Species distributions in anisopteran odonates: Effects of local and regional processes.

Abstract

Ecological communities are structured by processes operating at multiple spatial scales, including local species interactions and regional processes such as dispersal between sites. Determining the relative importance of processes at these two scales is critical to assessing the mechanisms which are predominately responsible for the structuring of ecological communities and for predicting how environmental changes will affect communities. Few studies, however, have examined the effects of processes operating at both regional and local levels on community organization within a single system. Using natural habitat surveys combined with lab and field experiments, I examined how processes operating at both local and regional spatial scales affect the breadth of species distributions in larval dragonflies. I contrasted the effects of predation (a local process) and dispersal limitation (a regional process) as mechanisms affecting the distributions of habitat specialist dragonflies that were confined to permanent lakes with sunfish (Centrachidae) top predators and habitat generalists that occurred in sites with a broad range of permanence levels and top predator types. I also related species' distributions to traits expected to affect performance. I determined that growth and activity levels were related to habitat distributions, the stronger the association with sunfish lakes the lower the activity levels and growth rates of species' larvae. However, results from lab experiments indicated that differential vulnerability to alternative top predator types was not a key mechanism restricting the distribution of habitat specialists from sunfish lakes. I also demonstrated dispersal limitation affected the richness and composition of larval dragonfly communities formed in a field mesocosm experiment. Dragonfly species were differentially affected by dispersal limitation and these differences indicate that this is a key mechanism in shaping species distributions and ultimately community composition in this system. Habitat specialists from permanent, sunfish lakes dispersed less often and shorter distances than habitat generalists and had lower morphological investments into structures related to flight performance. These habitat specialists were also less likely to colonize mesocosms, suggesting greater habitat selectivity than habitat generalists. These results suggest that dispersal limitation and habitat selection are critical factors restricting the distribution of habitat specialists from sunfish lakes. This research suggests that regional processes have important consequences for community composition and that they are likely to interact with local processes to determine community structure in aquatic systems.