Differential rates of immigration and extinction, in conjunction with mechanisms allowing local and regional species coexistence, are key factors determining biological diversity at the community (local) and metacommunity (regional) scale. How species sort and assemble into individual communities and complex metacommunities across time and space remains a critical question in ecology. Hutchinson’s concept of the niche as an “n-dimensional hypervolume” suggests numerous axes across which species may sort, facilitating coexistence and allowing greater local and regional diversity. However, our understanding of the dimensions across which species actually sort in nature is limited. Variation in the size of habitat patches is universal, and increasing abundance and diversity relative to area is one established law of ecology. Freshwater habitats are defined by the strong transition in predation risk between fish and fishless habitats. We directly and independently manipulated patch quality (fish presence) and patch size to examine their effects and possible interactions on the colonization dynamics of aquatic insects. We constructed 6 rectangular mesocosm arrays (blocks) of 6 pools each (N=36), completely crossing 3 pool sizes (1.13 m2, 2.54 m2 and 5.73 m2) with the presence/absence of an equal mixture of green sunfish, Lepomis cyanellus, and golden shiners, Notemigonous crysoleucus.
Results/Conclusions
Our experiment was colonized by 5782 individuals (51 species) of aquatic beetles and 1093 individuals (7 species) of aquatic true bugs. Both coleopterans and hemipterans generally avoided fish patches, but displayed an array of patterns with respect to patch size. Beetles generally favored fishless patches, but then sorted across the gradient of patch sizes. Hemipterans all strongly preferred fishless patches, and uniformly preferred large patches. Thus, in aggregate, beetles preferentially colonized small patches, while hemipterans preferred large patches, resulting in sorting between the two taxa. Patterns of abundance for the 14 most abundant "species" reinforce the importance of patch size and its contribution to perceived quality and resulting species sorting. Colonizing insects potentially reduce the intensity of interspecific interactions in fishless patches by behaviorally sorting across gradients of patch size, allowing increased regional diversity by decreasing the intensity of species interactions, both competitive and predatory. Habitat selection generates species sorting at the immigration stage that can play a preeminent role in community assembly. Establishing patch size as a niche dimension fundamentally changes how we view the role of patch size variation in supporting local and regional diversity, and the preservation of such variation as a driver of diversity.