TROPICAL FOREST ECOLOGY.
In July of 1984 I arrived on Barro Colorado Island (BCI) to begin my career as a tropical biologist. An important part of my early education was evening visits to the BCI office of Egbert Giles Leigh, Jr. to discuss tropical biology over a glass of whiskey (usually of brands I could not afford as a graduate student, therefore also part of the education). In Leigh, I found a man with very diverse interests ranging from sex allocation theory and group selection to plant architecture, mutualism, and the origin and maintenance of species diversity. In this delightfully well-written book, Leigh addresses the diverse ecological issues he has studied during his 30+ years as a biologist for the Smithsonian Tropical Research Institute. Using research conducted at BCI as a departure point, he marshals information from throughout the tropics to show what is interesting about tropical forests, what these forests have in common, and how we can use our knowledge of adaptation and natural selection to understand the interr elationships of plants and animals in tropical forests.
In the Introduction, Leigh establishes his adaptationist viewpoint and recognizes that not all ecologists agree on the relative role of competition and interdependence in the structuring of ecosystems. He states that his book seeks to answer three questions: (1) Do plants and their seasonal rhythms of fruit and leaf production limit populations of vertebrate herbivores? (2) Why are there so many kinds of tropical trees? and (3) What is the role of mutualism in the ecological organization of tropical forests?
In Chapter 1, Leigh introduces Barro Colorado Island, beginning with a brief discussion of the study area itself before departing on a fascinating description of the history of the biota with respect to the formation of the Isthmus of Panama, the climatic changes that occurred through the Pleistocene, and finally, the impacts of human populations on the forests of the region. Only relatively recently have ecologists begun to understand that we have only a snapshot view of systems that have been changing dramatically over time. Leigh embraces this viewpoint, providing us with the most appropriate introduction to the site.
In Chapter 2, entitled "Dramatis personae," Leigh sets out to summarize the natural history of a number of plants and animals that occur on BCI. I thoroughly enjoyed this section. It is full of witty observations, such as the fact that howler monkeys are supported by less energy than that of a 25-watt light bulb (sloths ranking just above that of a nightlight). For me, reading this section was very much like becoming reacquainted with old friends; however, I am not sure how this chapter would be read by someone unfamiliar with the site without some knowledge of how each of the players fits into the larger picture. Also, this approach easily brings the criticism that detailed information on some important taxa has been left out (e.g., euglossine bees). Finally, as I continued to read the book, I found myself wondering if an equivalent description of the important study areas and long-term experiments on BCI (Lutz Watershed, the Forest Dynamics Plot, the Irrigation Experiment) would not also have been useful ( perhaps as part of the previous chapter).
Leigh addresses the climate of tropical forests in Chapter 3. His main focus is on rainfall and evapotranspiration and the remarkable conclusion that sites which vary four-fold in rainfall (1510-5795 mm/yr) only vary 1.4-fold in evapotranspiration (1173-1675 mm/yr). Leigh suggests that the nearly constant rate of evapotranspiration in tropical forests is caused by a similar climate that provides for a similar vegetation structure which, in turn, provides a similar rate at which rainfall is recycled as water vapor. Following a short chapter on erosion and soil formation (Chapter 4), and a wonderfully illustrated discussion of tree architecture (Chapter 5), Leigh shows us the extreme similarity of vegetation structure among tropical forests in Chapter 6. The "constants of tropical forest," as he describes them (in forests with 1700 mm per year rainfall and four months dry season), are similarities in leaf fall (6-8 tons/ha/yr), leaf area index (6-8 ha/ha), tree mortality (1-2% per year), basal area (30 [m.sup.2]/yr), and biomass (300 tons dry weight/ha). Using a simple model of photosynthesis, he also shows that production is probably very similar among all tropical forests. Leigh's basic argument is clear: where rainfall (total or seasonality) and soils do not limit production, forest structure is driven by aspects of climate (temperature, humidity, and wind) that are very similar throughout the tropics.
In Chapters 7-9, Leigh returns to the questions raised in the Introduction. Taking advantage of various studies of the animals on BCI and their consumption of leaves and fruits, Leigh provides an analysis of the effect of the seasonality of leaf and fruit production on vertebrate and invertebrate herbivores. He explains that fruit consumption on the island is favored over folivory because mature leaves of tropical trees are so heavily defended. Along the way, Leigh uses this fact (and two other very pertinent ones) to lay to rest Terborgh's assertion that the lack of large predators on BCI results in an unusually high abundance of herbivorous mammals that negatively affect the vegetation.
In Chapter 7 I became fully aware that Leigh makes no use of graphs to present data in his book. All data are presented in tables, some of them quite large. I found myself staring at Table 7.1 for some minutes to convince myself that there was indeed a seasonal pattern in leaf flushing. This could have been determined at a glance from a line graph.
With regard to forest tree diversity (Chapter 8), Leigh again uses published data to give us a full picture of the patterns of diversity among tropical forests. He reviews the hypotheses that explain why tropical forests are so much more diverse than their temperate counterparts, rejecting the notion that chance plays a strong role in maintaining community diversity. The chapter on mutualism (Chapter 9) is a fascinating exposition on the paradox of competition and interdependence in tropical forests, applying the concepts of adaptation, group selection, and the evolution of mutualism to arrive at a resolution of the paradox. Addressing the harmonious nature of ecosystems, he brings up the "constants of the rainforest" again, suggesting these similarities tell us something important about the "commonwealth" of ecosystems.
In three moving pages, Leigh ends the book by decrying the spiritual crisis, caused by economic determinism, that has lead to the destruction of so much of the world's tropical forests. Clearly, humans do not share the commonwealth of the rainforest, but pillage its resources. No tropical biologist would disagree with Leigh's sentiments, although I doubt many approach the issue from Leigh's religious stance, which he so richly explains.
Leigh has produced a wonderful synthesis of our understanding of tropical forests, one that should attract a wide audience. With a few exceptions noted above, the information is straightforwardly presented and clearly organized. As one who has a bad habit of "humming the math," I greatly appreciated that much of the mathematical details were left for appendices after each chapter. I look forward to further editions of this book as our knowledge of the tropical forests of the world continues to grow--at least as long as there are any tropical forests left to study.
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|Author:||Zimmerman, Jess K.|
|Article Type:||Book Review|
|Date:||Aug 1, 2000|
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