Local vs Regional Controls on Diversity
in Marine Subtidal Communities
What controls the number of species living within a community? The species richness of local communities reflects the combined influence of ecological and evolutionary processes operating across a wide variety of spatial and temporal scales. Despite an increasing recognition that processes operating on large scales are important, the role of regional-scale processes in controlling local patterns of species diversity is poorly understood.
If local communities are simply a sink from the biogeographic (regional) species pool, then local species richness will be positively and linearly related to regional species richness over a broad range of diversities. Alternatively, if small-scale ecological interactions limit the number of species coexisting in local habitats, then regressions of local richness on regional richness should level off or saturate.
To investigate the relationship between the number of species in biogeographic regions and the number occurring in local communities, we quantified the number of sessile epifaunal invertebrate species occuring in 5-11 local sites in each of 11 biogeographic regions (e.g. Gulf of Maine, Pacific Northwest, Caribbean, Australia, New Zealand, Galapagos, Seychelles, Palau, Antarctica, South Africa etc.). Biogeographic regions were selected to represent a range in the size of regional species pool and to provide replicate levels of regional diversity. The map depicts different levels of regional diversity (low, intermediate and high by color) for shallow-water organisms and the location of some of our sampling sites.
Local diversity was quantified from 35mm photo quadrats (0.25m2) taken at random locations along transects placed horizontally across subtidal rock walls at approximately 10m depth (see adjacent photo).
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Seychelles |
South Africa |
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Gulf of Maine |
Palau |
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Above are representative photoquadrats from various sites in our study. The number of species in each photoquadrat was quantified. Quadrat photos were analyzed until a plot of the cumulative number of sessile invertebrate species at each location reached an asymptote, which represented our measure of local species richness. Regional species richness was estimated from published species lists and by consulting local experts at each site. Comparison of local and regional estimates of diversity, indicates a strong positive linear relation between regional and local richness (Type I relation) with 56% of the variation in local species richness explained by the number of species in the regional species pool. These findings suggest that small-scale local processes are not limiting the diversity of local assemblages and that regional-scale processes have a strong influence on the number of species within communities. It also suggest that conservation efforts, especially in marine environments, need to consider large-scale processes. The results from this research greatly enhances our ability to critically assess the importance of regional processes in structuring shallow marine hard-substrate communities and provides an important global database of the composition and diversity of subtidal rockwall communities. This work was done in collaboration with Jon Witman at Brown University. Support provided by NSF Biological Oceanography Program. |
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