Green turtles shape the seascape through grazing patch formation around habitat features: experimental evidence

Understanding how megaherbivores incorporate habitat features into their foraging behavior is key towards understanding how herbivores shape the surrounding landscape. While the role of habitat structure has been studied within the context of predator-prey dynamics and grazing behavior in terrestrial systems, there is a limited understanding how structure influences megaherbivore grazing in marine ecosystems. To investigate the response of megaherbivores (green turtles) to habitat features, we experimentally introduced structure at two spatial scales in a shallow seagrass meadow in The Bahamas. Turtle density increased 50-fold (to 311 turtles ha-1) in response to the structures, and turtles were mainly grazing and resting (low vigilance behavior). This resulted in a grazing patch exceeding the size of the experimental set-up (242 m2), with reduced seagrass shoot density and aboveground biomass. After structure removal, turtle density decreased and vigilance increased (more browsing and shorter surfacing times), while seagrass within the patch partly recovered. Even at a small scale (9 m2), artificial structures altered turtle grazing behavior, resulting in grazing patches in 60% of the plots. Our results demonstrate that marine megaherbivores select habitat features as foraging sites, likely as a predator refuge, resulting in heterogeneity in seagrass bed structure at the landscape scale.