Tuesday, August 7, 2007

PS 22-14: Community assembly yielded sequential dependency patterns and questions about effects of habitat size and quality

Denise A. Piechnik, University of California

Community ecologists seek to understand the mechanisms responsible for patterns of trophic structure by observing communities during assembly. Here I present results from a test of a modified version of the sequential dependency hypothesis by Holt et al. (1999). Holt and colleagues suggest that a specialist predator's colonization success is linked to prior successful colonization of its prey, and that generalist species will be early arrivers because they are less trophically constrained. Furthermore, predators require more area and energy on a per capita basis so they should have higher densities per unit area in either large or enriched plots relative to their prey. I observed sequential colonization of species from four trophic levels of an aphid-predator food web module on plots of California native annuals. In a field experiment, I manipulated plot size (1 m2 and 10 m2) and one level of enrichment, and I sampled arthropods several times during a growing season. I found support for the sequential dependency hypothesis, in some cases strong evidence, particularly for specialist predators at higher trophic levels. However trophic level responses to habitat size and enrichment were mixed.  During early colonization, there were higher prey and predator densities on enriched plots, but later samples yielded higher predator and prey abundances on small plots.  Although I found strong evidence for sequential dependency of trophic levels, similar to other studies the species-area response at this scale was equivocal.