Scared to Death
Fear and lethality in snowshoe hares: the deadly effects of non-consumptive predation risk (2018) MacLeod et al., Oikos 127(3)
When we think of a predator-prey relationship, many colorful examples of charismatic animals come to mind: the lion and the wildebeest, the orca and the seal, the owl and the mouse. We think of these organisms locked in an endless battle, with one needing to catch and eat, the other to escape and live. While these are definitely interesting and important aspects of the predator-prey relationship, prey species need to worry about more than just being eaten. These “non-consumptive effects” play into what is called the Ecology of Fear.
This study was an attempt to show that the perceived risk of predation itself was enough to reduce survival in prey species. Unlike previous studies on this question, MacLeod et al. were the first to conclusively show this effect in mammals.
How it Works
The authors used pregnant female wild-caught snowshoe hares in isolated enclosures, so they were able to test the effect of predation risk on both adult and offspring survival. The enclosures, while only 4 x 4 meters, had cover and hiding places so that the hares would be able to show their natural responses of either freezing or running and hiding.
Hares in the predator group were exposed to a trained dog once every other day for a few minutes to simulate their mammalian predators (like a coyote or a lynx). The dog, though recognized as a threat, was trained not to bark, whine, chase, or touch the hares. Control hares, while not exposed to the predator, still experienced regular disturbance in the form of a researcher entering the cage to check for mortality, check for offspring, and change the food and water.
Once the rabbit birthed a litter of leverets the exposure to the dog stopped, though the researchers continued to check daily for mortality.
Did You Know: Ecology of Fear and Yellowstone
In the United States, the near-extinction of the gray wolf allowed many prey species, like the elk, to reproduce and spread to previously-uninhabited areas. The reason for this was not only due to the lack of the wolf, but the elk’s lost of fear of encountering a wolf.
With the reintroduction of the gray wolves to Yellowstone National Park in 1995, the elk and other prey species have learned to be afraid again. Because they now avoid high-risk areas, the vegetation that had previously been obliterated by the elk, like aspen shoots, is able to grow again.
What They Found Out
Hares exposed to a predator had a higher mortality rate than the control hares. Across the two years of the experiment, 100% of the control hares lived while only 70% of the hares exposed to a predator survived. In addition, control hares successfully weaned about eight times as many offspring per mother as the predator-exposed hares.
Some reviewers of this paper took issue with the statistics in this study, specifically the p-values. When analyzing data, scientists rely on things like a p-value to determine if the results from two treatments, like the predator-exposed hares and the control hares, are statistically different from one another, or just a result of chance. In this study, the mortality rates of the two groups, while different, were not “different enough”, according to a widely-used cutoff. However, another thing that scientists rely on to determine how “important” an outcome was is an effect size. The larger the effect size, the more the difference between the two groups had an effect. In this case, the exposure to predators had a fairly large effect on adult mortality. So while some scientists may take issue with the p-value, the larger effect size is a strong argument for the effect predators have.
This is the first time that mortality due to non-consumptive effects (mortality due to something other than being eaten by the predator) has been experimentally shown in mammals. While non-consumptive effects are not a new concept to ecologists, it is super interesting that they are not limited to the world of invertebrates (insects, spiders, worms, etc.) or birds.
What this study has shown is that simply being afraid of a predator is enough to reduce survival and final group size of a population. Armed with this knowledge, we may be able to better predict the effects of reintroducing predators to their native ranges and how that may affect prey populations. For example, the ecology of fear has already been shown to have an effect on elk in Yellowstone. If we are able to bring back wolves to the rest of the United States, they may be able to help with excessive deer populations.