Spatial patterns of California amphibian declines and ecological limits: Why are frogs disappearing and what does it mean

California is a global hotspot of amphibian population declines. While some of these declines are due to obvious habitat destruction, many species have declined in protected areas for unexplained reasons. Chapter One examines declines of the federally threatened California red-legged frog ( Rana aurora draytonii). Historic locations for the species were mapped and their current population status determined. A geographic information system (GIS), was used to determine latitude, elevation, and land use attributes for all sites, and analyze the spatial pattern of declines. Observed patterns of decline were compared to those predicted by the climate change, UV-B radiation, pesticides and habitat alteration hypotheses for amphibian decline. Declines were not consistent with the climate change hypothesis, but showed a strong positive association with elevation, percent upwind agricultural land use, and local urbanization. Chapter Two examines the spatial patterns of decline for seven additional California species: Ambystoma californiense, Scaphiopus hamondii, Bufo californicus, B. canorus, R. caseadae, R. boylii , and R. muscosa. Unlike for the red-legged frog, all seven additional species had greater declines at lower elevations, the opposite of the pattern expected from the UV-B hypothesis. For three species extirpated sites had far greater surrounding urban land use than sites with present populations, consistent with habitat destruction as the primary cause of decline. For the other five species, urbanization was either moderate or not a factor. For four of these species, declines were strongly associated with the amount of upwind agricultural land use, consistent with the pesticide drift hypothesis. The association of declines with upwind agricultural land use strongly suggests that wind-borne agrochemicals may be an important factor in California amphibian declines. Chapter Three explores how we think about the loss of amphibian species and what is the relationship between a growing human economy and environmental quality. The dominant paradigm for understanding the relationship between economic growth and environmental quality for most biologists is the idea of biophysical limits to growth beyond which we face ecological and economic collapse. The paper argues that the concept of limits is ecologically and economically inaccurate and politically hinders conservation.