Discrete host mortality
Increasing predator density triggered overcompensation in total
cercariae output from infected hosts for both algae (Fig. 1) and
detritus (Fig. 2) resource scenarios. For algae, compared to no
predators (Fig. 1A), intermediate predator density generated peak
parasite outputs (Fig. 1B) before a successful predator threshold
eliminated infected hosts and parasites (Fig. 1C). Increasing predator
density promoted higher parasite yields until an intermediate density of
10 N m–2, after which predator densities
> 13 N m–2 successfully eliminated
infected hosts and parasites (Fig 1D). Similarly, for detritus, mean
cumulative parasite output peaked at 7 N m–2 before
successful predator densities of > 14 N
m–2 (Fig. 2).
Low (1 N m–2) to moderate (10 N
m–2) predator densities consistently failed to
suppress parasite output from all host size classes for both algae and
detritus (Fig. 3). Here, targeting small- to medium-sized hosts between
0 and 15 mm was unsuccessful in mitigating parasite emergence
irrespective of predator stocking density. That is, targeting infected
juvenile and adult hosts <15 mm failed to control cercariae
emergence for both algae and detritus. In contrast, targeting large
adult hosts >15 mm successfully eliminated parasites for
stocking predators >10 N m–2,
demonstrating a minimum threshold needed to control parasite output.
Combined, this unimodal backfire response of increasing control pressure
prior to reaching a sufficiently intense measure illustrates the risk of
moderate intervention.