Examination of the interaction between age‐specific predation and chronic disease in the Greater Yellowstone Ecosystem

Abstract

Predators may create healthier prey populations by selectively removing diseased individuals. Predators typically prefer some ages of prey over others, which may, or may not, align with those prey ages that are most likely to be diseased. The interaction of age‐specific infection and predation has not been previously explored and likely has sizable effects on disease dynamics. We hypothesize that predator cleansing effects will be greater when the disease and predation occur in the same prey age groups. We examine the predator cleansing effect using a model where both vulnerability to predators and pathogen prevalence vary with age. We tailor this model to chronic wasting disease (CWD) in mule deer and elk populations in the Greater Yellowstone Ecosystem, with empirical data from Yellowstone grey wolves and cougars. Model results suggest that under moderate, yet realistic, predation pressure from cougars and wolves independently, predators may decrease CWD outbreak size substantially and delay the accumulation of symptomatic deer and elk. The magnitude of this effect is driven by the ability of predators to selectively remove late‐stage CWD infections that are likely the most responsible for transmission, but this may not be the age class they typically select. Thus, predators that select for infected young adults over uninfected juveniles have a stronger cleansing effect, and these effects are strengthened when transmission rates increase with increasing prey morbidity. There are also trade‐offs from a management perspective—that is, increasing predator kill rates can result in opposing forces on prey abundance and CWD prevalence. Our modelling exploration shows that predators have the potential to reduce prevalence in prey populations when prey age and disease severity are considered, yet the strength of this effect is influenced by predators' selection for demography or body condition. Current CWD management focuses on increasing cervid hunting as the primary management tool, and our results suggest predators may also be a useful tool under certain conditions, but not necessarily without additional impacts on host abundance and demography. Protected areas with predator populations will play a large role in informing the debate over predator impacts on disease.

Modelling the role of predation on disease burdens of prey

“…Determining the relationship between predation and disease prevalence in prey species has the potential to inform monitoring and management of both predators and prey. For example, monitoring of the ratio of adult to juvenile predator mortality could be indicative of disease burdens in the population (Brandell et al, n.d.). An increase in predation rates of adults likely indicates a change in their vulnerability, potentially due to disease.…”

“…The GYE has a diverse predator population, is well studied providing empirical parameters for the model, and had CWD emerge in 2018 (Krumm et al, 2010; Miller et al, 2008; Smith et al, 2020). In this system, the novel model was used to determine the effect of predation by wolves and cougars on CWD prevalence in elk and deer given variation in both disease prevalence and predation with age (Brandell et al, n.d.). Unsurprisingly, the model indicated that predator cleansing effects were greatest when predators selected upon the age class with the most infections.…”

“…The ability to model when predators have a role in reducing disease prevalence establishes a clear framework for future empirical studies in large mammal systems (Brandell et al, n.d.). Much of the understanding of the predator cleansing effect has stemmed from aquatic systems, where elegant experiments coupled with field studies have revealed complexities and a lack of consistent patterns across species (Duffy et al, 2005; Lopez & Duffy, 2021).…”

“…However, the role this plays on the predator cleansing effect was largely unknown. To investigate this, Brandell et al (2022) developed models to explore how prey selection and disease distribution across age classes will influence predator-driven reductions in prevalence of Chronic Wasting Disease (CWD). CWD is an always fatal prion disease spreading in deer and elk populations across North America.…”

“…Thus, an increase in predator populations is something management agencies have the direct ability to influence. The model developed by Brandell et al (2022) indicates that increased kill rates, as a function of increased predator populations, reduces CWD prevalence; however, the authors caution that empirical data are needed as the model produces a range of results. Importantly, human tolerance for increased predators may be limited and disease reduction should only be considered among of suite of factors.…”