In any ecosystem, predators have the effect of reducing the numbers of prey by killing them for food. On the other hand, prey are fine-tuned to detect, avoid and escape predators, but this comes with costs; it takes time, effort and energy to avoid being a predator’s next meal. As a result, there is a delicate and complex balance between the populations of predator and prey species in an ecosystem.
Ecologists are familiar with counting the numbers of prey caught and killed by predators as a way of determining the impact of the predators on the prey population. However, a new study by scientists from the University of Western Ontario shows that the fear of predators on its own can also cause populations of prey to decrease, and this negative impact has never before been measured.
Professor Liana Zanette, her PhD student Marek Allen, and Michael Clinchy, all from Western University’s Department of Biology, monitored the population of free-living, wild song sparrows over three seasons and found that the fear of being attacked by a predator can halve the population of sparrows in five years or less. Avoidance of predation includes being cautious and vigilant, which can so impair parental ability to feed and care for offspring that fewer than half as many young reach adulthood.
“These results have critically important implications for conservation, wildlife management and public policy” explains Zanette, a renowned wildlife ecologist. “The total ecosystem benefits gained from conserving or rewilding native predators, and the full devastation wrought by introduced predators, must all now be re-evaluated.”
Zanette and her colleagues tested the impact of fear itself on the population growth rate of song sparrows by playing recordings of predator and non-predator vocalizations during three annual breeding seasons. They monitored the effects of these manipulations on song sparrow births and survival throughout each year, as well as collecting evidence indicating impacts on population growth beyond the parental generation.
Their findings, published today in the Proceedings of the National Academy of Sciences (PNAS), showed the significant impact that fear can have on population growth. In the presence of predator vocalizations, sparrows spent time watching out for predators, which prevented them from finding sufficient food for themselves and their young. This resulted in fewer young being born and fewer surviving each developmental stage on the way to adulthood, when compared with the data collected from parent sparrows that heard non-predator vocalizations.
The impact of fear even extended to subsequent generations, the study found. Some offspring that did reach adulthood showed evidence of impaired brain development that would be likely to decrease their chances of survival as adults. This represents a transgenerational impact of predation that could result in reduced population growth over several generations.
The study conclusively demonstrates that ecologists should not only focus on the number of prey killed by predators when assessing the impact of predators on prey populations. Although this is the conventional approach to studying predator-prey relationships, failing to consider the effects of fear itself could lead to significant underestimations of the total impact that predators have on prey population sizes. This could have disastrous consequences when reintroducing predators into wild areas from which they have been absent.
“Fear effects on prey population growth rates are likely the norm in birds and mammals because parental care is a fundamental characteristic of most birds and all mammals, and fear-induced reductions in parental investment and care are commonplace,” contends Zanette. “Having now demonstrated that fear itself can contribute significantly to the total impact predators have on prey populations, we expect this will be found to be true in most ecosystems.”