Stage-specific Biomass Compensation by Juveniles in Response to Increased Adult Mortality

Population size is often expected to decrease with increasing mortality. However, besides removing individuals from a population, mortality releases surviving individuals from competition, which may lead to enhanced growth, maturation or reproduction. Recently developed theoretical models of stage-structured consumer-resource systems show that stage-specific biomass compensation can arise in response to increased mortality. We parameterized a stage-structured population model to simulate the effects of increased adult mortality caused by a pathogen outbreak in the perch (Perca fluviatilis) population of Windermere (UK) in 1976. The model predicts compensatory biomass production of juveniles in response to high adult mortality due to a food-dependent increase in reproduction rate. This prediction is matched by our analysis of a 60-year time series of scientific monitoring of Windermere perch, which shows that the pathogen outbreak induced a strong increase in juvenile biomass while adult biomass decreased considerably. Age-specific adult fecundity was significantly higher after than before the disease outbreak. Taken together, these results suggest that the high pathogen-induced mortality released adult perch from competition, which increased fecundity and thus the production of juvenile biomass. By increasing juvenile biomass, the pathogen most likely promoted the predator pike (Esox lucius) that specializes on small perch, thereby altering community size-structure and dynamics. The results have general implications for predicting population and community level responses to increased size-selective mortality, for instance caused by human harvesting, biological invasions or disease outbreaks.