Habitat size, recruitment, and longevity as factors limiting population size in stage-structured species

Surprisingly little research has evaluated how habitat size may limit the population size of species that use different habitats at different stages of their lives. Here we develop simple discrete-time models to describe the population dynamics of species that use separate juvenile and adult habitats. Analytic solutions, model simulations, and elasticity and sensitivity analyses show that adult abundance is only limited by the size of the juvenile habitat when both adult habitat size and recruitment are much larger than juvenile habitat size. Juvenile habitat plays a marginally greater role in limiting population size for species with closed populations, where recruitment is proportional to adult abundance, versus open populations. Because adult populations often accumulate pulses of juveniles, adult habitat size can strongly limit population size over a broad range of parameter values, an effect that increases as the longevity of a species increases. Limited empirical research from a range of taxa supports these model predictions, although few studies were designed to actually test the limiting role of juvenile versus adult habitat. Future research must carefully evaluate whether and how processes at the juvenile stage affect adult abundance, and conservation efforts may be able to use this model to evaluate the cost-effectiveness, vis-a-vis increasing adult abundance, of time and money allocated to protecting juvenile habitats.