Evolution of a primary consumer in response to low and high food availability shapes life history traits and population demography

To test the hypotheses that individuals of rotifers adapted to high food levels are smaller at maturity than those adapted to low food levels, and that selection history has different effects on different life history traits and demographic variables of rotifers, we expose replicate populations of Brachionus angularis in low (LF) and high (HF) densities of Tetradesmus obliquus. Over 100 days (28 asexual generations) of selection, HF-selected populations have a significantly increased growth rate, but more rapidly decreased mictic ratio than LF-selected populations. Common garden experiments reveal that regardless of the density of food provided, populations with an LF-selection history have significantly lower reproductive and growth rates, but larger individuals than populations with an HF-selection history. Compared with LF treatment, HF treatment decreases reproductive and growth rates and individual size in populations with an LF-selection history, but these reaction norms are less pronounced (reproductive and growth rates) or opposite (individual size) in populations with an HF-selection history, indicating significant evolutionary effects. While the potential for evolution in rotifer populations may enable them to survive in oligotrophic and eutrophic water bodies, trait variation linked to food availability may lead to contrasting population dynamics in these two environment types.