Predator effects on aquatic community assembly: disentangling the roles of habitat selection and post-colonization processes

Top predators are known to play an important role in the assembly of communities via two mechanisms: (1) by altering the colonization (or emigration) patterns of prey through behavioral habitat selection, and (2) by altering vital rates (e.g. mortality, birth) of prey after colonization. While both these mechanisms act to determine assembly, research has focused on either their combined overall effects (confounding them), or examined them singly. As a result, it remains unclear how these mechanisms act to sequentially shape community structure. In this study, we experimentally disentangle habitat selection and post-colonization effects of predaceous fish to test their independent and combined influence on the assembly of insect and larval amphibian communities in experimental freshwater habitats. Specifically, we ask, 'do the behavioral choices of colonists continue to structure aquatic communities even after post-colonization processes have occurred?' Like previous studies, we found that colonization was strongly reduced by the presence of fish cues. More importantly, these effects of fish on prey colonization behavior combined independently with post-colonization processes to determine the overall effect of predators on community assembly. Although habitat selection and predation both reduced abundance and biomass of most taxa in the post-colonization communities, these factors had qualitatively different effects on aspects of trophic structure. Habitat selection altered the ratio of secondary to primary consumer abundance and biomass, while post-colonization predation drove strong trophic cascades not observed in response to habitat selection. Our results suggest that behavioral choices regarding habitat selection can have lasting and unique effects on the structure of aquatic communities.