Life-history Evolution in Harvested Populations: The Role of Natural Predation

Models and experiments of the evolution of ageand/ or size-at-maturation in response to population harvesting have consistently shown that selective harvesting of older and larger individuals can cause earlier maturation. These predictions, however, are all based on single-species considerations and thus crucially neglect the selective forces caused or mediated by species interactions. Here we develop simple models of phenotypic evolution of age-at-first-reproduction in a prey population subject to different types of predation and harvesting. We show that in the presence of natural predation, the potential evolutionary response of age-at-first-reproduction to population harvesting is ambiguous: harvesting can cause either earlier or later maturation depending on the type of predator interaction and its strength relative to the fishing pressure. The counterintuitive consequences of harvesting result from the indirect effects that harvesting of a prey population has on the selection pressure exerted by its natural predator, since this selection pressure itself typically depends on prey density. If harvest rates are high, the direct selection pressures considered in classical analyses prevail and harvesting decreases the age-at-first-reproduction, whereas at lower harvest rates the indirect, interspecifically mediated effects of harvesting can qualitatively overturn predictions based on simpler single-species models.