Fear of predators – not just the number of prey that predators directly kill – can significantly reduce the number of individuals in prey populations, according to a new Western University study.
In fact, fear can reduce wildlife populations by 50 per cent in five years or less, according to the findings by Liana Zanette, Michael Clinchy, and PhD student Marek Allen from Western’s department of biology. Their study was published today in Proceedings of the National Academy of Sciences (PNAS).
Zanette and her team experimentally demonstrated for the first time in any free-living wild animal that the fear predators inspire can itself reduce prey population growth rates. Their findings conclusively establish that focusing solely on the number of prey predators directly kill while failing to additionally consider fear, as conventionally done, risks dramatically underestimating the total impact predators have on prey population size.
“These results have critically important implications for conservation, wildlife management and public policy,” said Zanette, a Western biology professor and 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.”
In an experiment, the team tested the impact of fear on the population growth rate over multiple generations in free-living wild song sparrows. Fear was manipulated during three annual breeding seasons using playbacks of predator or non-predator vocalizations. The effects on births and survival were comprehensively quantified throughout each year, together with evidence indicative of impacts on births and survival beyond the parental generation.
The researchers found that watching for predators kept parents from finding food for themselves and their young. This had cumulative, compounding adverse consequences, from fewer young being born, to fewer surviving each stage towards adulthood. Those reaching adulthood showed evidence of impaired brain development likely to shorten their survival during adulthood – representing a transgenerational impact reducing population growth over generations.
“Fear effects on prey population growth rates are probably 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,” explained 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.”