Ecological theory about the dynamics of interacting populations is mainly based on unstructured models that account for changes in species abundances only and that represent species interactions by the positive or negative effect of one population on the population growth rate of another. These models hence ignore unique aspects of biological organisms, in particular individual life history and energetics and within-population variation due to individual development. Nonetheless, these models constitute the basis for our understanding of the functioning of ecological communities and ecosystems and their responses to environmental change, natural disturbances and human impacts. Structured models that take into account differences between individuals in age, stage or size have been shown to make predictions that can run counter to the predictions of unstructured analogues. In this presentation I address the question which biological mechanisms and processes give rise to these qualitatively different model predictions between structured and unstructured models.
Results/Conclusions
The combination of (i) demographic differences between juveniles and adults and (ii) the energy requirements to keep a living body alive (somatic maintenance costs) is sufficient to overturn the ecological rules-of-thumb derived from unstructured population models. Together, these two ubiquitous biological mechanisms result in positive relationships between (stage-specific) densities and mortality and thus to changes in population composition/structure with mortality. In turn, these mortality-induced changes in population structure fundamentally change our predictions about community structure and its response to perturbations. For example, classic theory predicts that when two species compete for a shared resource and are exposed to a shared predator, the inferior competitor will go extinct if it is also more sensitive to predation. In contrast, if basic maintenance costs are accounted for and the juvenile maturation rate of the inferior competitor is more resource-limited than its reproduction rate, an inferior competitor that is also more sensitive to predation will drive its opponent to extinction in the presence of the shared predator, despite its double handicap.