Herbivores can benefit both plants and their pathogens through selective herbivory on diseased tissue

Infectious disease can be a key driver of community structure,
particularly when pathogens affect foundation species. Seagrasses are
foundation species that form meadows along coasts worldwide, controlling
sediment deposition and biogeochemical cycling while supporting a diverse
community of fish and invertebrates. These plants are affected by wasting
diseases that cause necrotic tissue lesions. These lesions could alter
seagrass value as food, habitat, and mediators of ecosystem processes,
though biotic influences on disease dynamics are still not well
understood. We explored the role of one eelgrass herbivore in affecting
the development of wasting disease. We measured the severity and
prevalence of eelgrass wasting disease in a meadow across two summers
through repeated field surveys. We assessed the role of a common eelgrass
herbivore in affecting disease spread and growth using microcosm and
mesocosm experiments. We further explored herbivore preference in a choice
feeding trial, which was paired with chemical analysis of plant tissue and
analyzed using a structural equation model. While this herbivore
facilitates the growth of new lesions among isolated leaves, on balance
they reduce lesion severity by more than 50% in comparison to no-herbivore
controls in field-realistic settings. This was likely because this
herbivore strongly prefers to eat diseased rather than healthy tissue,
consuming nearly twice as much lesion area in choice trials. This
preference results from pathogen-driven changes in the host plant;
lesioned tissue requires less force to penetrate than non-lesioned tissue.
Additionally, as lesions increase in size, their polyphenolic
concentrations drop, which further increases the magnitude of preference
for lesioned tissue. Synthesis. These results suggest that these
herbivores could help maintain disease in this system at a high prevalence
(by facilitating disease development) but low severity (through
preferential consumption), which is consistent with our field observations
of nearly 100% prevalence and low severity in a natural bed where
herbivore density is high.   Understanding such
multi-species interactions in marine systems will advance our predictions
of future disease states beyond current understanding, which focuses
primarily on the influence of environmental change on pathogen outbreaks.