A long held paradigm in ecology is that environmental heterogeneity enhances diversity of communities. When increases in heterogeneity translate into increase in number of resources, such a pattern is a logical outcome of the competitive exclusion principle. Because number of limiting resources determines number of competitors, diversity begets diversity. Numerous published positive correlations between environmental heterogeneity and species diversity indicate ubiquity of this phenomenon. Nonetheless, most assessments of this relationship are phenomenological and provide little insight into how such patterns result. Increases in resource diversity should increase species packing within communities because a greater number of similar species can coexist. To this end, species richness should scale positively and species similarity should scale negatively with increases in resource diversity.
We examined interactions among resource diversity, species diversity and ecological similarity of species across 31 rodent communities in the central Mojave Desert. We used average phylogenetic distance of species within a community as an estimate of ecological similarity. Regression analyses and variance decomposition based on redundancy analyses were used to determine significance of hypothesis tests. We further examined the phylogenetic structure of communities as well as gradients in the degree of non-random structure of communities by comparing empirical phylogenetic distances to those based on random assembly from a regional species pool.
The relationship between species diversity and resource diversity was positive and significant. Relationships between species diversity and phylogenetic distance were negative and significant; more similar species are packed into communities with greater numbers of species. Similar patterns existed between resource diversity and phylogenetic distance. As resource diversity increased, more species that were more ecologically similar co-existed within communities.
Comparisons among empirical and randomly assembled communities provided further support that resource diversity enhanced coexistence and facilitated greater diversity. Variation existed regarding degree to which empirical communities fit a model of random assembly. Species poor communities coming from simple habitats were phylogenetically overdispersed and evince patterns consistent with competitively mediated structure. Species rich communities from complex habitats failed to evince such patterns. Standardized effect sizes described relative degrees of non-random structure within communities and were regressed against resource diversity. A strong and significant gradient emerged whereby effect sizes were large (phylogenetically overdispersed) in communities of low resource diversity and decreased (more clumped) as diversity of resources increased.
Results suggest that species diversity increases with resource diversity because more resources reduce effects of competition and allow greater species packing within communities.