High annual survival in infected wildlife populations may veil a persistent extinction risk from disease

Abstract

Abstract. Host response to emerging pathogens is variable, causing uncertainty about population-level impacts and challenging effective disease management. White-nose syndrome (WNS) has caused catastrophic declines in some bat species, while others appear less impacted. Developing predictive models based on observed survival patterns can generate testable hypotheses about mechanisms driving population dynamics and contribute to the development of targeted approaches to disease management. We conducted a mark-recapture study of federally endangered Indiana bats (Myotis sodalis) during 2011-2016. Annual survival decreased from 0.78 (95% CI: 0.59, 0.89) and 0.79 (95% CI: 0.70, 0.86) for females and males, respectively, in 2011 to 0.74 (95% CI: 0.33, 0.94) and 0.75 (95% CI: 0.53, 0.89) for females and males, respectively, in 2015. We then modeled two explanatory mechanisms potentially driving the observed patterns: (1) phased exposure to disease through the spatial spread of the pathogen within the hibernaculum; and (2) cumulative mortality risk from iterative yearly WNS infection. Under a phased exposure scenario, models suggest that infected individuals have an average survival probability of 0.68, and disease prevalence is predicted to reach 100% within 9 yr of disease emergence. Under the cumulative mortality risk hypothesis, survival probability of individuals decreases with each infection cycle. In either case, infected populations are predicted to stabilize at a negative growth rate. Results suggest that Indiana bats tolerate a pathogen load prior to onset of infection, leading to a less pronounced population decline than for other susceptible species. However, the long-term risk of WNS to Indiana bats may be more severe than current population trends suggest. To inform current conservation management, we performed a vital rate sensitivity analysis, which suggested that modest increases in survival (4-5%) through targeted intervention may return declining populations to stability (k = 1.0). Demographic modeling approaches coupled with continued population monitoring can highlight important differences in disease response, and ultimately extinction risk, in host species allowing conservation practitioners to tailor intervention actions so that they will be most effective.

Ecology and impacts of white-nose syndrome on bats

“…Some impacts of WNS are likely to last for many decades, while others may be permanent (Box 1). The slow population growth rates of heavily affected bat species, which usually give birth to only one young per year, means that it will take decades or longer for populations to recover to their original densities, even if they could return to pre-WNS growth rates 140,141,152,153 . As P. destructans can establish long-term environmental reservoirs, it is unlikely that this pathogen could ever be eradicated from North America, which has resulted in research being focused on preventing spread and mitigating impacts 30 .…”

“…For example, growth rates are still negative for many affected M. sodalis populations 117,141 . Coastal refugia have been implicated in the persistence of a few remnant M. septentrionalis 142 , although more research is needed to understand whether bats are actually surviving P. destructans infections in these habitats.…”

Management efficacy in a metapopulation model of white‐nose syndrome

“…Multiple modeling studies have explored various aspects of WNS disease dynamics via both continuous‐time and discrete‐time models (Erickson et al, 2014, 2016; Kramer et al, 2019; Langwig et al, 2017; Maslo et al, 2017; O'Regan et al, 2015; Reynolds et al, 2015; Russell et al, 2015; Thogmartin et al, 2013). Additionally, a few other modeling studies have considered the efficacy of several proposed control methods (Cornwell et al, 2019; Hallam & McCracken, 2011; Meyer et al, 2016).…”