We studied survival of radio-marked western gray squirrels (Sciurus griseus) and quantified causes of mortality including incidence and severity of notoedric mange in south-central Washington, 1998Washington, -2005. We used known-fate models in Program MARK to explore alternative hypotheses on factors related to survival and correlation analysis to investigate parameters that might be related to incidence of mange. The best-supported models indicated that survival varied by year and by sex and that survival of males was lower during the breeding season compared to the non-breeding season. We found little support for differences in survival between juvenile (5-12 months old) and adult squirrels, or for winter severity or size of the acorn crop as significant influences on survival. We determined the likely proximate cause of death for 81 animals; 63% were killed by predators and 37% succumbed to disease, with most disease deaths attributed to mange. Mange was documented in the population during all years and occurred more frequently in animals captured in spring than in animals captured in fall. Counter to our predictions, occurrence of mange was not correlated with 2 measures of winter severity but was strongly correlated with mildness of the preceding winter (number of days with mean air temperature !08 C). Sequential use of nests by individual squirrels during mild winters with temperatures conducive to survival of ephemeral, free-living mites may partially explain the periodic epizootics of notoedric mange in this western gray squirrel population. Continued deterioration of squirrel habitat through fragmentation will place additional stressors on the population and may compound the effects of mange on this threatened species. Ó 2013 The Wildlife Society.

Habitat loss and fragmentation can have detrimental effects on wildlife populations and where pervasive can create population isolates that may experience reduced genetic diversity and lower persistence. Diseases that cause epizootics also can reduce wildlife populations and may have disproportionate effects on small populations. We studied survival of radio-marked western gray squirrels (Sciurus griseus) using known-fate models in Program MARK and we quantified annual reproductive success by following females through the breeding season and counting young at natal nests. We used data on survival and productivity to model population growth rate and associated parameters using deterministic and stochastic approaches. Populations of western gray squirrels that we studied in an extensive, forested landscape and in a highly fragmented, urbanizing landscape in Washington, USA differed in their modeled growth rate. Adult survival was similar between populations although both were strongly affected by different endemic diseases with high epizootic potential. The demographic parameters that differed most between these 2 populations were related to productivity; litter size was marginally smaller and reproductive success was significantly lower in the urbanizing Puget Trough compared to rural Klickitat County. Results of our demographic modeling suggest that the larger Klickitat population is robust to immediate threats, whereas the smaller Puget Trough population is at risk because of its small size and low fecundity. Periodic outbreaks of notoedric mange in the Klickitat population reduce adult survival, although our models suggest that these epizootics would need to occur more frequently than observed to be of significant risk to the population. Continued degradation and fragmentation of western gray squirrel habitat in the Klickitat region along with mild winters resulting from climate change could increase the frequency and severity of mange epizootics and further threaten this population. Actions to retain and improve habitat resources may help ameliorate the effects of future mange epizootics and maintaining quality habitat should be a management priority in this region. The insular Puget Trough population experienced mortality due to tularemia each year of our study with infection rates !14%. Lack of characteristic histological indications in some affected squirrels may lead to underreporting of tularemia in animals submitted for routine necropsy and could complicate assessment of mortality risks in wildlife population studies. Given its small size and isolation, the Puget Trough population should be monitored closely for indications of decline in number or occupancy; this small population may need periodic augmentation to maintain genetic diversity. Increasing suitable habitat and maintaining connectivity between currently occupied range and potential habitat in the surrounding landscape will be crucial to the long-term viability of this population but will be challenging in the urbanizing landscape of the Puget ...