Professor Emeritus Lakehead University Thunder Bay, Ontario, Canada
Background/Question/Methods
Predation risk and its attendant fear are often invoked as major determinants of foraging and related behaviors, the dynamics of prey populations, and the structure of ecological communities. Evidence in favor of its importance in the dynamics of wild populations is mostly circumstantial, and studies of competitive interactions yield similar fear responses. Regardless of cause, adaptive responses to risk will depend on, and interact with, the density and spatial distribution of prey populations. Tests of that interdependence require experiments that can manipulate the benefits and costs of foraging in risky versus safe environments. I did so by creating food only, shelter only, food with shelter, and no-food-no-shelter treatments in an enclosed field population of meadow voles in Northern Ontario, Canada. I created safe sheltered areas in the otherwise low-food and risky habitat by covering the ground with straw, added (and later eliminated) rabbit chow to supplement the voles’ food requirements, assessed their risk response with giving-up densities (GUDs) in safe versus risky foraging patches, and confirmed their habitat choice by live-trapping all voles at the end of the experiment.
Results/Conclusions
GUDs were lower in safe patches than in risky ones (confirms that the voles foraged under risk). The difference in GUDs between paired patches was lower in the food with shelter treatment, and the GUDs higher, than in all others (voles foraged less when safe alternative foods were available). GUDs declined significantly when supplemental feeding stopped (they foraged more when there were fewer opportunities to obtain alternative foods). Voles’ assessment of risk increased through time during the food-addition phase, but declined during time when food was absent (it takes time to fully assess risk). Each of these results is consistent with theories of foraging under risk, but not necessarily only through the threat of predation. But by the end of the experiment the voles exhibited peculiar sex-dependent habitat selection. Pairs of males occupied sites with no cover; pairs of females occupied sites with cover. It is thus apparent that the foraging responses to risk are not due solely to predator-induced fear, but that they also interface with other adaptive behaviors such as habitat selection. Ecologists’ interpretations about the fear of predation need to change. We have much more to learn about trait-dependent indirect effects than we might otherwise have thought.
