Loss of an apex predator in the wild induces physiological changes in prey

Predators can impact prey via predation or risk effects, which can
initiate trophic cascades. Given widespread population declines of apex
predators, understanding and predicting the associated ecological
consequences is a priority. When predation risk is relatively
unpredictable or uncontrollable by prey, the loss of predators is
hypothesized to release prey from stress; however, there are few tests of
this hypothesis in the wild. A well-studied predator-prey system between
white sharks (Carcharodon carcharias) and Cape fur seals (Arctocephalus
pusillus pusillus) in False Bay, South Africa has previously demonstrated
elevated fecal glucocorticoid metabolite concentrations (fGCM) in seals
exposed to high levels of predation risk from white sharks. A recent
decline and disappearance of white sharks from the system has coincided
with a pronounced decrease in seal fGCM concentrations. Seals have
concurrently been rafting farther from shore and over deeper water, a
behavior that would have previously rendered them vulnerable to attack.
These results show rapid physiological and behavioral responses by seals
to release from predation stress. To our knowledge, this represents the
first demonstration in the wild of physiological changes in prey from
predator decline, and such responses are likely to increase given the
scale and pace of apex predator declines globally.