Larger Islands House More Bacterial Taxa

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Science  24 Jun 2005:
Vol. 308, Issue 5730, pp. 1884
DOI: 10.1126/science.1111318


The power law that describes the relationship between species richness and area size is one of the few generalizations in ecology, but recent studies show that this relationship differs for microbes. We demonstrate that the natural bacterial communities inhabiting small aquatic islands (treeholes) do indeed follow the species-area law. The result requires a re-evaluation of the current understanding of how natural microbial communities operate and implies that analogous processes structure both microbial communities and communities of larger organisms.

The relationship between species richness and area size is one of the few generalizations in ecology, but recent studies show that the slope of the relationship differs for microbes (1, 2). Here we show that the slope of the taxa-area relationship for natural bacterial communities inhabiting small aquatic islands is comparable to that found for larger organisms. The result implies that analogous processes structure both microbial communities and communities of larger organisms.

Several mechanisms explain how the number of taxa can increase with the size of the area. The number of taxa in a particular area results from the balance between the colonization of new taxa and the extinction of extant taxa. The size of the area influences the rate of colonization and extinction and so indirectly influences biodiversity. Alternatively, if taxa are adapted to a particular habitat, then larger areas likely contain more habitats and therefore more species. Finally, a taxa-area relationship will appear if more effort is devoted to sampling larger areas, because the number of taxa discovered increases with sampling effort (3). The relationship between diversity and island or sampling area size is well described by the equation S = cAz, where S is the number of species, c is an empirically derived taxon- and location-specific constant, A is the size of the area, and z is the slope of the line. The value for z is generally consistent across taxa but differs between islands (z ∼0.3) and areas of contiguous habitat (z ∼0.1) (3).

Recent work has suggested that, although there appears to be a similar relationship between microbial diversity and area, the slope z for microbial taxa (z ∼0.02 to 0.07) falls well below that observed for taxonomic groups of larger organisms (1, 2, 4). Many microbial taxa appear to be ubiquitous (5), so increasing the area of a survey results in only a marginal increase in the species richness of the sample. However, these studies have investigated the taxa-area relationship only within single contiguous habitats, where it is plausible that constant colonization from adjacent areas rapidly homogenizes the community. The slope of the species-area relationship is expected to be steeper on discrete islands, partly because they present a partial barrier to colonization. We predicted that the slope of species-area relationship for insular bacterial communities would be similar to that found for communities of larger organisms.

The “islands” that we used are water-filled treeholes, a common feature of temperate and tropical forests. Rainwater accumulates in bark-lined pans formed by the buttressing at the base of large European beech trees (Fagus sylvatica) to form small but often permanent bodies of water. Each of these islands houses a micro-ecosystem that derives its nutrients and energy from leaf litter. We measured the water volume (island size) and the bacterial genetic diversity (taxon richness) in 29 treehole islands, using denaturing gradient gel electrophoresis (DGGE) (6), a standard molecular technique in microbial ecology.

Bacterial genetic diversity in this system increased with increasing island size according to the familiar species-area power law (Fig. 1A). The slope z of the relationship (z = 0.26) is indistinguishable from published values for larger organisms (Fig. 1B). The data show that area size strongly influences the diversity of these microbial communities.

Fig. 1.

The species-area relationship for microbial communities. (A) Bacterial genetic diversity (the number of DGGE bands, S) in water-filled treeholes increases with increasing island size (volume, V) according to the power law S = 2.11V0.26. There is a similar linear relationship (not shown) between island surface area (A, in cm2) and bacterial genetic diversity (S = 3.30A0.28, R2 = 0.38, P < 0.001). Treehole volume and surface area are correlated (r = 0.71). (B) Slope of the species-area relationship for marine benthic ciliates and diatoms, salt marsh bacteria, and fungi inhabiting arid soil compared with slope from the current study. Black bars are microbial studies (1, 2, 7); gray bars are typical values for studies with larger organisms (3).

These results have implications for understanding how microbial communities operate and complement recent studies (1, 2) by indicating the conditions under which high microbial z values can occur. In relatively large areas of contiguous habitat, the slope of the species-area relationship appears to be reduced (Fig. 1B). Such communities might never approach equilibrium, because environmental conditions change faster than competitively inferior species become extinct. We suggest that treeholes and similar habitat patches are islands of relative stability where microbial communities can approach equilibrium. Under such conditions, the patterns of abundance and diversity of microbial communities would be similar to those found for larger organisms. It is possible that other mechanisms underlie the difference between our result and those of other microbial studies. Perhaps, for example, the treehole habitat is more heterogeneous, so diversity increases more rapidly with area size. What is evident is that, as for larger organisms, comparatively steep microbial taxa-area relationships are possible.

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