Grazer host density mediates the ability of parasites to protect
foundational plants from overgrazing
Abstract
Like many top consumers, parasites can regulate feeding of their prey
via trait-mediated means. If parasites modify the feeding behavior of
ecologically important grazers, they may have cascading effects on the
structure and functioning of whole plant communities. The extent to
which parasites can influence plant communities in this way is largely
dependent on the strength of their behavioral alteration, their
prevalence in host grazers, and the density of those hosts. Recent
experiments and comparative surveys in southeastern USA salt marshes
revealed that common larval trematode parasites suppress the per capita
grazing impacts of the marsh periwinkle (Littoraria irrorata),
generating a trophic cascade that protects foundational marsh plants
from drought-associated overgrazing. Here, we conducted a field
manipulation wherein we modified grazer host density while holding
infection prevalence constant at an ecologically relevant level (20%)
to determine whether the indirect, facilitative effects of parasites on
marsh plants varied with the density of grazers. We found that parasites
had significant positive impacts on marsh net primary productivity at
moderate densities of snails (≥50 snails/ 0.5 m2), but that the positive
effects of parasites were negligible at lower densities. Our results
confirm the findings of previous studies that parasites can protect
marsh plants from overgrazing at sufficiently high prevalence, but show
that their ability to do so depends on host density.