Can variation in prey density drive changes in predatory selection in natural systems? For salmon and brown bears, yes
Curry J. Cunningham, Gregory T. Ruggerone, and Thomas P. Quinn
How does the availability of food affect the selectivity of the consumer? It has long been known that survival of prey can depend on their density, as predators become satiated or cannot catch them all. In addition, many studies have shown that predators are selective, tending to kill and consume some species or sizes of prey over others, but very few are examples of prey density driving patterns of selection by predators. Brown bears inhabiting coastal ecosystems of the Pacific Rim obtain much of their annual protein and fat supply by consuming the Pacific salmon that return annually to streams to spawn, and they tend to kill the larger salmon among those available to them. However, this is a world of feast and famine; the number of returning salmon can vary greatly from one year to the next.
Utilizing 20 years of data including individual 41,240 salmon measurements from a small tributary of Bristol Bay, Alaska, scientists at the University of Washington have found that the strength of predatory selection by bears is inversely related to the density of their sockeye salmon prey. That is, contrary to the expectation that when the salmon are abundant the bears can choose the largest fish, the bears were more selective, tending to kill especially large fish, when salmon were scarce. This finding is especially interesting and important because these salmon populations are also exposed to a commercial fishery that is itself selective, tending to catch the large fish. The fishery has, therefore, both the direct form of selection and also intensifies size-selective predation by bears through the reduction in salmon density.