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.

Density and size-selection life-history data

Data in the two csv files were collected from individual Daphnia following the selection experiment. The column header abbreviations follow. ID (identification of an idividual), density (high (H) or low (L) density treated), selection (randomly culled (R) or size-selectively culled (S)), block (indicator of block), Am (age at maturity), max (estimated maximum size), growth (estimated growth rate measure), totegg_30 (total reproductive output up to day 30), size_mat (size at maturity), growth_i (size at first measurement), growth_f (size at third measurement), growth_rt (an alternate measure of growth, equal to mm grown per day), max_obs (max observed size).

ind.data.selection.csv

ind.data.alt.measures.csv

Variance within lines data

Average and standard errors for five life-history traits (age at maturity, size at maturity, estimated maximum size, days to half-maximum size, and total reproductive output up to day 30) for each of eight clonal lines.

lines.summary.csv

Density over time

Abundances from the microcosms for all treatments and blocks over the three-week experiment.

density.csv

R code for running analyses

R code Lyberger 2021.R