The direct effects that predators have on prey are obvious – predators kill and consume the prey they catch, and that prey individual’s chances to reproduce and pass on its genes are then over. Through these direct effects, predators have substantial effects on prey species demography and evolution. But for every prey animal a predator catches and kills, there are many that escape and live – perhaps they managed to outrun the predator, or simply hid successfully and managed to evade detection. What might be the long-term effects of these highly stressful encounters with predators? And could the stress of predator encounters provide an alternate, indirect route to prey mortality?
These were questions I set out to answer in my newly published article, “Fear and lethality in snowshoe hares: the deadly effects of non-consumptive predation risk”. This study is a good example of finding interesting results when you look at old data in a new way: Michael collected these data as part of his PhD thesis, in an experiment designed to test maternal predator exposure effects on reproductive output in snowshoe hares. As part of this experiment, which he conducted under the supervision of our co-authors Rudy Boonstra and Charley Krebs, captured pregnant wild snowshoe hares were exposed every other day to a predator that was not able to catch them (a trained dog), a treatment which stopped as soon as the hares gave birth. When I joined Michael’s lab as a postdoc to work on a project investigating maternal stress effects on offspring in lizards, we started to think about potential effects of this kind of predator exposure on adult mortality through the likely physiological stress it causes, and together revisited this data and developed new questions to test using it.
In this article we show that simply being exposed to a predator every other day resulted in death for six of the twenty experimental female hares, while hares in the control group (that were not exposed to a predator) suffered no mortalities at all. Even more interesting, when the surviving fourteen females that were exposed to the dog gave birth, their offspring were less likely to survive until weaning (when they stop suckling from their mother at around 4 weeks of age) than the offspring of females in the control group.
To sum up, predator exposure of pregnant adult females reduced their own survival, but also the survival of their offspring, which were never themselves exposed to predators. So, encounters with predators affected survival even when the predator could not kill the prey, and these effects passed on to the next generation too. The effect on group size was striking: the control group all reproduced successfully, and grew from 11 adults to a final group size of 30 adults and offspring. Meanwhile, the predator-exposed group, which was almost twice as large to begin with, dwindled from 20 adult females, to a final group size of 16 adults and offspring.
Our results suggest that just the perception of predation risk can lead to increased mortality – and that while the direct effects of predation are obvious, we should also be thinking about the less obvious indirect effects that predators can have on their prey.