Recovery of a cultivation grazer: A mechanism for compensatory growth of Thalassia testudinum in a Caribbean seagrass meadow grazed by green turtles

Abstract

Abstract Recovery of green turtles (Chelonia mydas), mega‐herbivores that consume seagrasses, is resulting in dramatic ecosystem‐wide changes as meadows are returned to a natural grazed state. The green turtle grazing strategy, with long‐term cultivation of meadows and high foraging site fidelity, is distinct from other terrestrial and aquatic mega‐herbivores and may affect seagrass compensatory growth responses. Identifying the mechanisms of compensatory growth responses to grazing is essential to understand the functioning of plant systems under natural grazing regimes. In a naturally grazed Caribbean seagrass ecosystem, we identify a mechanism for compensatory growth responses to grazing by evaluating relationships between Thalassia testudinum morphology and growth, grazing intensity, and canopy light dynamics in grazed and ungrazed areas. The morphological characteristics that explain variability in T. testudinum growth differed between grazed and ungrazed areas. In grazed areas, T. testudinum leaf linear growth, leaf area growth, and productivity:biomass (P:B) significantly increased as above‐ground biomass decreased; P:B also increased with shoot density. Mass growth in grazed areas exhibited an increasing trend with shoot density and was maintained above a threshold of 2.5 g dry mass m−2 above‐ground biomass. In ungrazed areas, trends for mass growth and P:B with above‐ground biomass and shoot density were opposite to those in grazed areas. In grazed areas, shoot density significantly increased with grazing intensity while above‐ground biomass decreased and leaf area index (LAI) was not affected. Light availability at canopy height was greater in grazed areas than in ungrazed areas, and canopy light attenuation increased with shoot density in grazed areas. Synthesis. Grazing removes above‐ground biomass, which increases light availability and stimulates leaf growth and turnover (i.e. compensatory growth). Shoot density increases with grazing intensity, maintaining LAI and canopy light harvesting potential. This maximizes the potential for leaf photosynthetic activity and provides the plant with the capacity to sustain mass growth and support a compensatory growth response to grazing. This study presents novel insight for assessing the underlying mechanisms of plant compensatory growth responses to cultivation grazing and proposes potential thresholds that may be used to evaluate the sustainability of in situ grazing pressure by a recovering mega‐herbivore.