Science Update: Another Chapter in Understanding Coyote Predation on White-Tailed Deer

Source: Newfoundland and Labrador Department of Environment and Conservation

So here’s the deal with this post. I don’t believe that science is detached from the social, cultural, and political implications of the knowledge it produces; however, these posts are intended to specifically focus on recent updates in scientific knowledge concerning species that hunters might be interested in. In an effort to keep these posts focused and concise, therefore, this post is a two-parter. The research paper I’m talking about here relates to a hotly debated and highly emotive issue, so I felt a bit compelled to also address the social and political aspects of the issue in a companion post. Onwards to the science though.

This post is seasonally timely with the Ontario white-tailed deer bowhunt opener on October 1. In a paper published in August 2016, researchers from the Ecological Research Center in Newton, Georgia reported on a study that began in 2003, to investigate the effects of mesopredators on white-tailed deer (Odocoileus virginianus) recruitment. The paper, titled Predator Exclusion As a Management Option for Increasing White-Tailed Deer Recruitment, reports on a study that used fenced exclosures to provide refuge for deer from predators to assess whether exclosures could be used as a management tool to reduce pressure on white-tailed deer from coyote (Canis latrans) and other mesopredator predation, particularly in cases where hunting coyotes is ineffective.

The researchers compared the neonate/adult female (fawn/doe) ratio in deer that used the exclosures to those that did not use the exclosures. Using neonate/adult female ratios gives an indication of recruitment – how many new individuals are brought into the breeding population. Since fawns are most vulnerable to predation, this ratio is generally a good indicator for population trends (i.e. if neonate survival is low, individuals are not surviving to reproduce, and combined with other mortality, this may lead to a decline in the population).

Exclosures are fenced areas that are just the opposite of enclosures: rather than containing a species, they are designed to prevent certain species from entering the fenced area. In this study, the researchers designed the exclosures with electric wire fencing so that deer were still able to jump the fence into the area, but coyotes, foxes (Vulpes spp.), and other predators were unable to get through the fence.

Grazing exclosure. Source: The Konza-Kruger Experiment

The effects of coyotes on white-tailed deer is one of the most hotly debated barstool topics among hunters. There are those who swear we should kill every coyote we see and there are those who insist that no matter how many coyotes we kill, we are only removing the transient individuals and having negligible effects on the actual local population. There is also the idea that coyotes will change their reproductive rate and behaviour in response to population changes, increasing litter size as populations decline. Then there is also a polarization of opinions on whether coyotes even present any real threat to white-tailed deer populations.

However, this paper notes that the massive population and range expansion of coyotes in North America throughout the last century has exposed white-tailed deer to increasing predation pressure that has affected neonate recruitment. Further, while there is evidence that removal of coyotes has had positive effects on white-tailed deer recruitment, the evidence also suggests that these effects are quite localized and difficult for most landowners to implement effectively. Coyote populations usually span a much larger area than individual hunters or landowners can affect, which means that individual animals that are killed will be replaced by others from the same population. In other words, coyote removal would need to occur over such large areas that for practical purposes at the property level, killing coyotes is generally not an effective tool to increase white-tailed deer recruitment.

The study found that the average neonate/adult female ratio was greater inside the exclosures than outside. Average neonate/adult female ratios within exclosures was 0.19 and outside exclosures this ratio was 0.09. To put this in more direct terms, this means that within exclosures, roughly speaking, there was 1 fawn for every 5 does; outside exclosures, there was 1 fawn for every 10 does. Hunter success also improved after the construction of exclosure areas. The study doesn’t report harvest statistics, so it’s difficult to know if hunters were killing more mature individuals, which would indicate that adult deer were also surviving longer.

In terms of management lessons, this study indicates that reducing predation pressure on white-tailed deer can increase neonate recruitment into the population. However, it’s notable that the effects of predators doesn’t always take the form of increased mortality. Decreased risk of predation can also result in behavioural changes in deer. Wildlife species are constantly balancing risk and reward – for example, how much energy will it take to locate a food source, and will the energy taken in by that food be greater than what was expended to find it. In terms of predation, prey species will seek refuge from predators, so predation risk can affect both foraging and reproductive behaviour (for a notable example of this, revisit the behavioural effects wolves had on elk in Yellowstone Park).

If increasing local deer population is a management goal, there needs to be an increase in neonate survival. In many cases, lethal control of coyotes is ineffective. In these cases, the authors suggest that predator exclosures may provide a viable option to manage predation risk by reducing “the need for energetically costly antipredator behaviors (e.g., increased vigilance while foraging) by providing year-round reduction in predation risk”. If exclosures provide more effective refuge from predators, it is likely that deer will use these areas to escape predation.

While this research is interesting, I can’t help but find larger questions to ask. For instance, while I have great respect and gratitude for the successes of the North American model of wildlife conservation, I still wonder: are we losing some essence of what it means to have wild life on these landscapes by managing them with the use of fences? At the end of the day, we need to find the best way to manage wildlife for the health of a wide variety of species and habitats; however, there’s a subjective element to this that I can’t escape, which makes me feel a bit uneasy about putting fences in wild places (even if those are on private properties). Then again, I can’t very well suggest that we continue to use potentially ineffective management strategies like lethal control of predators simply because it somehow feel more natural, can I? So uncertainties remain.

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