Evolutionary Consequences of Nonselective Harvesting in Density-Dependent Populations

There is now considerable empirical evidence that evolutionary changes in many phenotypic characters, such as body mass, age at maturation, and timing of breeding, often occur in populations subject to intense harvesting over longer periods. Here, we analyze the evolutionary component of the selection due to nonselective harvesting, which will operate even under selective harvesting and may generate a large evolutionary response. If phenotype affects susceptibility to density dependence-for example, through resource limitation-then nonselective harvesting can induce evolutionary change through its effect on population density. We provide a model for evolution of a quantitative character in such a fluctuating density-dependent population, using the diffusion approximation to describe jointly the temporal changes in mean phenotype and log population size. We show how nonselective harvesting in particular generates r-selection governed by genetic variation in the strength of density regulation and the magnitude of population fluctuations. We show that r-selection caused by nonselective harvesting is proportional to the mean fraction of the population harvested. We then compare the short-term as well as the long-term evolutionary impact of nonselective harvesting for different harvesting strategies by using the mean harvest fraction for different strategies. This comparison is performed for three different harvesting strategies: constant, proportional, and threshold harvesting. The more ecologically sustainable strategies also produce smaller evolutionary changes.