Effects of size selection versus density dependence on life histories: A first experimental probe

When prey experience size-based harvesting by predators, they are not only subject to selection due to larger individuals being preferentially harvested but also selection due to reductions in population density. Density-dependent selection represents one of the most basic interactions between ecology and evolution. Yet, the reduction in density associated with exploitation has not been tested as a possible driving force of observed evolutionary changes in populations harvested size-dependently. Using an artificial selection experiment with a mixture of Daphnia clones, we partition the evolutionary effects of size-based harvesting into the effects of removing large individuals and the effects of lowering the population density. We show that both size selection and density-dependent selection are significant drivers of life-history evolution. Importantly, these drivers affected different life-history traits with size-selective harvesting selecting for slower juvenile growth rates and a larger size at maturity, and low-density selecting for reduced reproductive output.