Ontogenetic diet shifters profit competitively from predation

When intraspecific individuals differ in resource intake, scramble competition occurs among inferior individuals growing food-dependently. Scramble can be released through predation mortality. As a consequence of this release, production rates in inferior individuals increase and biomass overcompensation in the subsequent life-stages may occur. When intraspecific individuals do not differ in their resource intakes biomass overcompensation does not occur. If an individual changes its resource intake over ontogeny, the balance of intake and losses, its energetics, will change over ontogeny. Furthermore, differences will arise between the energetics of different life-stages. The predominant volume of interspecific competition theory is based on studies assuming no stage-specific differences in energetics, neglecting the influence of ontogeny on community dynamics altogether. We study how an stage-specific differences in energetics affect expectations from conventional competition theory. We use a stage-structured biomass model consistently translating individual life history processes, in particular food-dependent growth in body size, to the population level. The stage-structured population can be reduced to an unstructured population, if the energetics of all individuals are assumed to be equal. The stage-structured model, however can also describe population dynamics when this equality is broken. We use the stage-structured biomass model to contrast the stage-specific differences resulting in a stage-structured population model, with an unstructured population model assuming no differences between stages. We show that stage-specific differences in energetics can affect competition on various trophic levels. I: In stead of outcompeting each other, a predator can be facilitated by another preying a scrambling prey life-stage of the same prey population. II: In coexistence with their prey, omnivores with an ontogenetic diet shift, where juvenile omnivores feed on resource and adults on prey, affect community structure only as predators, not as competitors to their prey. We show coexistence of omnivore and prey is not possible if the dominating interaction is competition. Feeding on prey, however, alleviates competition with prey and facilitates the introduction of omnivores. III: An ontogenetic diet shift creates niche partitioning, where without it this would result in neutral coexistence of two consumers competing for two resources. IV: Furthermore, predators can change resource requirements of diet shifters such that diet shifters can reduce resources to lower equilibria and sustain higher predator biomass than consumers without stage-specific differences in energetics.