Resource Competition Induces Heterogeneity and Beneficial Population Selection

We studied the effect of food competition on heterogeneity in growth and survival capacity. In a two-stage experimental protocol, four groups of juvenile guppies (Poecilia reticulata) were first treated with differing amounts of food and foraging competition for 21 days and then challenged with increased water temperature and starvation. Fitting the survival patterns obtained in the challenge stage with a vitality model that characterizes population survival capacity, we were able to infer the effect of treatment-stage growth and competition on the evolution of heterogeneity in population vitality. The model-derived group means and coefficient of variations in vitality obtained from challenge-stage survival curves were linearly related to the means and coefficient of variations of body mass measured in the groups at the end of the treatment stage. Our results demonstrate that foraging competition increases population heterogeneity in both mass and survival capacity (vitality). We suggest that under limited resources, competition-induced heterogeneity lowers selective pressure on high-quality individuals.  Uses of challenge studies coupled with vitality modeling are discussed in the context of examining dynamics of population selection.