Habitat Selection at Low Population Densities

Much of current habitat-selection theory assumes that individual fitness monotonically declines as a function of density, and that social interactions among settlers are entirely competitive. However, when animals settle at low densities, other fitness distributions (e.g., Allee effects) and positive interactions among settlers (e.g., conspecific cueing) are possible. We developed an individual-based simulation model to explore how habitat selection at low densities is influenced by three factors: (1) the relationship between density and fitness (negative density-dependence vs. Allee distributions), (2) intrinsic habitat quality, and (3) settlement costs. When two habitat patches are identical in quality, Allee and negative density-dependent models differ in two respects: in Allee models, (1) all individuals initially settle in one habitat patch, and (2) some individuals switch habitats as soon as the alternative habitat begins to be colonized. These patterns are accentuated if habitats differ in quality, and in this situation, Allee distributions and differences in habitat quality have comparable effects on settlement patterns. Settlement costs reduce the number of individuals switching habitat patches in Allee models, and if settlement costs decline as a function of density (as in conspecific cueing), settlers aggregate even if fitness declines monotonically with density after settlement is complete. Our results show that many of the patterns currently construed as supporting negative density-dependent habitat-selection models are also predicted by models that incorporate Allee effects or settlement costs and, more generally, suggest new approaches to habitat selection at low population densities.