MECHANISMS CREATING COMMUNITY STRUCTURE ACROSS A FRESHWATER HABITAT GRADIENT

Lentic freshwater habitats in temperate regions exist along a gradient from small ephemeral ponds to large permanent lakes. This environmental continuum is a useful axis for understanding how attributes of individuals ultimately generate structure at the level of the community. Community structure across the gradient is determined by both (a) physical factors, such as pond drying and winter anoxia, that limit the potential breadth of species distributions, and (b) biotic effects mediated by ecological interactions, principally predation, that determine the realized success of species. Fitness tradeoffs associated with a few critical traits of individuals often form the basis for species turnover along the gradient. Among species that inhabit temporary ponds, distributions are often constrained because traits that enhance developmental rate and competitive ability also increase susceptibility to predators. In permanent ponds, changes in the composition of major predators over the gradient limit distributions of prey species because traits that reduce mortality risk in one region of the gradient cause increased risk in other regions of the gradient. Integrated across the gradient, these patterns in species success generate distinct patterns in community structure. Additionally, spatial heterogeneity among habitats along the gradient and the fitness tradeoffs created by this heterogeneity may hold important evolutionary implications for habitat specialization and lineage diversification in aquatic taxa.