Evolutionary rescue in a host–pathogen system results in coexistence not clearance

Abstract

The evolutionary rescue of host populations may prevent extinction from novel pathogens. However, the conditions that facilitate rapid evolution of hosts, in particular the population variation in host susceptibility, and the effects of host evolution in response to pathogens on population outcomes remain largely unknown. We constructed an individual‐based model to determine the relationships between genetic variation in host susceptibility and population persistence in an amphibian‐fungal pathogen (Batrachochytrium dendrobatidis) system. We found that host populations can rapidly evolve reduced susceptibility to a novel pathogen and that this rapid evolution led to a 71‐fold increase in the likelihood of host–pathogen coexistence. However, the increased rates of coexistence came at a cost to host populations; fewer populations cleared infection, population sizes were depressed, and neutral genetic diversity was lost. Larger adult host population sizes and greater adaptive genetic variation prior to the onset of pathogen introduction led to substantially reduced rates of extinction, suggesting that populations with these characteristics should be prioritized for conservation when species are threatened by novel infectious diseases.

‘Resistance is futile’: Weaker selection for resistance during larger epidemics further increases prevalence and depresses host density

“…The implications of ‘resistance is futile’ for prevalence and host density raise broad, ecological concerns. When hosts evolve high resistance, infection prevalence remains small and epidemics depress host density little (Altizer et al 2003; Christie and Searle 2018; Duffy and Sivars-Becker 2007). If hosts instead evolve less resistance during large epidemics, these epidemics may depress host density significantly.…”

“…If this evolution of resistance occurs quickly enough, it should reduce the size of epidemics and, consequently, their impact on host density (Penczykowski et al 2011). Thus, rapid evolution of increased resistance by hosts is often expected to suppress prevalence and prevent declines of host density (Altizer et al 2003; Christie and Searle 2018; Duffy and Sivars-Becker 2007).…”

Species distribution models predict the geographic expansion of an enzootic amphibian pathogen

“…A recent assessment (Zumbado‐Ulate, Nelson, et al, 2019) found that 67 of 105 amphibian species screened for Bd in Costa Rica tested positive and confirmed that enzootic Bd is currently infecting species that exhibit multiple life‐history traits across all elevations. Fortunately, the lack of Bd ‐driven die‐offs detected since epizootics (i.e., before 2,000) suggests that current amphibian communities in Central America might be exhibiting shifts in host responses to combat enzootic Bd and recover from epizootic declines (Christie & Searle, 2018; Voyles et al., 2018).…”

Endemic Infection of Batrachochytrium dendrobatidis in Costa Rica: Implications for Amphibian Conservation at Regional and Species Level

“…However, it is also well known that some amphibian species suffered unexpected and unexplained declines in lowland environments (<700 m above sea level) during the 1980s and 1990s, likely due to chytridiomycosis [16,19,[29][30][31]. After the declines, the evolution of resistance and tolerance mechanisms in amphibian communities [32], and/or the evolution of less-pathogenic strains of Bd [33], might have allowed susceptible amphibians to persist with endemic Bd infection (i.e., enzootics) [22,[34][35][36]. However, susceptible species are still at a high risk of extinction under endemic infection if conditions shift in favor of the pathogen.…”