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Spatial subsidies alter species-area relationships on islands.
Anderson, Wendy1, Wait, David2, 1 2
ABSTRACT- We present a new hypothesis for predicting and describing patterns of species diversity on small islands and habitat fragments. We have modified the traditional island biogeography equilibrium theory to incorporate the influence of spatial subsidies from the surrounding matrix, which vary among islands and habitat fragments, on species diversity. The modification indicates three possible directions for the effects of spatial subsidies on diversity, which depend on where the focal community falls on the hypothesized unimodal curve of the productivity x diversity relationship. We test this hypothesis with plant richness data from islands in the Gulf of California, where small islands that are subsidized do not fit the species-area relationship found on larger or unsubsidized small islands. We assessed richness on 20 islands (10 subsidized, 10 unsubsidized) in two different wet years. On subsidized small islands, guano as a nutrient subsidy enhances productivity, and is associated with decreased richness. Indeed, the species-area relationship on subsidized islands is negative, while the relationship on unsubsidized islands is positive. Mechanisms for this pattern may be similar to those used to explain the descending side of the productivity x diversity unimodal curve, such as interspecific competition, although alternative explanations such as toxicity are possible also. Examining how the species-area relationship varies as a function of total island productivity (including subsidies) is an important step in understanding the interdependence of productivity, area, and diversity.
KEY WORDS: diversity, subsidies, biogeography, productivity