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Evolutionary dynamics of a rapidly receding southern range boundary in the threatened California Red-Legged Frog (Rana draytonii)
Authors: Richmond JQ, Barr KR, Backlin AR, Vandergast AG, Fisher RN | Date: 2013-02 | Outlet: Evolutionary Applications doi:10.1111/eva.12067 | Format: .PDF
Populations forming the edge of a species range are often imperiled by isolation and low genetic diversity, with proximity to human population centers being a major determinant of edge stability in modern landscapes. Since the 1960s, the California red-legged frog (Rana draytonii) has undergone extensive declines in urban-plagued southern California, where the range edge has rapidly contracted northward while shifting its cardinal orientation to an east-west trending axis. We studied the genetic structure and diversity of these front-line populations, tested for signatures of contemporary disturbance, specifically fire, and attempted to disentangle these signals from demographic events extending deeper into the past. Consistent with the genetic expectations of the Ďabundant-centerí model, we found that diversity, admixture and opportunity for random mating increases in populations sampled successively further away from the range boundary. Demographic simulations indicate that bottlenecks in peripheral isolates are associated with processes extending tens to a few hundred generations in the past, despite the demographic collapse of some populations due to recent fire-flood events. While the effects of recent disturbance have left little genetic imprint on these populations, they likely contribute to an extinction debt that will lead to continued range contraction unless management intervenes to stall or reverse the process.
The precarious persistence of the endangered Sierra Madre yellow-legged frog (Rana muscosa) in southern California
Authors: Backlin AR, Hitchcock CJ, Gallegos EA, Yee JL, Fisher RN | Date: 2013 | Outlet: Oryx - International Journal of Conservation (in press) | Format: .PDF
We conducted surveys for the endangered Sierra Madre yellow-legged frog (Rana muscosa) throughout southern California to evaluate their current distribution and status. Surveys were conducted between 2000 and 2009 at 150 unique streams and lakes within the San Gabriel, San Bernardino, San Jacinto, and Palomar mountains of southern California. Of the 150 survey locations only nine small, geographically isolated, populations were detected across the four mountain ranges. The nine R. muscosa populations all tested positive for the amphibian chytrid fungus (Batrachochytrium dendrobatidis). Our data show that when R. muscosa is known to be present, it is highly detectable (89%) from a single visit during the frogs active season. We estimate there were only 166 adult frogs that remained in the wild in 2009. From our research, it appears that R. muscosa populations in southern California are extremely vulnerable to natural and stochastic events and may become extirpated in the near future without intervention.
Disease in a dynamic landscape: Host behavior and wildfire reduce amphibian chytrid infection
Authors: Hossack BR, Lowe WH, Ware JL, Corn PS | Date: 2013 | Outlet: Biological Conservation 157: 293-299 | Format: .PDF
Disturbances are often expected to magnify effects of disease, but these effects may depend on the ecology, behavior, and life history of both hosts and pathogens. In many ecosystems, wildfire is the dominant natural disturbance and thus could directly or indirectly affect dynamics of many diseases. To determine how probability of infection by the aquatic fungus Batrachochytrium dendrobatidis (Bd) varies relative to habitat use by individuals, wildfire, and host characteristics, we sampled 404 boreal toads (Anaxyrus boreas boreas) across Glacier National Park, Montana (USA). Bd causes chytridiomycosis, an emerging infectious disease linked with widespread amphibian declines, including the boreal toad. Probability of infection was similar for females and the combined group of males and juveniles. However, only 9% of terrestrial toads were infected compared to >30% of aquatic toads, and toads captured in recently burned areas were half as likely to be infected as toads in unburned areas. We suspect these large differences in infection reflect habitat choices by individuals that affect pathogen exposure and persistence, especially in burned forests where warm, arid conditions could limit Bd growth. Our results show that natural disturbances such as wildfire and the resulting diverse habitats can influence infection across large landscapes, potentially maintaining local refuges and host behaviors that facilitate evolution of disease resistance.