MECHANISMS CREATING COMMUNITY STRUCTURE ACROSS A FRESH-WATER HABITAT GRADIENT

Lentic freshwater habitats in temperate regions exist along a gradient from small ephemeral ponds to large permanent lakes. This environment al continuum is a useful axis for understanding how attributes of indi viduals ultimately generate structure at the level of the community. C ommunity structure across the gradient is determined by both (a) physi cal factors, such as pond drying and winter anoxia, that limit the pot ential breadth of species distributions, and (b) biotic effects mediat ed by ecological interactions, principally predation, that determine t he realized success of species. Fitness tradeoffs associated with a fe w critical traits of individuals often form the basis for species turn over along the gradient. Among species that inhabit temporary ponds, d istributions are often constrained because traits that enhance develop mental rate and competitive ability also increase susceptibility to pr edators. In permanent ponds, changes in the composition of major preda tors over the gradient limit distributions of prey species because tra its that reduce mortality risk in one region of the gradient cause inc reased risk in other regions of the gradient. Integrated across the gr adient, these patterns in species success generate distinct patterns i n community structure. Additionally, spatial heterogeneity among habit ats along the gradient and the fitness tradeoffs created by this heter ogeneity may hold important evolutionary implications for habitat spec ialization and lineage diversification in aquatic taxa.