering the northern half of Eurasia during the ice ages.

The unusual chemical composition of these waters may have a greater influence than their absolute salt content. This happens in waters that are very rich in gypsum or borates or are unusual for their disproportionately high content of other ions. Excess salt may cause deposition of the calcium carbonate and magnesium carbonate contained in incoming waters, forming tufa and concretions as beautiful as those of Lake Mono in California (see Fig. 218) and those in the pools in Mexico occupying former volcanic craters.

Life always finds a way to invade and adapt to waters of the most extraordinary compositions. There are always, at least, bacteria, cyanobacteria, and even unicellular chlorophytes, such as Dunaliella. If the input of freshwater increases (like the input of the River Jordan into the Dead Sea), the less salty water floats on the denser water and prevents or reduces the free vertical mixing that would take oxygen to the deeper layers and in general permit greater biological activity. The fact that mixing is difficult means that the bottom of the lake has even less visible life. The waters of the Dead Sea have circulated a little more in the last few years; this should not, however, be seen as an improvement but recognized as a natural consequence of the fact that the consumption of water from the Jordan is greater.

The heat regime and fertility

The heat regime of lakes depends basically on the greater specific heat capacity of water than of air and on the fact that freshwater is densest at 39[degrees]F (4[degrees]C). During the hot season, the water in the upper layers of a lake warms up, and generally at a relatively shallow depth there is a point at which the downwards transmission of heat ceases, and a layer forms in which there is a sharp change in temperature, the thermocline. Vertical thermal profiles show this discontinuity, which is closer to the surface in lakes than in the oceans; the smaller the lake is, the closer the thermocline is to the surface. Normally, the thermocline forms at a depth of between 33 and 98 ft (10 and 30 m), and during the more productive hot season it effectively isolates the deep layers, where the dissolved oxygen may be exhausted.

When the surface water cools down, its density increases, and it falls by convection to the bottom of the basin, thus breaking the stratification in a time much shorter than the time it took to form. The best way to see this is to compare how the surface temperature varies over the course of the year in relation to the accumulated heat in the water column, a ratio that naturally describes an irreversible trajectory. The lake thus acts as a heat accumulator during the hottest season and gives up this heat in the cold season, so that it locally buffers, on its banks, the general climatic fluctuations; this explains why the favorable local climate on the shores of the large lakes on the edges of the Alps allows the growth of a relatively thermophilous vegetation. The lake's annual heat balance, the heat exchanged over the year, can be quantified. It is between 20,000 and 40,000 cal/[cm.sup.2] in the large lakes in temperate latitudes, such as those in the forests in the Alps and the Great Lakes in North America; in mountain lakes, these values are usually between 5,000 and 10,000 calories/[cm.sup.2], and they are even smaller in tropical lakes. In reservoirs, these values are between 2,000 and 17,000 calories/[cm.sup.2] that is to say, they contribute very little to regulating the local climate.

The formation of the thermocline is important, because to some extent, it isolates or separates an upper compartment within the lake that for a time may gradually lose its nutrient content, and this determines many aspects of the biology of the entire system. Vertical mixing leads to upwelling to the light of nutrients (nitrogen, phosphorus) that had ended up in the deepest layers and thus normally starts a new ecological succession. This usually starts with a large quantity of diatoms, such as Aulacosira, Tabellaria, and the beautiful star-shaped Asterionella. They are followed by chrysophytes (Dinobryon, etc.) and dinophytes (Peridinium cinctum, Ceratium hirundinella). In summer there are also numerous chlorophytes, and towards the end, shortly before vertical mixing occurs, cyanobacteria are frequently dominant. The depth of the lake defines its capacity as a system as a reserve of heat and nutrients.

Many lakes, called meromictic lakes, do not mix all the way down to the bottom. Sometimes, the cause of meromixis may be that the lake contains salty water or suspended sediments at the deeper levels, but it is normally due to the fact that there is a large mass of water at a temperature near 39[degrees]F (4[degrees]C), at which its density is greatest, that does not mix easily if it is very deep, that is to say, far from the effects of the wind. This water accumulates nutrients until they are recycled, which only occurs in very cold winters. This happened in some lakes in the Mediterranean Basin in the winters of 1963 and 1970 and was also to some extent reflected in the Mediterranean Sea itself.

Life in lakes

Below the thermocline, in the deep layers of the lakes, the dissolved oxygen in the water may be exhausted if the upper illuminated layer produces a large quantity of phytoplankton that falls to the bottom and decomposes, consuming oxygen. Thus, any excessive production due to discharges from outside the lake (not simply internal recycling) lead to a loss of water quality in the deeper layers. This loss is even greater if the organic matter input enters directly from the affluents into the deeper layers of the lake or reservoir, where it contributes to the consumption of oxygen. Communities living on the shores of a lake that extract their drinking water from the deeper layers of the lake are very aware of this. This loss of water quality is the most noticeable effect of the eutrophication of lakes and of freshwater in general. Today, eutrophication is ruining the water quality and beauty of lakes everywhere. Lakes associated with karstic systems, which receive underground waters, may be relatively infertile but enjoy some protection against eutrophication. In addition to the phytoplankton, lake vegetation includes many algae and vascular plants on the shoreline. Vascular plants have colonized freshwater very successfully. The relatively calm surroundings, very different from the pounding by the sea on the coastline, and the ability to make use of the air, water, and soil, have given rise to great taxonomic and morphological diversification in these invaders from dry land. Often their contribution to the life of the lake, in terms of the production of living matter, is far greater than that of the phytoplankton. Many forms have been introduced throughout the world, such as Elodea [=Anacharis] canadensis (Hydrocharitaceae) and the beautiful but troublesome water hyacinth (Eichhornia, Pontederiaceae), which only grows in waters that do not freeze in winter.

In freshwater there are also communities of terrestrial origin, especially insects, pulmonate molluscs, and amphibians. Insects and amphibians often spend their juvenile stages in the water, while the adults form part of terrestrial ecosystems. This kind of evolutionary duality is also important from an ecological point of view because it involves the transfer of organic material from the water the land. One just has to remember the vast numbers of chironomids that, looking like columns of smoke, often spiral or rise up from the water.

Whereas the epiplankton of continental waters is comparable with that of the sea in terms of diversity, that of lakes is frankly poor. There are in fact only protoctists, rotifers, and crustaceans (mainly copepods, such as the genus Cyclops, and cladocerans, such as Daphnia) and some insect larvae (Chaoborus). This poverty does not prevent them from having truly original features in their biology, such as the ease with which local changes in body morphology occur (the appearance of spines or protuberances) in response to both the passing of the seasons (the short generations last between a few days and a few weeks) and to the presence of potential predators.

Both cartilaginous fish and bony fish certainly originated in freshwater, where they are both highly diversified.

There is an immense diversity of fish in the rivers of South America and in the lakes of Africa. Regions that have suffered recent glaciations, such as the whole of northern Eurasia and North America, have lakes with poorer faunas. In these areas, the invasion of species that spend part of their life in the sea, such as salmon and eels, has been especially important.

188 Bora Bora atoll in the Society Islands (Polynesia) is a good example of a doubly isolated system, as it is an island within a fringing reef, as this satellite photo clearly shows. Atolls are more or less circular islands, of variable size, consisting of a barrier reef of coral around a central lagoon of salt water. The barrier reef may be continuous or contain passages joining the central lagoon to the open sea. As these reefs consist of the skeletons of corals, which need high temperatures and a very stable environment to grow, they only occur in tropical seas. In fact, they are occur almost exclusively in the Pacific and Indian Oceans.

[Photo: Spot Image / CNES / Explorer]

189 Julia sets may serve as an example to explain how the process of speciation begins. A Julia set is generated by drawing a line on a surface separating the points of the plane, fulfilling a given property from those that do not (a line generated in this way has a fractal structure). The mathematical equation defining the Julia set on the left is the simple equation [X.sub.n + 1] = [X.sup.2.sub.n] + C, when [X.sub.n] and [X.sub.n] + 1 are two points on the plane and C is a constant. Varying the value for C in this formula gives a very different Julia set, such as the one on the right. This model might be compared with the first stage of speciation. The Julia set on the left might represent a species' area of distribution. If, for whatever reason, a single environmental constant varies (which would be equivalent to a change in C), the area of distribution might become that of the Julia set to the right. In this case, the area of distribution is not uniform, but divided into different populations that are not in contact. This is the first step towards the formation of a new species. Unless a population is divided into two or more populations separated by some sort of barrier (geographical, ecological, genetic, etc.) that prevents the respective gene pools from crossing, it will never be possible for the two populations to diverge and for different species to appear eventually. This is why isolated systems play such an important role in the process of speciation and biological evolution.

[Drawing: Jordi Corbera, from several sources]

190 Bivalve molluscs colonized freshwater systems from estuaries and river mouths. Some species did so long ago, as shown by the fact they are widely distributed and highly specialized, but others only colonized freshwater recently. One new arrival in freshwater is Dreissena polymorpha, which recently began to increase its area of distribution. It was originally from the areas around the Black, Caspian and Aral Seas, and around 1820 began to spread west on the hulls of boats. It rapidly colonized most of the freshwater ecosystems of western Europe. It reached London in 1820, Rotterdam in 1826, Hamburg in 1840, and Copenhagen by 1840. By 1865 it had reached France and was moving south to-wards the Iberian Peninsula.

[Photo: Alain Le Toquin / Jacana]

191 The first finches, the ancestors of the honeycreepers (Drepanididae), must have been blown to Hawaii by a hurricane, as otherwise these small birds would have been unable to cross the 1,864 mi (3,000 km) separating the islands from the American coastline. When they arrived, they found a very agreeable site--food was abundant, there were predators, and several ecological niches were available for them to occupy. These conditions allowed spectacular divergence into a large number of species, a phenomenon known as adaptive radiation (see figure 199). The species of the genus Hemignathus are a good example of adaptive radiation. These drawings date from the late 19th century, and are from F.W. Frohawk's work Aves Hawaiienses (S.B. Wilson and A.H. Evans, 1892). They show two of the three species of Hemignathus, the akialoa (H. obscurus, formerly H. procerus) and the nukupuu (H. lucidus, formerly H. hanapepe), two more or less similar nectar-feeding species, as shown by their long curved beaks. The akiola is probably extinct, as it has not been sighted since 1967 on Kauai Island, although it had formerly lived on the islands of Ohau and Lanai too. The situation of the nukupuu is slightly more favorable; there are still some specimens in the forests on Kauai and Maui, but they no longer occur on Ohau Island.

[Photo: Robert Harding Picture Library]

192 The great diversity of cichlid fish in the great lakes of Africa is the most spectacular example of diversification among the vertebrates. It is easy to see why different lakes may house different species, but it is extraordinary that so many species have been able to evolve and coexist in a single lake (there are, for example, more than 400 species in Lake Malawi). This great diversity would not have arisen if the cichlids had not adapted to widely varying sources of food, as shown by the fact that the main morphological differences between species are in the mouth parts. Feeding adaptations, together with their wide range of size, colonization of sandy and rocky shores as well as open waters and deep waters: All make it possible for many species to coexist in a single lake. The six species in the drawing are good examples. Copadichromis borleyi, with a maximum size of 6 in (16 cm) in the male and 5 in (13 cm) in the female, prefers rocky lake bottoms and mainly eats plankton. Dimidiochromis compressiceps may reach 8 in (21 cm) and lives in shallow areas covered with vegetation, where it lurks waiting for small fish. Labeotropheus trewavasae is about 4 in (11 cm) long, lives in the top 16 in (40 cm) and feeds on the algae covering the rocks. Male Melanochromis auratus reach a maximum length of 4 in (10 cm) and live on rocky or mixed bottoms by grazing on the algae covering the rocks. Pseudotropheus zebra may reach a length of 5 in (13.5 cm), and only lives on rocky bottoms, preferably free of sediment, where it grazes on the algae covering the rocks, but in open waters it also eats plankton. Rham-phochromis lucius which may reach 16 in (40 cm) long, occurs in open waters, often at great depth, and preys on other fish.

[Drawing: Jordi Corbera]

193 The variability of the species Geochelone elephantopus, the Galapagos giant tortoise, is so great that this chelonian has been essential in understanding the possible range of variation within a single species. A total of 15 subspecies have been described, four of them now extinct and one (G. e. abingdoni, on the island of Pinta) represented by a single male (1994). The five volcanoes of Isabela Island each have a different subspecies of tortoise, and the other species occur on the smaller islands. An initial classification of the tortoises is made on the bases of the morphology of the carapace, which may be saddle or dome shaped. The tortoises living on the smaller, low, islands (Espanola, Pinzon, Pinta, etc.), where there is no moist layer, and those restricted to the more arid areas of the large, high islands (such as those on Wolf Volcano on Isabela Island) have saddle-shaped carapaces. The front of the shell is raised so that they can stretch their long neck, and as they also have long legs, they can reach the highest leaves of the shrubs and the cacti that form the vegetation where they live. The most typical species with a saddle shaped carapace was G. e. phantastica, on Fernandina, which is now extinct. The tortoises living in the wet highlands of the larger islands (Santa Cruz, Isabela) have domed shaped shells, like ordinary tortoises, and short legs and necks. In the highlands where vegetation is abundant almost all year round, and the tortoises can find food nearer the soil, they do not need long legs and a high neck, and so their carapace has not changed. The clearest examples of dome shaped carapaces are the subspecies G. e. darwini, on Santiago, and G. e. vandenburghi, from Alcedo Volcano on Isabela. Some-times it is difficult to distinguish whether a tortoise has one or other type of carapace, because there are also intermediate forms.

[Drawing: Jordi Corbera, from several sources]

194 Geochelone elephantopus vandenburghi lives on Alcedo Volcano on Isabela Island. The population now consists of about 5,000 specimens, making it the most common of all the subspecies of the Galapagos tortoises.

[Photo: Peter Ryley / Natural Science Photos]

195 Islands far from the coastline, such as the Colombretes in the western Mediterranean, that did not have very intense contacts with the mainland have a very taxonomically poor composition when compared with mainland biomes. This is because not many organisms managed to reach them, and even fewer managed to settle them, either owing to lack of adequate habitats or because of the presence of other organisms that competed with them. Something similar happens to a greater or lesser extent in other isolated habitats, so the composition of the flora and fauna of these environments is very unusual.

[Photo: Jaume Altadill]

196 Anak Krakatau is one of the few islands that has formed in recorded history. It appeared in the former submerged crater of Krakatoa when the volcano erupted again in 1927, thus providing scientists with the opportunity to study the unusual phenomenon of the formation of an island and its colonization by living organisms. Years before, in August 1883, after three months of eruptions, Krakatoa exploded and annihilated the entire living population of the island it formed, Rakata. Half the volcano completely disappeared, and the other half, together with the neighboring islands of Sertung and Lang, was covered with a layer of volcanic ash between 98 and 197 ft (30 and 60 m) thick. It can be stated with almost total certainty that Rakata's entire flora and fauna was annihilated. It was rapidly repopulated. Within a few weeks many plants had arrived. After nine months, researchers found a spider, and within a few years, a python. It is estimated that within 30 years of the explosion the island's fauna had once more reached an equilibrium. More recently, similar studies were performed when the island of Anak Krakatau appeared. In December, 1927 a new under-water eruption of Krakatoa began and a volcanic cone formed. This gradually grew until in 1930 it rose above sea level, thus forming a new island.

[Photo: Alain Compost / Bruce Coleman Limited]

197 The colonization of the Pacific Islands by flowering plants. The red isolines surround areas with the same number of genera. The further east of Asia the island is, the fewer plants have reached and colonized it. The plant diversity of an island is basic for the dispersal of other organisms, as it determines the availability of ecological resources, and thus the possibility that a greater or a lesser number of animal species can colonize it. Many flowering plants have adaptations to ensure their seeds are dispersed and germinate far from the mother plant. Some seeds have a pappus for wind dispersal. Other seeds are within fleshy fruits that are eaten by birds and may thus travel long distances. There are also seeds covered with small hooks or sticky glands that fix them to the feathers of birds, and are thus carried with them on their voyages. Plants showing these types of adaptations manage to reach distant islands more easily.

[Drawing: Editronica, based on B. Cox, D. Moore and P. Whitefield, 1989]

198 The common kiwi (Apteryx australis) is one of New Zealand's three species of kiwi. It lives in the forests in the rainiest sectors of North Island, South Island and the Stewart and Kapiti islands but also occurs in scrub and croplands with high vegetation. Kiwis are flightless nocturnal terrestrial birds. Their wings are rudimentary, and their feathers lack barbules, which is why they look like hairs. They have short, strong legs and feet with very robust toes, as they are good runners. Compared with other flightless ground-living birds, such as the ostrich (Struthio camelus), the rhea (Rhea) and the emu (Dromaius novaehollandiae), they are small birds (about the size of a chicken) that were able to evolve because of the absence of mammals from New Zealand.

[Photo: Frances Furlong / Bruce Coleman Limited]

199 The Madiinae subtribe of the Asteraceae, endemic to the Hawaii Islands, provides a remarkable example of adaptive radiation. There are 28 woody species in three genera (Argyroxiphium, Dubautia, and Wilkesia). They include growth forms as different as trees, shrubs, spine cushions, rosettes and a liana. They occupy a wide range of habitats, from some of the wettest on earth to extremely dry ones. All these plants originated from a single species that colonized the islands from North America. The silversword (Argyroxiphium sandwicense), the species shown in the photo, grows only on volcanic ash at very high altitudes. This plant may reach a height of 5.9 ft (1.8 m), and its dense basal rosette represents an adaptation to extreme temperatures, aridity and the intense sunshine. Its generic name means "silvery sword" in Greek and refers to the color and shape of the leaves. This species shows clear morphological convergence with other single-stemmed tuft-rosette trees, such as the giant Senecio and Lobelia in Africa (see photos 182 and 183), the tree ferns Cyathea and Blechnum of New Guinea, the Puya, Blechnum and Lupinus, and especially the Espeletia of the paramos and cloud forests of the Andes (see photo 136). The fact that the Asteraceae disperse with relative ease has allowed the members of this family to colonize the Hawaii Islands before other trees (see photo 191).

[Photo: Bryan L. Sage / Natural Science Photos]

200 The dodo (Raphus cucullatus) was a plump bird that could not fly. It was discovered by hungry Dutch sailors who caught it without any trouble in the forests of Mauritius in 1598. Hunting pressure on the species was so intense that shortly afterwards, around 1660, it was extinct. All that is left are descriptions and drawings from the time, such as this 1774 bookplate, to give us an idea of what this strange bird looked like. The dodo is now the paradigm of the effect of human predation.

[Photo: G.I. Bernard / NHPA]

201 Eleven species of terrestrial snail of the genus Partula, most of them now in danger of extinction, live or lived on the island of Moorea. This high species diversity is due to island's unusual relief. A crescent-shaped mountain ridge, which is part of the wall of a former volcanic crater, runs from the northwest to the northeast of the island, with its convex face to the south. Perpendicular to the main axis of this ridge, there are many valleys separated by mountain ridges. The relative isolation of each valley largely determines the distribution of the vegetation, which in turn provides suitable habitats for the different species of snail. A single species may occur in several valleys, with different subspecies or varieties in each one, or it may be restricted to a given area within a single valley. The distribution of the different species, subspecies and varieties was studied by Andrew Garrett from 1866 to 1888. H.E. Crampton reproduced Garrett's distribution map (upper map) in his work "Studies on the variation, distribution and evolution of the genus Partula" (Carnegie Institution of Washington, 1932), together with an improved version (lower map), as in Garrett's time, detailed maps of Moorea were not available. The map that Garrett drew is clearly incorrect; the western part of the island, which he studied in detail, is disproportionately large. In the same work, Crampton showed a sample of the great diversity of the forms of the subspecies P. taeniata simulans) (numbers 1 to 38) and P. t. striolata (numbers 39 to 64), two of the most widespread species. Depending on their site of origin, the snails vary in size, width of the opening of the shell, color pattern, and so on. Numbers 4 and 23 are mutants with shells that coil to the left

[Photo: Courtesy of the Museu de Zoologia de Barcelona / Jordi Vidal]

202 The only natural population of the Aldabra tortoise (Aldabrachelys gigantea) is on Aldabra Atoll in the Seychelles Archipelago, north-west of Madagascar. There are estimated to be 150,000 of them. In 1978, a small colony was introduced into the central Seychelles, in the center of the Indian Ocean as a tourist attraction, and some specimens still survive. The Aldabra tortoise occurs in a wide range of habitats, from meadows to shrub areas and mangroves. Their intense pressure on the plants has led to a dwarf vegetation that covers much of the island where this giant chelonian lives.

[Photo: Jean-Yves Grospas / Bios / Still Pictures]

203 One of the most outstanding features of the surprising endemic flora of Madagascar is the genus Pachypodium (Apocynaceae), with 17 species, most endemic to Madagascar and the rest endemic to southwest Africa. Some species of this genus are small plants, others are shrubs, and others are trees up to 26 ft (8 m) tall. These curious plants are sometimes known as "elephant's feet," because their single vertical trunk seems like an inflated column crowned with a terminal tuft of leaves. Its generic name comes from the Greek words pachys, thick, and pous meaning foot. The photo shows a splendid specimen in the spiny scrub in the southwest of the island, with a specimen of a leafless Euphorbia also in the foreground.

[Photo: Michel Gunther / Bios Still Pictures]

204 The chameleontids are the most typical reptiles of Madagascar. They are very well adapted to arboreal life, as shown by this male Chamaeleo pardalis, one of Madagascar's largest chameleons. The laterally flattened body allows them to climb the thinnest branches. The tail is prehensile, and the feet consist of two opposing sets of fused digits and are excellent for grasping. Chameleons are best known for their ability to change color, although they are not the only lacertids that can change their color.

[Photo: Jacqueline Kauffmann / WWF / Still Pictures]

205 One remarkable example of a relict endemic group is provided by the tuataras (Sphenodon punctatus and S. guntheri), the only living representatives of the order Rhynchocephalia, reptiles that were most widespread during the Jurassic era but are now restricted to New Zealand. Until the early 19th century, tuataras were abundant throughout New Zealand, but after the arrival of Europeans, their domesticated animals (dogs, cats, and pigs) eliminated them from many places. They now only survive on a few islands, such as the volcanic islet of Little Barrier, where this specimen of S. punctatus) was photographed.

[Photo: Gerald Cubitt / Bruce Coleman Limited]

206 One of the few New Zealand localities where it is still possible to find the kakapo (Strigops hapbroptilus) is on Maud Island. The kakapo is a flightless, nocturnal bird that belongs to the same family as parrots and cockatoos (Psittacidae). With its yellowish green plumage its goes unnoticed in the Nothofagus forests where it lives. In the mid-19th century it was still abundant, but the destruction of its natural habitat and predators introduced from Europe led to a rapid decline in its numbers. Several reintroduction programs have failed, and attempts to breed them in captivity have also failed.

[Photo: Gerald Cubitt / Bruce Coleman Limited]

207 The New Caledonia Oine (Araucaria columnaris) is one of the commonest conifers on New Caledonia and the neighboring islands, where it often forms very large forests, such as this forest on the Loyalty Islands. It is a thin, straight tree that can grow to a height of 195 ft (60 m). Although most species of Araucaria are endemic to New Caledonia, some grow in South America, which is where the name comes from. Araucaria is the name of the Araucanian people of central Chile, where the species Araucaria araucana grows.

[Photo: M. Baileau / Explorer]

208 Pachnodus is a genus of terrestrial snails endemic to the Seychelles. This archipelago consists of islands of two different geological types, some being continental and granitic, whereas others are of coral origin; each type of island has its own fauna of terrestrial snails. The coral islands, however, have virtually no snails, and the species present are small enough to be dispersed by the wind. The specimen in the photo is probably P. ornatus.

[Photo: Jean-Yves Grospas / Bios / Still Pictures]

209 The Mediterranean islands' herpetological fauna typically contains a very high number of species of lizard, the result of a process of adaptation to different island environments. This is clearly shown in the genus Podarcis, which in the Mediterranean islands alone is represented by a dozen species and up to a hundred subspecies. A single species may also show geographical variants, as for example the Tyrrhenian lizard (Podarcis tiliguerta, upper photo), which is endemic to Corsica, Sardinia, and the neighboring islets. The basic dorsal markings of the males of this species are striped or reticulate, but specimens from different localities may vary greatly in their tonality and in their details. The male in the photo, photographed in Scandola (Corsica), is light in color, but on some islets some males are much darker. There are species, like the dwarf algyroid (Algyroides fitzingeri, lower photo) which show little geographical variation. This male from Sardinia is almost the same as those from the island of Corsica.

[Photos: J.C. Malausa / Bios / Still Pictures and Tristan Lafranchis / Bios / Still Pictures]

210 Situations of double insularity increase the levels of endemism, as is the case in Cuba of the vegetation on serpentine outcrops (rocks with high levels of magnesium that form oligotrophic soils). Serpentinicolous vegetation is spiny and microphyllous or sclerophyllous and forms islands in the middle of the basophilic, seasonal evergreen, or the semideciduous vegetation that grows on the limestone soils most common on the island. This double isolation explains why serpentinicolous patches show such high rates of endemism. The map shows the distribution of these species and the relative abundance of endemic species that is proportional to the length of the bars.

[Drawing: Editronica, based on Borhidi, 1993]

211 The alamiqui of His-paniola (Solenodon paradoxus) is a shrew-like insectivore, but is larger and stouter. It lives in the forests of the island of La Hispaniola. Together with the other species of alamiqui in the Antilles, the Cuban alamiqui (S. cubanus), it is an island relic of a primitive group.

[Drawing: Helmut Diller / WWF / Still Pictures]

212 The monkey-eating eagle (Pithecophagus jefferyi), is endemic to the Philippines and is one of the world's most endangered animals. It lives in the remaining virgin forests of the islands of Luzon, Samar, Mindanao and perhaps also in Leyte, at elevations of up to 3,937 ft (1,200 m). It got its name because its diet consists of up to 90% mammals of medium size, including monkeys. It has not been possible to breed it in captivity, considerably increasing the threat to the survival of this protected species. Its survival is also threatened by logging and hunters, who hunt the birds for export.

[Photo: Gunter Ziesler / Jacana]

213 The most local and least studied of the species endemic to the Canary Islands is probably the Fuerteventura stonechat (Saxicola dacotiae) which lives exclusively on this island and on Alegranza and Montana Clara, two islands to the north of Lanzarote. This small birds can be seen in fields, scrub and open areas in general, and its populations appear to fluctuate widely. The photo shows a male in the Vega del Rio de Palmas, in Fuerteventura.

[Photo: A. Greensmith / Ardea London Ltd.]

214 The extreme youth of the relief of the Hawaii Islands is clearly shown by these cliffs on the Na Pali coast on Kauai Island. The deeply eroded gullies are the result of the modeling of the rugged site by torrential rainstorms. Located in the middle of the Pacific Ocean and south of the Tropic of Cancer, the Hawaiian Archipelago consists of eight main islands, almost all of them formed by overlying layers of basaltic lava. The islands' arrangement in a line from east to west indicates the archipelago's development. The islands are formed as a result of volcanic activity at the eastern tip of the island arc and are then displaced westwards and gradually sink under the waters; thus, the eastern islands are younger than the western ones. This process started only five million years ago and is still continuing. A new island, Loihi, is slowly rising beneath the sea from the hot point where they all arose, at the easternmost point of the chain, and one day it too will form part of the largest island chain in the Pacific.

[Photo: AGE Fotostock]

215 The terrestrial iguanas (Conolophus subcristatus) of Plaza Sur Island in the Galapagos have managed to survive in the island's poor environment for countless generations. The population is now about 330 individuals. As one third of the island's surface is covered by shrubby cacti of the genus Opuntia, the iguanas have been forced to make their fruit, prickly pears, their basic foodstuff. They normally eat the ones that have fallen to the soil but they sometimes also climb up the prickly pear "trees." Prickly pear fruit have the major drawback of being covered in spines that would stick into their tongue and gums. To get rid of the prickles the iguanas roll the fruit around the soil, or simply pull out the largest prickles with their teeth. These precautions often do not prevent them from getting prickles stuck in their mouth, but this does not appear to cause them any great inconvenience.

[Photo: Robert de Armas Marrero / WWF / Still Pictures]

216 Lakes emphasize the scenic qualities of a landscape, contain fascinating plant and animal communities, and play an important role in the economy of many human populations. The good health of a lake is essential for the life of all the organisms that live in it or depend on it. Although they are to some extent separate ecosystems, lakes are profoundly affected by the phenomena that take place in the surrounding area, and even those at great distances. There are different types of lakes; volcanic, endorheic, tectonic, and glacial--such as this lake in Pitztal Valley in the southern Tyrol (Austria)--but it general a lake is simply a depression in the earth's surface that is full of water.

[Photo: AGE Fotostock]

217 Hot water springs in hydrothermal areas, such as the ones in Waimangu, in Rotorua (New Zealand) are proof of the great volcanic forces that are still at work under the surface rocks. These springs are a volcanic phenomenon very similar to geysers: The subterranean magma heats the water in the aquifer, which emerges and forms a lake that is much hotter than the surrounding area, sometimes even emerging as steam.

[Photo: Josep Pedrol]

218 The calcareous concretions in Lake Mono, in the mountains of the Sierra Nevada in California, formed when the very alkaline water of the lake mixed with a subterranean phreatic water table that is very rich in calcium carbonate. When the level of the lake goes down, the curiously shaped calcareous concretions stand above the surface.

[Photo: Rich Kirchner / NHPA]

219 Depth-time diagrams show how the temperature of a lake varies with depth over the course of the year. The lines join together points at the same temperature, thus clearly distinguishing the periods of the year and the levels at which temperatures change rapidly from those showing more constant temperatures. These diagrams show the existence of the three basic annual heat cycles, the basis for classification into dimictic, monomictic, and amictic lakes. Dimictic lakes, such as Lake Nuguren in Norway, typically show two periods of vertical mixing, when the temperature is equalised at all depths. Monomictic lakes have a single period of continuous vertical mixing, such as the shallow El Plateado lake in the Andes. Amictic lakes are permanently stratified, although small differences between day and night allow limited vertical mixing of the water in the surface layers, and this mixing is of greater biological importance if it exceeds the thickness of the photic zone (between 66 and 164 ft [20 m and 50 m]). In addition to these three basic types, there are other lakes that follow very different heat patterns, such as Banyoles Lake in Catalonia, which is meromictic. In meromictic lakes, the vertical circulation gradually decreases and the deepest layers do not mix. In tropical regions, there are many lakes that are polymictic, that is to say, they have a frequent regime of stratification and mixing, which may even occur every day. It is clear that, all other factors being equal, fertility is relatively greater in lakes in which there is some circulation and the waters are mixed frequently. Thus, fertility is higher in monomictic lakes than it is in amictic lakes and is even higher in dimictic lakes. The more frequently vertical mixing occurs, the greater the diffusion of nutrients towards the surface layers, and thus the higher the fertility, as happens in the marine pelagic environment.

[Drawing: Jordi Corbera, based on Margalef, 1983]
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Publication:Encyclopedia of the Biosphere
Date:Jan 1, 2000
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