Disturbances propagate through ecosystems, extending and magnifying their impacts. Propagation can occur through trophic links, as is the case for trophic cascades. The strength of a trophic cascade can determine how vulnerable or resilient an ecosystem is to change. Classically, trophic cascades are considered to occur in predator-prey-resource systems. Some predictors for the strength of these cascades have been established. Recent work has characterized many examples of trophic cascades in parasite-host-resource systems but predictors of the strength of these cascades have not been established. Some candidate predictors, susceptibility to infection and primary productivity, likely follow patterns analogous to those of capture rate and primary productivity in predator-driven cascades.
These patterns are analyzed with models that calculate the effect of parasites on equilibrium densities of hosts and their resources. The dependence of this cascade strength on the susceptibility and productivity parameters is compared to the dependence of predator-driven cascade strength on capture rate and productivity. These theoretical predictions are tested with a mesocosm experiment in which productivity, susceptibility to parasites, and the presence of parasites were varied.
Results/Conclusions
Modeling shows that more susceptible hosts show stronger parasite-driven trophic cascades, both in terms of decreased host density and increased resource density. This result is analogous to the effect of capture rate on predator cascade strength. The effect of productivity, however, is more complex. Increased productivity was found to increase resource response but host response was maximized at intermediate resources. This result differed from the effect of productivity on predator-driven cascades, which predicts higher prey and resource response at higher productivity.
Mesocosm experiments verified the predicted effect of susceptibility on cascade strength. More susceptible populations showed larger decreases in host density and increases in resource density caused by parasites. As predicted, productivity increased resource response to parasites. Too few productivity treatments were used to verify or falsify the predicted curve shape of host density response but the general pattern was consistent with that modeled for parasite-driven cascades.
This work establishes that the strength of parasite-driven trophic cascades shares some predictors with the classic, predator-driven cascades. Some patterns, however, differ qualitatively between parasite cascades and predator cascades. Further work is required to predict how disturbances will propagate through parasite-driven cascades and how this differs from or mirrors predator-driven cascades.