Modeling the role of stage-structured agonistic interactions in ontogenetic habitat shifts

Ontogenetic habitat shifts are often associated with habitat-dependent changes in competition and/or predation risk. Although inter- and intraspecific interactions are known to influence the distribution of size classes among habitat types, the mechanistic role that agonistic behaviors play in driving apparent ontogenetic habitat shifts has yet to be rigorously examined. We used an individual-based, correlated random walk model which permitted behavioral interactions among individuals to test whether spatial variation in juvenile occupancy of different habitat types can be produced by (1) mortality (i.e., agonistic interactions between adults and juveniles result in juvenile mortality) or (2) escape responses during agonistic interactions (i.e., juveniles fleeing after interactions with adults). We used the signal crayfish (Pacifastacus leniusculus Dana), a well-studied stream invertebrate that experiences an ontogenetic habitat shift from riffles to pools, as a model organism. When modeled juveniles experienced mortality during adult–juvenile agonistic interactions or when interactions produced escape behaviors, the population distributions became size-structured (juvenile occupancy of riffles greater than pools). Although both mortality and escape behaviors produced size-structured distributions, the effect of escape behaviors was greater than the effect of mortality on the distribution of juveniles. Furthermore, maintenance of an asymmetric distribution of juveniles between the habitat types was strongly dependent on a lack of underlying preferences for the pool habitat in juveniles. Our results suggest that juvenile escape responses during agonistic interactions with adults may be a key mechanism contributing to observed ontogenetic habitat shifts, particularly in species for which the juveniles show greater niche flexibility than the adults.