Species → Green Sea Turtle
Green Sea Turtle
Chelonia mydas
What if it disappeared?
Impact HighWithout green turtles in abundance, a seagrass meadow would keep a different structure from the one their grazing imposes today where turtles concentrate (Lakshadweep, India) — canopy height, shoot length and density, reduced flowering — an effect that can also shift the site's species composition.
DocumentedDetail
Lal, A., Arthur, R., Marbà, N., Lill, A.W.T. & Alcoverro, T. (2010). Implications of conserving an ecosystem modifier: Increasing green turtle (Chelonia mydas) densities substantially alters seagrass meadows. Biological Conservation, 143(11), 2730–2738.
The study draws on a lagoon in the Lakshadweep Islands (Indian Ocean) where a stable, high-density turtle aggregation serves as a natural laboratory for observing what happens as green turtle populations recover elsewhere in the world.
Across the five atolls monitored since 2005, every high-density turtle aggregation eventually exhausted the seagrass meadow within 2 to 6 years, down to bare sand — a scenario that repeated atoll after atoll, and had wiped out every large seagrass meadow in the archipelago by 2019.
DocumentedDetail
Gangal, M. et al. (2021). Sequential overgrazing by green turtles causes archipelago-wide functional extinctions of seagrass meadows. Biological Conservation.
This long-term monitoring complements and qualifies the first point: green turtle grazing isn't only a positive force for seagrass. Without a limit on turtle density or site fidelity, repeated grazing at the same spot can lead to complete collapse rather than a simple change in structure.
The recovery of green turtles could keep improving seagrass health elsewhere in the world — but mainly where sharks, their main predators, aren't themselves being decimated by overfishing: it's predation pressure that has historically limited turtle density and site fidelity, and its absence could reproduce the collapse scenario observed at Lakshadweep elsewhere.
ExtrapolatedDetail
Heithaus, M.R., Alcoverro, T. et al. (2014). Seagrasses in the age of sea turtle conservation and shark overfishing. Frontiers in Marine Science, 1, 28.
The authors bring together experimental data from several ocean basins showing that turtles' negative impact on seagrass is dampened where shark populations remain intact — the mere presence of predation risk changes turtle grazing behavior (a "landscape of fear" comparable to the wolf's in Yellowstone). But this is a synthesis and a forward-looking caution, not a site-by-site confirmation at global scale.
What's missing to confirm it: Comparative monitoring between lagoons where shark populations remain intact and lagoons where they're overfished, as turtle populations keep growing worldwide, would confirm whether the Lakshadweep pattern generalizes or remains a regional case.
Documentation
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Oceanic Society — Global Conservation Efforts Lift Green Turtle from Endangered Status (2025)
Accessed August 7, 2026
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Lal et al. (2010) — Implications of conserving an ecosystem modifier, Biological Conservation
Accessed August 7, 2026
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Gangal et al. (2021) — Sequential overgrazing by green turtles causes archipelago-wide functional extinctions of seagrass meadows, Biological Conservation
Accessed August 7, 2026
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Heithaus et al. (2014) — Seagrasses in the age of sea turtle conservation and shark overfishing, Frontiers in Marine Science
Accessed August 7, 2026
Last verified: August 7, 2026