Do predators create healthier prey populations?

This blog post is provided by Ellen E. Brandell and tells the #StoryBehindThePaper for the paper “Examination of the interaction between age-specific predation and chronic disease in the Greater Yellowstone Ecosystem”, which was recently published in the Journal of Animal Ecology.
Photo credit: Yellowstone Wolf Project, National Park Service

The debate about if and how predators kill their prey to result in “healthier” prey populations has been ongoing for the past three decades, but current wildlife disease issues have shone an even brighter light on it. Chronic wasting disease (CWD) is a fatal infection of deer species (deer, elk, moose) that is spreading geographically and increasing in prevalence across North America. CWD has no cure and animals are infectious long before they show symptoms or die – this is what makes it so hard for wildlife agencies to control. Some claim that the best solution for controlling CWD already exists in nature: predators. They claim that predators, like gray wolves and cougars, have heightened senses that allow them to detect and kill infected animals, thereby removing infection and preventing transmission in prey populations. This is supported by studies demonstrating that predators, especially predators that chase down their prey, select for the easiest individuals to kill, which are often the very young, very old, sick, or injured. However, other studies have shown that the “cleansing effect” of predators is modulated by other factors like prey behavior; for example, the presence of predators may cause prey to group together for protection, and within groups, high contact rates can lead to more disease transmission than would occur if there were no or fewer predators.

Age is a defining feature of both prey selection by predators and infectious disease patterns. In the case of chronic wasting disease, middle-aged deer tend to have higher prevalence than other ages because the young have not been exposed yet and the old have already succumbed to the disease. Similarly, predation has an age signature; many predators prefer to kill very young prey because they are highly vulnerable. For this reason, we suspected that the ability of predators to remove infected prey and thereby create healthier prey populations depends on the extent to which both predation and disease occur in the same ages.

The Yellowstone Ecosystem is an exciting area to study this because there is a rich predator community and CWD has just started to infect elk and deer, so there is much to be learnt about how predators, prey, and disease interact here. However, CWD is a very slow disease and it will be many years to decades before we can measure the impacts in the field. In the interim, we used data collected from predator studies in Yellowstone to develop a model of potential scenarios. More specifically, we built a mathematical model based on CWD, predators (wolves, cougars), and prey (elk, mule deer) in the Yellowstone Ecosystem to examine predator-prey-disease relationships. Our model, although based on this system, was generic and meant to explore the relationship between age-specific predation and disease. We built in realistic ecological components including prey age and sex, increasing CWD transmission rates as the infection progressed, a predator population that fluctuated with prey abundance, and predator selection of prey based on their age and infection severity. This work is now published in the Journal of Animal Ecology

Wolves hunting an elk calf in Yellowstone National Park. (Photo credit: Yellowstone Wolf Project, National Park Service)

Results from our model suggest that predators can reduce CWD prevalence in a prey population under realistic conditions. Predators are more effective at removing CWD when they select for prey ages with the most infections, which in this case is younger adults. However, this comes at a cost because younger adults are also responsible for population growth via reproduction and high survival rates when uninfected. Typically, wolves prefer to kill juvenile or old prey and cougars prefer juvenile prey – therefore, these predators must switch their preferred prey age class to effectively remove CWD. This could happen if CWD makes deer and elk more vulnerable to predation, which has been observed in certain settings. We demonstrate that prey selection by age and infection severity is very important when considering predator-prey-disease relationships, and predator cleansing effects can be complex. The utility of models like this is a deeper understanding of complex ecological relationships, but models require simplifying certain processes as well and should be interpreted with some caution.

Our objective was to better understand the interaction between disease and predation that is characterized by prey age. Our work does not show that predators will reduce disease burdens in prey populations for all types of infectious diseases. And in fact, there are many areas where CWD-infected deer and elk coexist with cougars, coyotes, and black bears. However, wolves and CWD-infected prey are just starting to overlap in the Yellowstone Ecosystem, Great Lakes region, and soon in Colorado. Our model demonstrates the possibility of predator cleansing effects, yet whether or not wolves will actually have a detectable effect on CWD in these areas will likely take decades of data collection to determine. For example, the rate at which predators select infected individuals compared to healthy individuals is a key element that requires many years of data collection to estimate. Protected areas like national parks will be critical to study because wolves and other predators occur at relatively higher densities than managed lands, and our results show that at least moderate predator abundance and kill rates are required to reduce disease prevalence in prey. Given more data and stronger estimates for our model, we can move towards predicting predator-prey-disease outcomes rather than identifying a range of potential scenarios.

Read the paper

Read the full paper here: Brandell, E.E., Cross, P.C., Smith, D.W., Rogers, W., Galloway, N.L., MacNulty, D., Stahler, D.R., Treanor, J. and Hudson, P.J. (2022), Examination of the interaction between age-specific predation and chronic disease in the Greater Yellowstone Ecosystem. J Anim Ecol. Accepted Author Manuscript.

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