Habitat bottlenecks in stage-structured species: hermit crabs as a model system

Abstract

The availability of habitat at one stage of a species' life history may limit the ultimate population size of that species. In particular, species that use distinct juvenile habitats may experi- ence bottlenecks at an early life stage that have important and potentially unexpected consequences for adult population sizes. I used intertidal hermit crabs Pagurus samuelis and P. hirsuitisculus as a model system to test experimentally the effect of increasing juvenile and adult habitat (snail shells, Littorina sp. and Tegula funebralis) availability on adult population size. Despite strong evidence from these experiments that juvenile hermit crab population size was limited by juvenile habitat availability, adult population size was not. Instead, results indicated that adult populations were probably limited by recruitment and adult habitat availability. Consequently, these results (1) demon- strate that identifying true habitat bottlenecks can be challenging, (2) show that identifying true bottlenecks requires one to evaluate the effect of bottlenecks on adult population size, and (3) offer a rigorous empirical test of the assumption that hermit crab populations are limited by the availability of large shells. These results also highlight how conservation and management strategies designed to protect species with stage-structured life histories must consider the actual role different habitats play in controlling adult population size if they are to be most effective.