Warming and grazing independently and interactively impact plant defenses and palatability

The ecological impacts of multiple stressors are hard to predict but
important to understand. When multiple stressors influence foundation
species, the effects can cascade throughout the ecosystem. Gulf of Mexico
seagrass ecosystems are currently experiencing a suite of novel stressors,
including warmer water temperatures and increased herbivory due to
tropicalization and conservation efforts. We investigated the impact of
warming temperatures and grazing history on plant performance, morphology,
and palatability by integrating a mesocosm study using the seagrass
Thalassia testudinum with feeding trials using the sea urchin Lytechinus
variegatus. Warming temperatures negatively impacted T. testudinum
tolerance traits, reducing belowground biomass by 34%, productivity by
74%, shoot density by 10%, and the number of leaves per plant by 24%, and
negatively impacted resistance traits through 13% lower toughness of young
leaves and a trend for reduced leaf carbon:nitrogen. Lytechinus variegatus
individuals preferred to consume plants grown under heated conditions,
which supports findings of enhanced palatability. Simulated turtle grazing
impacted more plant traits than grazing by other herbivores, potentially
diminishing plant resilience to future disturbances through reduced
rhizome non-structural carbohydrate concentrations and increasing
palatability through reduced fiber content and 23% lower leaf
carbon:phosphorus. Simulated turtle, simulated parrotfish, and urchin
grazing reduced leaf carbon:nitrogen by 11%, also potentially increasing
nutritive value. Interactions between warming temperatures and grazers on
plant traits were additive for 16 out of 19 response variables. However,
the stressors non-additively impacted the number of leaves per plant,
fiber content, and epiphyte load. We suggest that the impacts of grazers
on leaf turnover rate and leaf age may vary based on water temperature,
potentially driving these interactions. Overall, increased temperatures
and grazing pressure will likely reduce seagrass resilience, structure,
and biomass, potentially impacting feedback systems and producing negative
consequences for seagrass cover, associated species, and ecosystem
services.