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

Background/Question/Methods

Top predators are known play an important role in the assembly of communities via two mechanisms: (1) by altering the colonization patterns of prey through behavioral habitat selection, and (2) by altering the birth and/or mortality rates (or emigration) of prey post-colonization.  In this study, we report the results of an experiment designed to measure the independent and combined effects of habitat selection and subsequent predation by fish on structure and diversity of freshwater communities in experimental pools.  As in previous studies, we found prey colonization was strongly influenced by the presence/absence of ‘non-lethal’ fish, suggesting an important role for ‘non-consumptive’ influences of predators on aspects of community assembly.  More importantly, our experimental design allowed us to examine the relative importance of predation and habitat selection on post-colonization communities and to ask the question: “Do these behavioral responses of prey to the presence of predators affect the final structure of a community?” 
Results/Conclusions

We found that the strong effects of habitat selection during colonization persisted post-colonization to the end of the study.  Post-colonization fish predation also had dramatic effects on final community characteristics, but these effects differed qualitatively from the effects of habitat selection.  Habitat selection, but not subsequent predation, altered the ratio of secondary to primary consumers, while post-colonization predation, but not habitat selection, resulted in a trophic cascade.  We found little evidence for habitat selection modifying subsequent predation except among herbivores, where the presence of free-roaming predators increased the effects of habitat selection in the final community.  Our results confirm the importance of predation as an important post-colonization process shaping aquatic communities observed in past studies, but also show that shifts in habitat selection can have large, persistent and sometimes novel effects on community assembly and structure.