Infection pathology and competition mediate host biomass overcompensation from disease

Predators can increase the biomass of their prey, particularly when prey life stages differ in competitive ability and predation is stage specific. Akin to predators, parasites influence host population sizes and engage in stage‐structured interactions, yet whether parasites can increase host population biomass remains relatively unexplored. Using a stage‐structured consumer–resource model and a mesocosm experiment with snails and castrating trematodes, we examined responses of host biomass to changes in infection prevalence under variation in host pathology and resource competition. Equilibrium adult host biomass increased with infection prevalence in the model when parasites castrated hosts and adults were superior competitors to juveniles. Juvenile biomass increased with infection prevalence whether parasites caused mortality or castration, but only when juveniles were superior competitors. In mesocosms, increases in infection by castrating trematodes reduced snail egg production, juvenile abundance, and adult survival. At high competition, juvenile growth and total biomass increased with infection prevalence due to competitive release. At low competition, juvenile biomass decreased with infection due to reduced reproduction. These results highlight how disease‐induced biomass overcompensation depends on infection pathology, resource availability, and competitive interactions within and between host life stages. Considering such characteristics may benefit biocontrol efforts using parasites. Although natural enemies exert negative effects on individual victims, they can sometimes have counterintuitive positive effects on their resource's overall population abundance (i.e., “hydra effects”; Abrams 2009, Schr？der et al. 2014). For instance, an increase in population density can occur when mortality reduces the amplitude of population cycles, is temporally separated from density dependent growth, or decreases consumption rates leading to increased resource productivity (Abrams 2009). Increases in mortality can also increase biomass of specific life stages due to the relaxation of population growth bottlenecks in reproduction or maturation (i.e., “stage‐specific biomass overcompensation”; Persson and De Roos 2013). Positive effects of mortality have been associated with important ecological consequences, including the magnitude of trophic cascades (Cortez and Abrams 2016), patterns of energy flow in lakes (Jansson et al. 2007), and responses of fisheries to harvest (Reichstein et al. 2015). Positive mortality effects from species interactions have been