Community consequences of overcompensation in stage-specific biomass due to size-selective mortality: Emergent Allee effects and emergent facilitation

The main body of theory concerning population dynamics and community structure is based on population models that only account for reproductive processes and for mortality, either from predation or from other causes. A major process that is not considered by these classical approaches is the fact that most if not all individual organisms develop during their life history and often grow significantly in body size during this development. In addition, this growth in body size is often food-dependent. If both reproduction as well as development depends on food-density, one of these two processes may turn out to limit population growth more than the other. As a consequence, a population in equilibrium can be regulated in two distinct ways: either through total population reproduction as limiting process or through total population maturation. We will show that in case of reproduction regulation the population is dominated by adult individuals and that any increase in mortality may lead to an increase in juvenile biomass. Similarly, in case of maturation regulation the population is dominated by juvenile individuals and increases in mortality may increase adult biomass. This overcompensation in biomass occurs with increases in both random and stage-specific mortality, even when the latter targets the stage exhibiting overcompensation. Such biomass overcompensation in prey species may have important community consequences, as it may give rise to emergent Allee effects for stage-specific predator species or to facilitation between two stage-specific predator species if these predators exploit different stages of prey.