Association between environmental variation and severity of host population declines following pathogen invasion varies across species in a multi-host pathogen system

Alexander T. Grimaudo, Joseph R. Hoyt and Kate E. Langwig, Department of Biological Sciences, Virginia Tech, Blacksburg, VA, Skylar R. Hopkins, Department of Applied Ecology, North Carolina State University, Raleigh, NC, Carl J. Herzog, New York State Department of Environmental Conservation, Albany, NY

Background/Question/Methods
Emerging infectious diseases have had severe impacts on host populations across a wide range of host taxa. However, within host species, there is often substantial variation in population response to novel infectious diseases, in which some populations are more heavily impacted than others. It is well recognized that environmental conditions can both mitigate and exacerbate the severity of disease, a relationship that can scale up to a population-level response. It is poorly understood, however, how variation in environmental conditions may contribute to disease dynamics. Here, we investigate how variability in environmental conditions, specifically temperature and humidity, drives population-level responses of five North American bat species to white-nose syndrome (WNS). WNS presents a tradeoff for hibernating bats because the environmental conditions most suited to satisfying physiological requirements associated with hibernation may not always be those that mitigate disease severity. Pairing environmental data with longitudinal population data, we explore how variation in environmental conditions has influenced the severity of host population declines of several species. We hypothesize that a higher degree of variation in environmental conditions mitigates the population-level impacts of WNS by allowing individuals to transition between microclimates that satisfy physiological requirements and those that minimize disease severity.
Results/Conclusions
Our data suggest a significant role of variation in both temperature and humidity conditions in driving the population-level impact of WNS on bat populations. Two of the five species examined appeared to have benefited from temporal variation in thermal conditions within hibernacula, whereas the remaining three exhibited more severe declines associated with this variation. Temporal variation in humidity conditions seemed to exacerbate population declines in three species but had little association with population response in the remaining two. Of the four species for which there was sufficient spatial data on thermal conditions within hibernacula, three showed positive associations between spatial variation and severity of declines and the fourth showed a negative association. Broadly, this suggests that hosts interact directly with the variation in available environmental conditions, which can scale up to a population-level response if the disease is highly environmentally dependent. However, for multi-host disease systems, we should not expect all host species to share the same relationship between environmental variation and disease severity.