Endemic diseases affect population dynamics of tree squirrels in contrasting landscapes

Abstract

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 ...

Differential Resource Use between Native and Introduced Gray Squirrels

“…Although females of both species excluded other conspecific females from their high‐use areas, competitive exclusion from preferred habitat was not apparent between species. Annual survival of female western gray squirrels on the Base (0.62) was comparable to that measured for a much larger population in southern Washington (0.64; Vander Haegen et al 2018). In contrast, annual productivity of female western gray squirrels on the Base (0.94 female young/breeding age female) was lower than for females in southern Washington (1.3; Vander Haegen et al 2018).…”

“…Western gray squirrels do not inhabit urban areas in western Washington, but areas surrounding the Base have become increasingly urbanized and support source populations of eastern gray squirrels. With a small, insular population imbedded in a largely urban and suburban landscape (Vander Haegen et al 2018), western gray squirrels on the Base may be vulnerable to competition from invading species like eastern gray squirrels and to further habitat loss caused by changing forest structure and composition.…”

“…Intensive efforts to study and recover western gray squirrels on the Base began in 2007 because of increasing evidence that this population was vulnerable to extirpation (Ryan and Carey 1995, Bayrakçi et al 2001, Fimbel and Freed 2008). From 2007–2012, biologists augmented the population by releasing western gray squirrels translocated from other populations in Washington and Oregon, USA, and monitored their movements alongside resident squirrels with radio‐telemetry to investigate squirrel ecology and evaluate augmentation success (Vander Haegen and Orth 2011, Vander Haegen et al 2018). Concurrently, Johnston (2013) radio‐tracked eastern gray squirrels and experimentally removed some individuals to evaluate competitive interactions with western gray squirrels based on spatial partitioning, resource use, fitness correlates, diet (Johnston et al 2019), and behavior.…”

Diets of native and introduced tree squirrels in Washington

“…We trapped and radio‐collared eastern and western gray squirrels at 5 study sites on the Base that were designated as units for experimental removals of eastern gray squirrels from April 2007 to April 2012 (Johnston ). In addition, state biologists trapped and radio‐collared western gray squirrels at nearby sites for a concurrent project to study and augment the population as part of a recovery strategy for this species (Vander Haegen and Orth , Vander Haegen et al ). In October 2010, we began marking Douglas’ squirrels and radio‐tracked several individuals alongside eastern gray squirrels from January 2011 to March 2012.…”

“…In the western United States, eastern gray squirrels have invaded some areas that support native western gray squirrels ( Sciurus griseus ) and Douglas’ squirrels ( Tamiasciurus douglasii ), where they may compete for food resources (Byrne , Linders and Stinson , Gonzales et al ). Western gray squirrels are state‐listed as a threatened species in Washington, and intensive efforts to recover a population that coexists with eastern gray squirrels in western Washington began in 2007 (Vander Haegen and Orth , Vander Haegen et al ). Eastern gray squirrels were introduced to this region in the early twentieth century and quickly spread throughout urban areas around Puget Sound (Linders and Stinson ).…”