Effect of seagrass nutrient content and relative abundance on the foraging behavior of green turtles in the face of a marine plant invasion.
Invasions may succeed because they are not controlled by natural enemies in a novel environment (enemy release hypothesis [ERH]). If local consumers preferentially feed on native species, they can facilitate invasions, while preferential or even non-selective feeding on an invader could minimize its impact. Halophila stipulacea, a seagrass native to the western Indian Ocean, has spread to the Mediterranean and Caribbean. As locally abundant large-bodied grazers, green turtles Chelonia mydas could either facilitate or attenuate the H. stipulacea invasion depending on their foraging patterns. We tested a priori predictions about green turtle foraging behavior in the presence of H. stipulacea and native seagrasses off the west coast of Guadeloupe, French West Indies, to investigate the ERH. Using video-assisted individual focal follows (n=45), we assessed individual- and population-level foraging preferences. Turtles foraged across a wide range of microhabitats including monospecific seagrass meadows and a range of compositions within mixed-species meadows. Green turtles showed clear preference for native Syringodium filiforme over the invasive seagrass. Both H. stipulacea and macroalgae were generally avoided. However, individual turtle foraging preferences varied with ca. 12% of individuals foraging on H. stipulacea at rates above those predicted by availability. No differences in nutrient content were observed between the native and invasive seagrass, suggesting that other factors can drive forage selection. The preference shown by most green turtles in Guadeloupe for native over invasive seagrass suggests green turtle foraging likely facilitates the H. stipulacea invasion at this stage of its spread, regardless of relative abundance and nutritional value.