SPECIES IN THE TAIL OF RANK-ABUNDANCE CURVES.
A fundamental question of ecology concerns the abundance and distribution of species. In particular, there has been considerable interest in the distribution of abundance among species within local communities. A common observation is that at a given location some species will be very abundant, while many species will have small- to moderate-sized populations (Darwin 1859, Preston 1948, MacArthur 1960, Whittaker 1965, May 1975). Abundance distributions within communities have been represented usually as rank-abundance or dominance-diversity curves, with one axis of the curve representing species rank in a community and the other representing species abundance.
Both theoretical and empirical studies have investigated the structure of rank-abundance curves for many different communities. Theory has concentrated primarily on the structure of rank-abundance curves and on within-community explanations for their shapes (e.g., Preston 1962, May 1975, Pielou 1975, De Vita 1979, Stenseth 1979, Sugihara 1980, Hughes 1986, Wilson 1991, Gregory 1994, Wilson et al. 1996). Empirical work on local abundance has focused on fluctuations in the numbers of individuals of a species through time at one location, or among a small number of communities, and has also tried to support or reject the various theories regarding curve structure (e.g., Preston 1962, Sugihara 1980, Diamond and Case 1986, Grubb 1986, Mitchley and Grubb 1986, Chesson and Huntley 1989, Tokeshi 1990, Wilson 1991, Wilson et al. 1996). This work has clarified a number of important issues; however, broad generalizable patterns between particular curve structures and particular underlying mechanisms have yet to be found. The present paper investigates rank-abundance curves from a different perspective, employing a macroecological approach to explore how the local abundance of species is related to abundance throughout entire geographical ranges.
Some empirical studies have investigated species' abundances across entire or nearly entire geographical ranges (Hanski 1982, Hengeveld and Haeck 1982, Rapoport 1982, Brown 1984, Rabinowitz et al. 1986, Schoener 1987, Maurer 1994, Brown et al. 1995). Generally, it has been observed that species are found in low abundance at most sites, throughout their respective geographical ranges while achieving high abundances only at a small number of sites (e.g., Brown et al. 1995). The recurring rank-abundance pattern within local communities of a small number of abundant species together with a long tail of lower-abundance species might be associated with a geographical-scale pattern whereby most species are of low abundance in most parts of their range but of higher abundance at some locations [ILLUSTRATION FOR FIGURE 1A OMITTED]. Alternatively, the rank-abundance pattern might be associated with most species being of low abundance throughout their geographical ranges and thus in the tail of rank-abundance curves wherever they occur [ILLUSTRATION FOR FIGURE 1B OMITTED].
We test which alternative best describes patterns of abundance throughout geographical ranges for flowering plant species of dry sclerophyll woodland and temperate rain forest. We ask the simple but important question whether most species in the tail of rank-abundance curves can be found in high abundance at some other location within their respective geographical ranges. In addition we compare a total of 10 attributes between the two groups of species to assess any traits that consistently distinguish somewhere-abundant from everywhere-sparse species through the ability to achieve high abundances given the opportunity.
The basic approach was to locate a set of species that contributed low cover, i.e., were in the tail of the rank-abundance curve, within focal communities, and to determine how many and which of these were substantially more abundant at other locations or in some other environment throughout their geographical range, versus how many and which occupied low cover wherever they occurred. Only species whose entire geographical ranges fell within Australia were considered. The methods necessarily made use of two types of evidence. The study began at the level of the local community, first procuring abundance information for all flowering plant species within focal sites by standard quadrat methods of 1 ha in dry sclerophyll woodland. Having identified certain species as being of low abundance at focal sites, the second phase was to visit other communities within the same general vegetation type where the species were known or thought to occur. At these secondary sites, similar sampling of abundance occurred, which gave an initial listing of species that were low in abundance at the focal sites and distinctly greater in abundance at the secondary sites. For species that still had not been found to be abundant anywhere at this stage, no sampling procedure was capable of assessing abundance throughout their geographical ranges, which for many species extended far beyond the particular vegetation type in which the focal site was situated. Accordingly, the third phase of investigation drew on a different type of evidence, the firsthand knowledge of field-survey ecologists and herbarium botanists having many years of experience in the relevant regions, including authors D. A. Keith, P. J. Myerscough, J. Howell, A. G. Floyd, and K. Mills. These contributors and others (see Acknowledgments) were surveyed to find out whether they had seen any of the low-abundance species from within the focal hectares in high abundance, and if so, where this was observed. This evidence, drawn from personal knowledge, field notes, and unpublished surveys, represented cumulatively [greater than]200 person-yr of field botanical experience in the region. We consider it the most reliable available with regard to abundance throughout wide areas and whole geographical ranges. To assess the generality of patterns found in dry sclerophyll woodland, the relationship between local abundance and abundance throughout geographical ranges was also investigated for flowering plant species of temperate rain forest, using data on species abundance within communities of 15 ha of undisturbed vegetation obtained from Turner and Vernon (1994).
Dry sclerophyll woodland
Four focal sites, each a replicate of dry sclerophyll woodland, were situated within the Sydney region, New South Wales, Australia. Focal sites A, B, and D were in Ku-ring-gai Chase National Park, while focal site C was in Garigal National Park. All sites were on infertile Hawkesbury sandstone. A focal site had to be 1 ha of vegetation that had not been burned or cleared in any manner for a minimum of 10 yr. The site had to appear homogeneous in the sense that there was no particular tendency for one side to be different from the other side. Focal sites were square (100 x 100 m). Within them were located nine 2 x 5 m quadrats in a 3 x 3 grid. Each quadrat was [approximately]30-40 m from the next. The percent canopy cover of all flowering plant species was estimated in each of the nine quadrats within the hectare. Mean cover was calculated for each species across the nine quadrats and used as an estimate of canopy cover over the hectare. The rest of the hectare was searched for species not found in the nine quadrats, and canopy cover over the whole hectare was estimated for these species.
Because maximum potential canopy area covered by an individual varies between plant species, species were assigned to three size classes. Species were placed in the large size class (size class 1) if their maximum potential cover exceeded 2 x 2 m (mostly trees such as Eucalyptus spp.), in size class 2 if maximum potential canopy was between 0.2 x 0.2 m and 2 x 2 m (mostly shrubs such as Grevillea spp.), and the small size class (size class 3) if maximum potential canopy never exceeded 0.2 x 0.2 m (mostly herbs, e.g., Lomandra spp.; and small woody dicotyledons, e.g., members of the Euphorbiaceae). Given that the present study compared abundances of species at a particular site, between sites for each species, and between species for geographical ranges, the quantification by size class emphasizes the relative nature of comparisons. For example, some species have very large individuals, and thus the smallest percentage canopy cover an adult can have is quite large.
Rank-abundance curves were created for each focal site [ILLUSTRATION FOR FIGURES 2A-D OMITTED]. A species was considered to be of low abundance (i.e., in the tail) if it fell below a threshold value of percent cover. Cutoff points were placed subjectively where the tail section of curves became perceptible. The threshold value was different for each size class (size class 1 = 1% canopy cover, size class 2 = 0.4% canopy cover, and size class 3 = 0.01% canopy cover). The average number of all species per focal site was 66 (A, 76; B, 51; C, 69; D, 69), with a total of 43, 20, 26, and 19 tail species at each focal site, respectively. In all, 78 species were identified as being of low abundance across the four focal sites, with some species within the tail of the rank-abundance curve at more than one focal site. Of the 78 tail species, six were identifiable to genus only; these species were discarded, as species-level identification was required. Two of the 78 species were exotics (Chrysanthemoides monilifera ssp. rotundata and Lantana camara), and were given no further consideration, as abundances within the native geographical ranges of species were the topic. Two further species, Cassytha pubescens and Themeda australis, were discarded because their geographical ranges extended outside Australia, to New Zealand and New Guinea, respectively. This left 68 native species of low abundance to be studied.
Habitat descriptions and extent of geographical ranges were obtained for all tail species. Sources included Australian Government Publishing Service (1981), Harden (1990-1993), Hnatiuk (1990), Robinson (1991), Benson and McDougall (1993, 1994, 1995), Carolin and Tindale (1994), Fairley and Moore (1995), and species lists obtained from the Royal Botanic Gardens, Sydney (courtesy of R. Coveny). From these sources it was possible to construct a list of eight prospective locations within the Sydney region where each of the focal species might occur in high abundance. Abundances of focal species at the eight prospective locations were determined by estimating percent canopy cover. All cover values were comparable directly to focal site abundances. A species was considered to be significantly more abundant at a location compared to its abundance at a focal site if it met both of two criteria. First, its abundance had to fall above the threshold value established at the focal site for the relevant plant size class. Second, abundance had to be at least 10 times greater than that recorded at the focal site. Tail species that were found to be more abundant somewhere else within their geographical range were termed somewhere-abundant species. If a species was found to be much more abundant before all eight locations were visited, it was not necessary to visit remaining locations, since this indicated that the species could be classified as being more abundant elsewhere (i.e., somewhere-abundant). Species that were abundant during early post-five years (various authorities; B. R. Murray, personal observation) but not in established vegetation 10 years or more after fire were categorized as sometimes-abundant.
When the eight locations had been visited for all tail species where necessary, and any species remained that were not found to be more abundant, ecologists and botanists experienced in the Sydney flora (including D. A. Keith, P. J. Myerscough, and J. Howell) were consulted further about remaining species. In particular, people consulted were asked whether they had ever found the species to be abundant anywhere within its geographical range according to the criteria described previously. Given the many combined years of field experience of those consulted, this information was more reliable than what could have been obtained by any fresh sampling. Species that were not found to be abundant in established vegetation (somewhere-abundant species), nor identified as being abundant only during early post-fire years (sometimes-abundant species), were considered to be of low abundance throughout their geographical range, and were termed everywhere-sparse.
Temperate rain forest vegetation
To provide a further test of the predictions, a second vegetation type was examined. Turner and Vernon (1994) provided information on abundances for a large area of floristically and structurally well developed, undisturbed temperate rain forest vegetation in New South Wales. All flowering plant species at 46 sites (totaling [approximately]850 ha, median area = 15 ha) were described, and their abundances rated as one of four categories (very common, common, occasional, and rare) at each site. These 46 sites were considered equivalent to the four focal sites of dry sclerophyll woodland, in that they represented sites at which local abundances were estimated. We considered a classification of either occasional or rare at any of the 46 sites as denoting low local abundance (i.e., tail species, in our terminology) at those particular sites. From the 46 rain forest sites, 116 native species were identified as being tail species at one site at least. As with the tail species of dry sclerophyll woodland, the next step was to ask whether the tail species of local sites in temperate rain forest could be found in much greater abundance in similar or other vegetation types within their respective geographical ranges. This was assessed from information also provided in Turner and Vernon (1994) (e.g., a species of low abundance at one of the 46 sites may have been abundant at one or more of the remaining sites listed) and from referencing other sources of abundance information of temperate rain forest species (e.g., Mills and Jakeman 1995). Again, the third phase in distinguishing somewhere-abundant from everywhere-sparse species was consultation with botanists and ecologists (including A. G. Floyd and K. Mills) having extensive field experience in this type of vegetation.
Attributes were compared between somewhere-abundant and everywhere-sparse species. Data were not available for all the attributes that might have been desired. Those compared were dispersal morphology (dry sclerophyll woodland [w], temperate rain forest [R]), flowering length (w, R), flowering season (w, R), fruiting period (R), fruiting season (R), geographical range (w, R), maximum potential height (w, R), plant size class (w, R), regeneration after fire (w), and seed size (w, R). Attribute information for comparisons for one or both vegetation types was obtained from various authorities and a plant attribute database established and maintained in the School of Biological Sciences at Macquarie University. Seed masses were expressed as dry mass in milligrams for dry sclerophyll woodland, and included embryo, endosperm, and seed coat structures, while any dispersal structures (e.g., elaiosomes) were removed before weighing. For plants of temperate rain forest, seed sizes were presented as largest linear dimension (millimeters), rather than as mass, since more data were available in this form from literature sources. Dispersal morphologies were assessed using structures present on the seeds indicative of a particular mode of dispersal (e.g., Westoby et al. 1990). Flowering information was a broad measure of the months of the year spent flowering. These months were then translated into seasons of the year spent flowering (e.g., a species that flowers from August to October flowers in austral winter-spring). Species methods of regeneration after fire were recorded as fire-killed and relying on a stored seed bank (obligate-seed-regenerator) or capable of vegetative regeneration (resprouter). For geographical range, the Census of Australian Plants (Hnatiuk 1990) divides the Australian continent into 97 regions, based on biogeographical zones recognized by the relevant herbaria in each state. Presence or absence of all species in each region is listed, so that the number of regions occupied provides a broad, continental-scale measure of total geographical range.
Data analyses: cross-species and phylogenetic approaches
Data for seed mass (but not seed length), geographical range, and plant height were all log-transformed for statistical analyses (Sokal and Rohlf 1981). To identify which attributes discriminated most strongly between somewhere-abundant and everywhere-sparse species, a stepwise discriminant function analysis (DFA) was employed (Dixon 1992). While DFA was used to look at cross-species comparisons and consider questions about present-day ecological relationships among species, phylogenetically independent contrasts (PICS, Burt 1989) were also used to compare somewhere-abundant and everywhere-sparse species. Phylogenetic analyses test whether differences in the attributes compared arise consistently across replicated phylogenetic divergences between somewhere-abundant and everywhere-sparse species or clades. Pairs of species for PICs were selected to be as closely related as possible. In dry sclerophyll woodland, a total of four contrasts was possible, two at species-within-genus level, one at genera-within-family, and one at subclasses-within-class. For temperate rain forest, six contrasts were established, two at species-within-genus level, one at genera-within-family, two at families-within-order, and one at orders-within-subclass.
Dry sclerophyll woodland
Of the 68 tail species studied, 13 species were found to be significantly more abundant only at sites [less than]10 yr since fire (e.g., Acacia suaveolens, Cassinia denticulata, Gompholobium glabratum). These sometimes-abundant species are probably abundant within focal site communities 10 yr after fire, but in the form of a seed bank rather than as above-ground cover. Of the remaining 55 species, 50 (91%) were found to be considerably more abundant somewhere within their geographical range (Table 1). Of these 50 somewhere-abundant species, the higher-abundance sites for most (45) were in the same vegetation type (dry sclerophyll woodland), while the other five species had their higher-abundance sites in habitat that was different from the focal sites. Because the original search for these species as abundant at some other location was not aimed at investigating all dry sclerophyll woodland sites first, it is possible that further searches may find these five species in high abundance at some woodland sites.
Only five (9%) of the 55 tail species were found to be abundant nowhere (Bossiaea ensata, Bossiaea scolopendria, Dampiera purpurea, Hovea linearis, Logania albiflora). All of these everywhere-sparse species fell into size class 2, the intermediate-sized species. Focal sites A, B, C, and D had three, zero, two, and three everywhere-sparse species, respectively, within the hectare, indicating that 96%, 100%, 97%, and 96% of species within the tail of rank-abundance curves for these four local communities (A, B, C, and D, respectively) could be abundant elsewhere within their range.
Temperate rain forest vegetation
The proportion of tail species in temperate rain forest that were found to be in high abundance at other locations was similar to the equivalent proportion in dry sclerophyll woodland. In total, 110 temperate rain forest species (95%) were found to be abundant at one or more locations, and were considered to be somewhere-abundant (e.g., Acmena smithii, Geijera parviflora, Pittosporum revolutum, and Syzygium australe). There was no clear indication that any of the species found to be in high abundance somewhere were actually sometimes-abundant species, i.e., abundant only after fire or other disturbances. Only six tail species (5%) were found nowhere in high abundance and were considered everywhere-sparse (Cayratia eurynema, Dysoxylum rufum, Emmenosperma alphitonioides, Passiflora herbertiana ssp. herbertiana, Sambucus australasica, and Symplocos thwaitesii). Of the 110 somewhere-abundant species, 105 could be found in greater abundance in similar vegetation to the original locations where they were identified as tail species (i.e., temperate rain forest). The other five species could only be found in greater abundance in a different habitat.
Flowering season duration was the best single discriminator for dry sclerophyll woodland (DFA: [F.sub.1,51] = 5.44, P [less than] 0.05). Somewhere-abundant species on average flowered across a longer period of the year than everywhere-sparse species. After this was entered into the discriminant function, no further attributes were found to be significant discriminators between the two groups. The majority of both somewhere-abundant and everywhere-sparse species in dry sclerophyll woodland were dispersed by ants, were capable of vegetative regeneration, and flowered in winter-spring. For rain forest, geographical range was found to be a significant discriminator between the two groups ([F.sub.1,109] = 5.01, P [less than] 0.05), with somewhere-abundant species having more widespread distributions. After this was entered into the function, no further attributes were found to be significant discriminators between the two groups. Both groups of temperate rain forest species were dispersed primarily by vertebrates, flowered in spring and summer, and had their fruit ripe mainly during summer-autumn.
Phylogenetic contrasts were generally consistent with the cross-species patterns described by DFA. For dry sclerophyll woodland, there was a not-quite-significant trend for somewhere-abundant species to spend a longer period of the year flowering compared with everywhere-sparse species (t = 2.18, P = 0.058, df = 3). This concurred with cross-species DFA analyses. For temperate rain forest species, the only significant result was for geographical range to be smaller in everywhere-sparse species (t = 2.92, P = 0.05, df = 5), again matching cross-species (DFA) comparisons.
For both dry sclerophyll woodland and temperate rain forest, tables listing species used for phylogentically independent contrasts and their attributes are available in Ecological Archives, along with a table listing all 116 somewhere-abundant and everywhere-sparse species of temperate rain forest (see Appendix).
Patterns of abundance throughout geographical ranges
How abundance is distributed among species within local communities has been a much-investigated question (e.g., Preston 1948, MacArthur 1957, Cohen 1968, May 1975, De Vita 1979, Hughes 1986, Wilson et al. 1996). [TABULAR DATA FOR TABLE 1 OMITTED] Nevertheless, the relationship between abundance within a local community and abundance elsewhere throughout the complete geographical range of species has been investigated rarely (see Brown et al. 1995). Most models that predict species abundance distributions at local scales invoke within-community mechanisms alone, foregoing processes operating at wider scales. Yet since around 96% to 100% of species within the tail of a given rank-abundance curve can be abundant at other locations or times (as shown for dry sclerophyll woodland in the present study), explanations as to why species are in the tail that are based on within-community interactions without consideration of wider-scale patterns will be incomplete for most species, because any explanation ought to account for different outcomes in different places.
Of the 55 tail species identified in dry sclerophyll woodland (and not classified as sometimes-abundant), and the 116 tail species found in rain forest, 91% and 95%, respectively, were found to be substantially more abundant at other sites. For the purpose of this study, substantially more abundant meant that they were at least 10-fold more abundant than at the focal site, and also that their abundance exceeded the threshold defined for their size-class. Under some circumstances, 1% cover could constitute "substantially more abundant." Hence, the results should not be interpreted as meaning that all these species attained cover of 3050% somewhere.
Mainly, somewhere-abundant species were more abundant at other locations within the same general vegetation type. Of the two alternatives outlined in the introduction, these results support the first [ILLUSTRATION FOR FIGURE 1A OMITTED] as the best description of patterns of abundance throughout geographical ranges. If this finding is combined with the results of Brown et al. (1995), then the most commonly observed distribution of species abundance at a local scale (i.e., few abundant species and many species of low abundance) arises as follows. First, it is clear that the majority of species in the tail of rank-abundance curves have locations where they can be found in much greater abundance. Second, most if not all species are of low abundance throughout much of their geographical range (Brown et al. 1995). Thus, the distinctive distribution of species abundance within local communities simply reflects that sampling at any given point within the landscape will capture most species at low abundance and a few species at high abundance. The question why certain species are found in the tail of rank-abundance curves, so often asked from a within-community perspective, can now be approached from a different perspective. Rather than focus on factors constraining abundance of particular species within a site, it is more pertinent to ask how species are able to switch positions in the rank-abundance curve across wider scales. That is, what allows somewhere-abundant species to become abundant at some sites within their geographical range? Any explanation should explain also why the few everywhere-sparse species are not able to become abundant anywhere within their respective geographical ranges.
One can distinguish two ways a somewhere-abundant species might achieve high abundance at some sites but low at many sites: First, particular sites may favor high abundance for that species on a continuing basis (e.g., remnant populations, Eriksson 1996). Second, somewhere-abundant species may achieve high abundance opportunistically and temporarily at any of a range of sites. Under the second scenario, somewhere-abundant species would be characterized by attributes that permitted them to become abundant given a suitable opportunity, and these attributes would be absent in everywhere-sparse species. These two scenarios are of course extremes of a spectrum, rather than strict alternatives. The real world might be somewhere between these two extremes, with some sites where a species is nearly always abundant, others where it may become abundant opportunistically quite often, and others where the opportunity for high abundance occurs relatively infrequently. In this scheme, sometimes-abundant species represent a sampling or life-history complication: Species conspicuous during early years after fire in dry sclerophyll woodland are actually abundant in later years, but as a seed bank rather than as canopy cover.
There is some evidence that particular sites within the geographical ranges of somewhere-abundant species favor high abundance on a continuing basis. Using long-term data ([approximately]30 yr) for North American birds, Brown et al. (1995) showed that a few sites supported consistently higher abundances of particular species than much of the rest of their geographical ranges. Many other studies, varying from few to many sites and short to long time periods, have similarly indicated conservation of high abundance by particular species at certain sites (e.g., MacArthur 1972, Moyle and Vondracek 1985, Mitchley and Grubb 1986, Lawton and Gaston 1989, Bengtsson et al. 1997). On the other hand, a number of studies have found evidence that high abundances are not maintained by species at particular sites, or have found that differences in site environmental conditions, latitudinal position, and climatic conditions determine whether species' abundances remain constant (Coull and Fleeger 1977, Jarvinen 1979, Taylor and Taylor 1979, Grubb et al. 1982, Grubb 1986, Bethke 1993, Bohning-Gaese et al. 1994, Wilson et al. 1996). Evidence for high abundance being possible at a number of locations throughout geographical ranges can thus be found. In these cases, it would appear that there was a shifting cloud of abundance (sensu Hubbell and Foster 1986), with opportunities for high abundance being taken as they arose across the landscape. The concept of a shifting cloud of abundance relies heavily on stochastic, or lottery models (e.g., Sale 1977, Chesson and Warner 1981, Fagerstrom 1988, Pacala 1996). Rather than particular sites favoring high abundance by certain species (e.g., Brown et al. 1995), "the relative abundance of . . . species . . . at any site is largely a result of the chance recruitment of young to that site and will change from time to time" (Sale 1977:354).
If continuing high abundance sites do exist for somewhere-abundant species, existing evidence suggests they will most likely be found toward the centers of species' geographic ranges (Grinnell 1922, Bock et al. 1977, Hengeveld and Haeck 1981, 1982, Brown 1984, Bart and Klosiewski 1989, Hengeveld 1994, Carey et al. 1995). Therefore, testing the hypothesis that sites of long-term high abundance exist for these vegetation types could be carried out by first establishing long-term monitoring sites towards the centers of geographic ranges. It is also worth considering that high abundances may be expected in conditions that are more similar to those in evolutionary centers.
The proportion of everywhere-sparse species was small but not dismissable. Five species of dry sclerophyll woodland (9%) and six species of temperate rain forest (5%) were categorized as everywhere-sparse. If these percentages can be extrapolated roughly across all species in each different vegetation type, then the absolute number of everywhere-sparse species may be quite large. For example, [approximately]2000 species in total are found in dry sclerophyll woodland on Sydney sandstone (Carolin and Tindale 1994), so it might be estimated that around 180 of these (9%) are everywhere-sparse. This is a substantial number of species that are apparently unable to make use of opportunities for high abundance. Note that everywhere-sparse species are not expected to be distinguished by attributes that enable them to persist at low abundances. Somewhere-abundant species are similarly at low abundances throughout most of their geographical ranges. Therefore, the distinction in this study is not centered on the persistence of everywhere-sparse species at low abundances or on possible adaptations for existing at low abundances, but rather on the potential for becoming abundant in somewhere-abundant species versus the absence of that potential in everywhere-sparse species. None of the everywhere-sparse species identified in the present study are considered rare or threatened (Briggs and Leigh 1996). We have avoided the term rare here for this reason, and also because "rare" has been used in many different senses (see Rabinowitz et al. 1986, Gaston 1994). It is possible that with even more complete data about abundance throughout the geographical range, some everywhere-sparse species might be reclassified to somewhere-abundant. The effect of this would be to increase further our estimate of the proportion of tail species that are somewhere-abundant.
Of the attributes available to be compared between somewhere-abundant and everywhere-sparse species, only one was significantly different in each vegetation type as cross-species tendencies and as replicated phylogenetic divergences. In dry sclerophyll woodland, somewhere-abundant species flowered on average across a longer time during the year than everywhere-sparse species. In temperate rain forest, somewhere-abundant species occurred throughout wider geographical ranges than everywhere-sparse species. Since neither pattern was repeated in the other vegetation type, there was no consistent support for any differences between somewhere-abundant and everywhere-sparse species in any of the attributes compared here. In separate field-studies on two phylogenetically-independent comparisons (Murray and Westoby, unpublished data), provisional evidence has been found for a lower potential for increase in everywhere-sparse species compared to somewhere-abundant species.
The principal conclusion reported here is that most ([greater than]90%) species in the tails of local rank-abundance curves are substantially more abundant somewhere else. If this pattern extends to other vegetation types and to other taxa besides plants, it indicates that models of population interactions within communities will not by themselves provide a complete account of rank-abundance patterns. The second principal conclusion reported here is that there exists a small but still interesting group of species that are everywhere sparse. If this conclusion extends to other vegetation types and taxa, it must be asked what attributes might account for some species' reaching high abundance at some times or places, and for others' being confined to low abundance everywhere.
We are very grateful to other botanists who shared their wealth of field information regarding the abundances of numerous plant species: D. Benson, F. Burrows, R. Coveny, A. Downing, M. Dunlop, L. Rodgerson, L. McDougall, M. Reed, J. Turner, S. Vernon, and G. Williams. Thanks also to Lesley Hughes, Carlos Fonseca, Will Edwards, Bill Lee, Jeremy Smith, and Rob Whelan for critiquing earlier drafts of this work and to the Ecology Discussion Group for helpful comments. Charles Canham and two anonymous referees provided encouraging and helpful advice, for which we thank them wholeheartedly. This work was carried out while B. R. Murray was in receipt of an Australian Postgraduate Award, and he would like to thank personally Darryl Nelson, Carlos Fonseca, Jake Overton, Chris Chambers, and Susan Murray for their support and friendship. This is contribution 265 from the Centre for Biodiversity and Bioresources, Macquarie University.
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Tables presenting the species used for phylogenetically independent contrasts and their attributes are available in ESA's electronic data archive. Separate tables are posted for dry scherophyll woodland (Ecological Archives E080-011-A1) and for temperate rain forest (Ecological Archives E080-011-A2). A third table, listing all 116 somewhere-abundant and everywhere-sparse species of temperate rain forest examined is also provided (Ecological Archives E080-011-A3), along with a bibliography (Ecological Archives E080-011-A4).
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|Author:||Murray, Brad R.; Rice, Barbara L.; Keith, David A.; Myerscough, Peter J.; Howell, Jocelyn; Floyd, Al|
|Date:||Sep 1, 1999|
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