Habitat as a limiting factor in stage-structured populations

This thesis addresses the role of habitat availability in limiting the population size of species that use distinct juvenile and adult habitats. Several approaches are used to accomplish this, including a meta-analysis of data from studies of applied situations, theoretical explorations using model simulations, and two empirical studies designed to test model predictions.; I performed a meta-analysis of empirical data from 89 no-take marine reserves from around the world to test how habitat-based management efforts have affected marine populations. Results were striking, showing that on average populations within reserves compared to populations outside reserve boundaries were twice as dense, had three times the biomass, were a third larger in average individual size, and were 25% more diverse. Furthermore, these effects were independent of reserve size and trophic group.; A simple 2-stage population model was then developed to test the role of stage-specific habitat size in controlling adult population size. Model simulations and sensitivity analyses show that the size of the juvenile habitat can play a dominant limiting role in controlling adult population size, but only given a relatively small set of possible parameter values. Because adult populations can store pulses of juveniles, the size of the adult habitat can have a strong impact on population size over a broad range of parameter values, an effect that increases as the longevity of a species increases.; Finally, empirical work was conducted in two different field systems to test model predictions. First, I evaluated how mangrove stand size and proximity affected the population size of two Caribbean coral reef species, Lutjanus apodus and Gerres cinereus, that use mangroves as an obligate juvenile habitat. Results indicated that the distance between reef and mangrove stand and the size of a single mangrove stand was not correlated with the number of adult fish on the nearest reef patch, but that island-wide fish population sizes correlated with the total extent of mangroves around an individual island. I further tested these ideas with manipulative experiments on hermit crabs and the availability of the snail shells they use as homes. By adding empty shells of different size classes, I found that juvenile populations responded significantly to the addition of juvenile-sized shells, but that adult population size did not. Instead, results suggest that adult population size is limited by recruitment and/or a seasonal limitation of adult habitat. Results from this hermit crab work in particular demonstrate the importance of measuring the effect of potential habitat bottlenecks on adult population size and that the interpretation of habitat bottlenecks is not always straight-forward.