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Biodiversity in the scales

Scaling coefficients can reveal nonlinear relationships among biological variables. The approach has now proved fruitful in exploring the relationship between diversity at different taxonomic levels.

Species diversity is central to ecological studies. But it is a challenging parameter to measure because diversity is organized hierarchically: individual organisms are classified into species, species into genera, genera into families, and so on. As a consequence, patterns of diversity at one level (genera, for instance) are linked statistically and by evolutionary history to the patterns at higher and lower levels (such as families and species). On page 610 of this issue1, Enquist and colleagues describe tight scaling relationships between species richness and diversity among the higher taxa (genera or families), in both living and fossil plant communities. These patterns should eventually help in understanding how species diversity is controlled and how total biomass is partitioned among coexisting species.

Enquist et al.1 used allometric scaling equations that have traditionally been applied to problems of body size and relative growth. In mammals, for example, a power function2 describes the relationship between brain size (x) and body size (y):

The exponent z is the scaling coefficient, and describes whether y is increasing faster (z > 1) or more slowly (z < 1) than x. On a log–log scale, these relationships plot as straight lines for which the scaling coefficient is the slope. In previous work, Enquist and colleagues3,4,5 successfully applied allometric models to general patterns of biomass, body size and growth form in plant and animal assemblages. Here, they extend this approach to describe patterns of taxonomic and biomass partitioning in different tree communities. They analysed a high-quality global data set, collected by the late Alwyn H. Gentry, that includes 227 sites, each one-tenth of a hectare in area, from six continents. A database of plant fossil communities from the Tertiary period included 29 samples from sites in western North America.

For both the living and fossil tree communities, Enquist et al. plotted the logarithm of the number of higher taxa against the logarithm of the number of species, and discovered a tight relationship that explained more than 90% of the variance (although the fit to the Gentry data was not strictly linear). In other words, the number of higher taxa in a community is highly predictable from the number of lower taxa. In one sense, this is not new. Ecologists and biogeographers established long ago that the diversity of higher taxa changed predictably in large communities compared with small ones6. As more species are drawn randomly from a regional source pool, the number of higher taxa rises steeply at first as common taxa are added, and then more slowly as progressively rarer taxa are added7. The power function transforms this sampling curve into an approximately linear allometric relationship over limited regions of the curve.

Could taxonomic scaling be the inevitable consequence of random sampling from a large regional or global species pool? The answer seems to be no: the exponents for randomly constructed communities were consistently larger than those found by Enquist and colleagues. This result means that higher-level diversity in communities accumulates relatively slowly. Put another way, communities consist of slightly more species per genus or species per family than would be expected by chance, a pattern first noted in the 1940s by the statistical ecologist C. B. Williams8.

If statistical sampling cannot entirely account for the pattern, what is the mechanism controlling taxonomic diversity? Previous applications of allometric scaling laws have been successful because they have often been derived from first principles of physical scaling laws, such as the hydrodynamic constraints that control the flow of fluids through plants5. Here, there are no such underlying first principles, and the mechanisms controlling taxonomic diversity are elusive.

Closely related species have similar dispersal abilities, so that communities might come to be dominated by families or genera that contain good dispersers, which would decrease the scaling coefficient. However, Enquist and colleagues' null model assumed that species colonize randomly and with equal probability, so it did not incorporate differences in dispersal ability among species or differences in total abundance among sites, which ranged from 52 to 1,005 individuals. Moreover, the fixed plot sizes in the Gentry data set will have sampled different proportions of species-poor and species-rich communities. As a consequence, the allometric coefficients measured by Enquist et al. are not constants and will change with plot size or sampling design. Variations in annual precipitation, site elevation and community age also contributed to deviations from the null-model predictions. Enquist et al. propose that local and regional processes control the pattern, but much more analysis will be necessary to identify the specific mechanisms responsible for this tight association.

Taxonomic scaling may influence other patterns. In previous analyses of the Gentry data, Enquist and Niklas9 established that total biomass per plot varied surprisingly little across different communities. Consequently, as more species are added to a community, the amount of biomass partitioned to each species must inevitably decrease. This result appears to contradict studies in which total biomass increases when species are experimentally added to communities10. But there is enough scatter in the biomass partitioning data to accommodate both patterns. In contrast to the tight taxonomic scaling curves, the allometric relationship between biomass per taxon and total species richness explained little more than 50% of the variance in the data. For a given number of species in a community, biomass per species can vary tenfold. At smaller spatial scales, adding species to a community may indeed increase total biomass, even though biomass per species falls when communities are compared across large biogeographical regions.

The application of allometric scaling laws to patterns of taxonomic diversity has revealed surprising regularities in the diversity of plant communities. Moreover, the existence of such patterns in both fossil and living plant assemblages from different regions suggests that diversity might be regulated more by local processes, such as species interactions, than by historical factors, such as dispersal barriers or speciation.Teasing apart the specific biological and statistical mechanisms responsible for these patterns is a task for the future.


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Correspondence to Nicholas J. Gotelli.

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