Competition for resources can increase the survival rate of a population

Background/Question/Methods

The effects of variability in mass on survival have remained elusive, despite the well demonstrated increased survival caused by increased mean size or mean growth in fish populations. In this study, we examined the effects of food competition on variability in body masses and vitality, which is an abstract measure of survival capacity. In a two-stage experimental protocol, four treatment groups (fractional factorial design) of juvenile Trinidadian guppies (Poecilia reticulata; N = 681) were first reared at differing levels of food availability (low, moderate or high) and degrees of foraging competition (presence or absence) for 21 days. They were then challenged with increased water temperature and an absence of food resources. We recorded the time to mortality for each individual during the challenge stage, and then used the vitality model to fit survival curves and vitality densities. Linear relationships were determined between mass, time to mortality, and vitality, as well as between variability in mass and heterogeneity in vitality among the four treatment groups. This experimental design helped to determine the effect of food availability on average mass and average time to mortality, and to examine how competition altered variability in mass and heterogeneity in vitality of the guppies. 

Results/Conclusions

The model-derived group means and coefficients of variation (CVs) in vitality obtained from the challenge-stage survival curves were linearly related to the means and CVs of body mass measured in the groups at the end of the treatment stage (all linear relationships tested: n = 4, p < 0.01). In the moderate growth groups, the “competition” group had significantly lower average mass (p < 0.001) and survival (p < 0.001) than the group without competition. Despite this impediment, the “competition” group had 45.5% of individuals with greater vitality at the onset of the challenge, and equivalent survival on day 12 of the challenge compared to the “no competition” group. Our results demonstrate that food resource competition increases population heterogeneity in both mass and vitality, and that heterogeneity can buffer against mortality. We suggest that under limited resources, high-quality individuals in a population can experience higher survival at a later age or life stage with greater competition-induced heterogeneity. Finally, we suggest that challenge studies and vitality modeling together provide a unique way to view and quantify natural selection processes.