Award: OCE-1737144

Global climate change is altering the broader ecology of terrestrial and marine habitats. Some examples include the warming-induced range expansions of megaherbivores such as green turtles. In areas where they have become established, these turtles have dramatically increased the amount of seagrasses consumed and can alter the functioning of seagrass-dominated ecosystems. While prior work has often addressed the direct effects of climate on organismal functioning, fewer studies have considered how alterations in biotic interactions (via shifts in species ranges and food web dynamics) might obscure, or even amplify, the direct effects of climate change on ecological structure and function. The major goal of our project was to determine how herbivory by newly-arrived green turtles and existing herbivores interacts with nutrient supply to affect turtlegrass growth and abundance in St. Joseph Bay, FL, and in other areas experiencing large increases in green turtle numbers. To achieve this goal, we tested how herbivory at different intensities was influenced by increased nutrient enrichment, something occurring in many areas experiencing human population increases. We found that after 4 months high levels of herbivory reduced the number of leaves per turtlegrass shoot and after 7 months, high levels of herbivory resulted in reduced shoot densities. Additionally, after 7 months, caging effects were present (partial cages had lower seagrasss cover than control plots), which is likely a result of herbivores being attracted to structure, thereby increasing herbivory. Simulated grazing treatments also reduced turtlegrass percent cover, becoming more severe the higher the grazing intensity. Nutrient enrichment appeared to have mitigated some of the negative effect of herbivory on the size of turtlegeras leaves, but there were no important interactive effects between grazing and nutrient supply Combined with grazing by other species such as sea urchins, herbivory by the recently-arrived green turtles reduced turtlegrass growth and the physical structure of turtlegrass meadows. As megaherbivore like green turtles shift their distributions poleward in response to ocean warming, in combination with other resident herbivores, consumption by these large consumers may lead to overgrazing and dramatic reductions in the health of seagrass meadows. Last Modified: 11/08/2021 Submitted by: Kenneth L Heck