Grazer host density mediates the ability of parasites to protect foundational plants from overgrazing

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.