An underwater Serengeti: Seagrass‐mediated effects on intake and cultivation grazing behavior of a marine megaherbivore

Abstract

Populations of green turtles (Chelonia mydas), a megaherbivore that consumes seagrasses via cultivation grazing, are recovering worldwide. Information on plant‐mediated effects on herbivore foraging behavior is critical to understanding plant–herbivore interactions and sustainability of grazing as ecosystems continue to change. In a Caribbean seagrass ecosystem, we use stationary cameras and benthic surveys to evaluate the effects of seagrass morphology and leaf nitrogen content on green turtle grazing behavior. Thalassia testudinum leaf morphology has significant effects on forage intake (in milligrams of dry mass [DM] per minute) for green turtles, whereas leaf nitrogen content has no effect. Intake increases in grazed areas with shorter leaves and higher leaf biomass concentration (in milligrams of DM per cubic centimeter), indicating more efficient foraging under these conditions. Bite rate (in bites per minute) increases in grazed areas with short leaves, a result of reduced search time. Bite size (in milligrams of DM per bite) increases in grazed areas with short but dense canopies, because a turtle crops more shoots with each bite. Increased foraging efficiency and reduced search time in grazed areas with high biomass concentrations collectively maximize intake. Ingested leaves are shorter than the mean height of all available leaves in grazed areas, indicating herbivore selection for shorter leaves. Our estimate for daily intake is 86.1 g DM day−1 33‐kg turtle−1. Our study provides a novel contribution on the effects of plant‐level cues on the grazing behavior of a marine megaherbivore, and how cultivation grazing behavior optimizes the green turtle foraging strategy by maximizing foraging efficiency and intake.

Warming and grazing independently and interactively impact plant defenses and palatability

“…grazing → increased leaf nutritional quality → increased grazing pressure). This phenomenon has been observed with the green turtle C. mydas , where C. mydas selectively consume new growth and discard older tissue, returning to the same grazing plots to re‐crop the blades (Bjorndal 1980, Gulick et al 2022). This grazing strategy increases seagrass growth, increases leaf nutrient and protein content, and minimizes consumption of lignin and epiphytic calcium carbonate (Bjorndal 1980, Zieman et al 1984, Moran and Bjorndal 2007).…”