Long-Term Impacts of an Emerging Disease, White-Nose Syndrome: Drivers, Mechanisms, and Conservation Action Influencing the Persistence of North American Bats
Emerging infectious diseases can place severe pressures on wildlife populations, leading to major population declines, local extirpation, and species extinctions. However, variability in disease impacts, existing among species and across a spatial and temporal scale, can help us identify species or populations persisting with disease either via resistance, tolerance, pathogen evasion, or by existing within environmental refugia. Understanding mechanisms leading to host persistence can inform conservation management priorities and strategies. White-nose Syndrome (WNS) is a recently emerged disease caused by the fungal pathogen, Pseudogymnoascus destructans (Pd), that has led to severe declines in hibernating bat populations in North America. This work examines patterns and mechanisms associated with variability in WNS impacts with implications for the conservation of affected species. My first chapter investigates spatial heterogeneity in initial impacts of Pd spread across half of continental North America. We found that WNS-related impacts were lessened in the southwestern regions of North America, suggesting potential spatial refugia from WNS-related impacts but only for Perimyotis subflavus. We found that annual air surface temperatures driving Pd growth explained, in part, this spatial variation in WNS-related impacts. Despite evidence for lessened WNS-related declines in the southwest, impacts to bat populations are severe throughout North America for most bat species. My second chapter examines colonies of M. lucifugus that have experienced variability in declines over time, persisting potentially due to host-specific responses. Specifically, I investigate if differences in early winter fat reserves could explain survivorship and persistence of M. lucifugus colonies with WNS. We found that bats persisting with WNS in 2016 were significantly fatter than bats colonies sampled during WNS arrival in 2008 and 2009 at four out of our six sampled sites. At another two sites, we found that bats were either fatter in 2008 and 2009 compared to 2016. We used hibernation energetic models to estimate the amount of fat afforded to survival and found that increased fat reserves from bats measured in 2016 could reduce mortality by 65%. These data suggest that increased fat reserves can explain, in part, the persistence of M. lucifugus colonies with WNS. Lastly, my third chapter experimentally investigates one possible cause of variability in WNS impacts, variation host susceptibility via protective bacteria in the skin microbiome. In this chapter, I explore the efficacy of using a probiotic bacterium, harvested from the skin of a species experiencing lessened WNS impacts, Eptesicus fuscus, as a conservation tool applied to a more highly affected bat species, M. lucifugus. We found relative increases in survival for probiotic-treated groups compared to our sham control group. We also found evidence for decreased fungal infection and severity in probiotic-treated groups. Our results suggest that probiotic treatment can reduce incidence of White-nose Syndrome in M. lucifugus although timing of treatment is an important factor. Together, this work finds that variability in spatial, species-specific, and temporal impacts from WNS can inform conservation efforts. Namely, this work suggests that bat conservation should involve a multi-pronged approach that protects colonies where bats are persisting with WNS via habitat restoration, and potentially treating bats for threatened populations not persisting with WNS. Given the continued threat of WNS to bats as it spreads throughout North America, using a variety of tools to combat this disease may be critical to prevent disease-induced extinction and the local extirpation of affected bat species.