Assessing Habitat Quality and Anthropogenically-Mediated Change in Habitat Quality for the Mojave Desert Tortoise.

摘要

Resource distributions on the landscape have strong influences on community structure. Overall ability of a population of species to persist is dependent on a number of habitat characteristics, including forage availability and shelter. However, quality of the habitat patch may not be the only driver of species presence, abundance, or persistence, all of which may be affected by factors external to the habitat patch. In this thesis, I examined the relationship between habitat characteristics and individual measures of fitness in the Mojave desert tortoise (Gopherus agassizii). In addition, I looked at the effects of landscape attributes, in particular those mediated by anthropogenic activity, on habitat quality. Specifically, I studied how roads and climate change may exert pressures on desert tortoise habitat and populations. Finally, to explore indirect effects of a changing landscape I looked at the relationship between maternal body size and offspring quality. I found that within desert tortoise habitat the annual vegetative community is the best predictor of individual measures of fitness. Both the presence of roads and the occurrence of climate change appear to exert negative effects on desert habitat that have direct and indirect effects on desert tortoises. These effects will likely scale-up to have population-level effects. In particular, both appeared to reduce the mean size of individuals. I also found that smaller female tortoises produce lower quality offspring than do larger females. Thus factors that reduce individual size will likely have additional indirectly mediated negative effects on tortoise populations.