Conservation of endangered systems: Using modeling and empirical investigations to assess the dynamics of endangered species and their threats

Chris Wilcox's dissertation, entitled “CONSERVATION OF ENDANGERED SYSTEMS: USING MODELING AND EMPIRICAL INVESTIGATIONS TO ASSESS THE DYNAMICS OF ENDANGERED SPECIES AND THIER THREATS”, focuses on processes related to the persistence and extinction of populations, combining theoretical and empirical approaches. In chapter one I examine the spatial distribution of fishing activity relative to California's Big Creek Marine Ecological Reserve. Using shore-based observations and landings data we found no aggregation of fishing effort near the reserve. I discuss the factors driving the spatial distribution of fishing, and their relevance to marine reserve success. In chapter two I use an artificial pond experiment and surveys of natural ephemeral pools to study the effect of habitat size and isolation on colonization patterns of predatory aquatic insects. Insect abundance was related in a positive and nonlinear way to pond size and negatively to isolation. These effects of size and isolation may explain some of the invertebrate community differences that are not explained by hydroperiod or food resources. In chapter three I explore habitat occupancy, population density and population turnover in two fairy shrimp species (Branchinecta lynchi and Linderiella occidentalis) in the same wetlands analyzed in chapter 2. B. lynchi populations are spatially aggregated, while those of L. occidentalis are not. B. lynchi appears to have primarily local dispersal via abiotic factors, while avian predators may disperse L. occidentalis over longer distances. Using information on these mechanisms and the observed spatial structure, I discuss ongoing conservation efforts. In chapter four I study the effects of stochasticity on the accuracy of the diffusion approximation for predicting extinction for different life history strategies. Diffusion predictions differ significantly from simulated data, and the amount and direction of the error is not systematic. I suggest that the diffusion approximation should be used only with caution when assessing extinction risk or making management decisions. In chapter five I explore including density dependent catastrophes in models of threatened populations and show that there is a relative increase in persistence at intermediate population sizes. I illustrate these results with data from crabeater seals. These results are qualitatively different from past results, and have important implications for the conservation of species that may experience catastrophes such as disease outbreaks or starvation.