The Unified Neutral Theory of Biodiversity and Biogeography (MPB-32) by Hubbell Stephen P.; Hubbell Stephen P. P.;

Author:Hubbell, Stephen P.; Hubbell, Stephen P. P.;
Language: eng
Format: epub
Publisher: Princeton University Press
Published: 2001-04-07T04:00:00+00:00

FIG. 6.17. Species-area relationships at large spatial scale for a metacommunity of 201 × 201 local communities, J = 4. Curves are drawn for θ = 0.1, 1.0, and 10.0, for a low migration rate (m = 0.005). The correlation length L decreases with increasing θ for small m. Mean of one hundred simulation runs.

FIG. 6.18. Species-area relationships at large spatial scale for a metacommunity of 201× 201 local communities, J = 4. Curves are drawn for θ = 0.1, 1.0, and 10.0, for a high migration rate (m = 0.5). The correlation length L increases with decreasing θ for large m. Mean of one hundred simulation runs.

The correlation length L is a really important concept and number because it measures and defines the natural length scale of a biogeographic process over which metacommunity events are dynamically and evolutionarily connected. It is an especially important number for conservation biology because it quantifies the size and region within which observed metacommunity biodiversity evolves, lives, and dies. Thus, the unified theory assumes that, besides relative abundance distributions, species-area curves also contain information about both the fundamental biodiversity number and mean dispersal rates throughout the metacommunity. Although the slope of the species-area curve does not functionally determine the values θ and m, z does constrain θ and m to lie along a single isocline in θ-m parameter space (fig. 6.15). Conversely, knowing θ and m, according to the unified neutral theory, is sufficient to predict the slope of the species-area power law relationship expected on intermediate spatial scales.

The theory developed here for species-area curves makes the assumption that speciation rate is independent of dispersal rate. In general, one would expect speciation and migration rates to be inversely related (Mayr 1963, Slatkin 1977, 1980). High rates of dispersal should promote panmixis and therefore inhibit the speciation rate, at least for some modes of speciation. As we have seen, however, even assuming no interaction between dispersal and speciation, high rates of dispersal lead to lower slopes of species-area curves. A negative effect of dispersal on speciation would reduce the slopes of species-area curves still further. In the absence of quantitative data on the effect of dispersal on speciation rate, there is little beyond this qualitative statement that can be said here.

Before I conclude this chapter, it is useful to illustrate the application of the theory to interpreting species-area curves. My example consists of species-area relationships in twenty seven families of Panamanian trees and shrubs, compiled from regional checklists in the Flora of Panamá (D’Arcy 1987). Species-area curves were constructed for trees and shrubs achieving stem diameters > 1 cm dbh. The areas within which nested species counts were made ranged from the 50 ha plot (0.5 km2) on Barro Colorado Island (BCI) at the low end, to all of BCI (15 km2), to the area of the former Canal Zone (103 km2), to the province of Panama (2· 104 km2), and finally to the entire country of Panama (7.5 · 104 km2). From these data I computed the z values of the species-area curves for each family.

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