Linking cumulative density effects across ontogeny with population growth
The effects of density on survival and reproduction, and in turn population growth, have been studied for decades. However, density effects can change over ontogeny with physiology and environmental conditions, making the overall effect of density on population growth difficult to predict. In plants, positive density dependence (facilitation) is more likely in the juvenile stage, when abiotic stress tolerance is low. In contrast, negative density dependence (competition) is more likely in the adult stage as individuals accumulate biomass and deplete resources. While ontogenetic shifts in density dependence are known, their long-term effects on population growth and extinction are unclear. We develop and use a novel model to explore how ontogenetic switches in density dependence regulate populations. Specifically, we test whether ontogenetic shifts from positive to negative density dependence can stabilize populations and reduce extinction risk in the face of environmental stochasticity.
Simulations reveal the ontogenetic balance of density dependence indeed can regulate population growth. Weak positive density effects on seedling survival can prevent extinction when adults experience negative density effects. Feedbacks between juvenile and adult stages disrupt population stability and magnify the contribution of density-dependent processes to population growth. In contrast to our predictions, environment-induced correlations between density-dependent and -independent processes increase susceptibility to extinction. Overall, our results reveal that the cumulative effects of density may be important but underappreciated regulators of population size.