Compensatory effects of recruitment and survival when amphibian populations are perturbed by disease

Abstract

Summary1. The need to increase our understanding of factors that regulate animal population dynamics has been catalysed by recent, observed declines in wildlife populations worldwide. Reliable estimates of demographic parameters are critical for addressing basic and applied ecological questions and understanding the response of parameters to perturbations (e.g. disease, habitat loss, climate change). However, to fully assess the impact of perturbation on population dynamics, all parameters contributing to the response of the target population must be estimated. 2. We applied the reverse-time model of Pradel in Program mark to 6 years of capture-recapture data from two populations of Anaxyrus boreas (boreal toad) populations, one with disease and one without. We then assessed a priori hypotheses about differences in survival and recruitment relative to local environmental conditions and the presence of disease. 3. We further explored the relative contribution of survival probability and recruitment rate to population growth and investigated how shifts in these parameters can alter population dynamics when a population is perturbed. 4. High recruitment rates (0AE41) are probably compensating for low survival probability (range 0AE51-0AE54) in the population challenged by an emerging pathogen, resulting in a relatively slow rate of decline. In contrast, the population with no evidence of disease had high survival probability (range 0AE75-0AE78) but lower recruitment rates (0AE25). 5. Synthesis and applications. We suggest that the relationship between survival and recruitment may be compensatory, providing evidence that populations challenged with disease are not necessarily doomed to extinction. A better understanding of these interactions may help to explain, and be used to predict, population regulation and persistence for wildlife threatened with disease. Further, reliable estimates of population parameters such as recruitment and survival can guide the formulation and implementation of conservation actions such as repatriations or habitat management aimed to improve recruitment.

Fragile coexistence of a global chytrid pathogen with amphibian populations is mediated by environment and demography

“…Further mechanisms for coexistence may operate at the population level, such that even observable negative effects of Bd on the individual host do not automatically translate to negative effects at the population level [4,43]. In our case, both populations appeared relatively stable over the study period.…”

“…In the face of such uncertainty, maintaining the capacity of host populations to compensate reduced survival (regardless of its cause) through recruitment is crucial for their long-term persistence. These considerations have been made for amphibian hosts in epizootic situations [4,48,49], but they also hold for endemic, temporarily stable host -pathogen systems, particularly where environmental factors that currently mediate coexistence might change in the future. To counter this risk, several studies have suggested that long-term population coexistence may be promoted by actively optimizing the environmental conditions, for example through habitat manipulation [49 -51].…”

“…Secondly, the pathogen may be parasitic but have compensatory, rather than additive, effects on the vital rates of individual hosts; that is, the individuals that die of disease would also have had higher mortality rates in the absence of the pathogen [13]. Alternatively, the parasitic pathogen may inflict additive mortality, but this is compensated at the host population level, for example through increased recruitment and a shift in age structure [4,14,15]. Understanding whether, and by what mechanisms, host populations truly coexist with a pathogen is crucial for predicting and eventually managing the potential impacts of a disease.…”

“…However, the impact on population persistence may be equally significant, but less obvious and more difficult to assess, when the pathogen has a long-term presence in the population [3]. Overall, the impact of wildlife diseases is highly variable and most often depends on a complex interplay between multiple host, pathogen and environmental factors [4].…”

Lability in Host Defenses: Terrestrial Frogs Die from Chytridiomycosis under Enzootic Conditions

“…In other species with enzootic infections, the absence of negative effects of Bd in population growth rates, survival, and recruitment may indicate the evolution of tolerance (Muths et al, 2011;Tobler et al, 2012). However, this is not the case for E. coqui because populations are still declining, Bd infection hinders frog survival and juveniles are more susceptible than adults .…”