Extinction in fragmented landscapes: Demographic mechanisms and predator-prey interactions

We are still burning, clearing and "developing" tropical rainforests and other habitats around the world. This causes habitat loss and habitat fragmentation. Although much lip service has been paid to the effects of habitat fragmentation on extinction risk, little is actually known. Could we limit the effects on biodiversity by leaving habitat remnants to a higher degree adjacent to each other in fewer and larger blocks, and less subdivided and isolated?; I investigate this question using a variety of different approaches. In chapter I, I use a simple model and available data on the shape of density dependence in population growth rates for different species to present a numerical/statistical mechanism which affects the overall population growth rate of a metapopulation in a deterministic fashion relative to a single continuous population. In chapter II, I use the limited data on extinction rates on islands in archipelagoes to calibrate a model of extinction probabilities as a function of habitat area and degree of fragmentation. In chapter III, I use simple communities of bacteria and protozoa to conduct laboratory experiments where I observe the effect of habitat loss and fragmentation on species viability, with and without dispersal between subpopulations. In chapter IV, I give exact numerical solutions to stochastic birth-death models with density dependence in the demographic rates, for continuous populations and for populations of the same overall size that have been subdivided into isolated subpopulations. In chapter V, I use computer simulations to study the effects of habitat loss and fragmentation under different regimes of demographic and environmental stochasticity and different degrees of subpopulation isolation. In these models, individuals are represented individually in the data structure and the population trajectories of single populations and metapopulations modelled explicitly. Chapter VI can be read as a summary chapter where I review and critique the work that has been done on the effects of habitat fragmentation on population viability and extinction. I also try to look ahead to problems that still need to be resolved before we have an operative understanding of how habitat subdivision impacts the risk of losing species from our ecosystems.