Beneath the clear, shallow waters of the Lakshadweep, lie large, uninterrupted stretches of bare sand. Although it is almost impossible to tell right now, these vast strands are underwater deserts. Fifteen years ago, they looked rather different. Lush meadows of long-lived seagrass carpeted the lagoon, giving shelter and food to a vast array marine life, including several important fish species that artisanal fishers relied on, crabs, octopuses, sea cucumbers, urchins and turtles. Over the years, these meadows have lost most of their flowering plants, and with them, the habitats they provided. This trajectory of decline from healthy, structured seagrass ecosystems to swathes of sand, a new 15-year-long study finds, has been caused by intense grazing by endangered green sea turtles.
The green turtle is one of conservation’s rare runaway success stories, and it shows that if we try hard enough, it is possible to bring species back from the brink. Decades of collective efforts by people and organisations all over the world to protect nesting beaches, raise hatcheries, reduce bycatch, and understand sea turtle migration and behaviour have helped green sea turtle populations recover smartly. While these efforts are proving to be conservation successes in many pockets—with turtles numbers growing in some parts of the tropics from Bermuda and Mexico to Australia, Borneo and Mayotte—they also bring along with them a unique set of dilemmas.
Researchers of the study tracked five lagoons in the Lakshadweep islands with extensive seagrass meadows to monitor annually—Kavaratti, Kalpeni, Kadmat, Agatti, and Bangaram. They found that green turtle populations moved sequentially between atolls across the archipelago in large aggregations—turtles would persist in a meadow for a few years, consuming almost all the seagrass it had to offer. Once they had depleted resources there, the aggregation regrouped on another meadow in the archipelago.
In 2010 when I first heard about turtle overgrazing, I thought it was a phenomenon only from the Agatti lagoon. But as years passed, we started observing overgrazing in other lagoons as well. Turtles would find lagoons with good seagrass cover, aggregate in large numbers, graze out all the seagrass and then move somewhere else, leaving behind a lagoon of bare sand.Mayuresh Gangal, lead author
Often, when we measure the success of species conservation , we think in terms of their numbers. Conserving species, however, goes beyond bringing numbers back to healthy levels. It is equally important to understand how the species interacts with its ecosystem. The delicate balance of an ecosystem is maintained by various species interacting with each other and the environment they are a part of. Some species, like the green sea turtle, are what scientists like to call ‘ecosystem engineers’. These species modify their environments in significant ways—they have a strong control over the structure and functioning of entire ecosystems. They can create, change and even destroy habitats.
Green sea turtles eat a lot of seagrass, and with their populations increasing, there has been a rise in seagrass overgrazing in turtle feeding grounds in Mayotte island, Bermuda, Akumal in Mexico, Borneo, and in the Lakshadweep islands as well—sometimes even leading to the collapse of entire intricate ecosystems.
Seagrasses have had a long evolutionary history of grazing, and are able to cope when sea turtles graze in moderation. In fact, turtles along with other herbivores can help keep seagrass meadows healthy by controlling the epiphytes grow on seagrass leaves and compete with the seagrass for light. A moderate amount of grazing may actually promote more productive growth. And just as turtles keep seagrass under control—trimmed and well-distributed across the seabed—large predatory sharks (and the fear of sharks) keep sea turtle numbers in check.
However, with the growing success of collective efforts to protect sea turtles around the world, combined with the rampant overfishing of sharks (one of the primary predators of sea turtles), there has been an increase in concentrated feeding by green sea turtles, and seagrasses aren’t being able to keep up with this.
As turtles increase in number and start consuming massive quantities of plants in a short time, seagrass meadows soon shift from tall verdant stands of long-lived species to ragged patches of short-lived species. As resources decline, turtles are compelled to take extreme measures. Green sea turtles have been observed to dig up rhizomes and roots with their front flippers once most of the leaves have been consumed. This can be devastating for the meadow because it urges along a process of desertification, giving little chance to these plants to recover and grow again. Within a few years of intense grazing, what remains of a once healthy meadow is a few remnant patches that turtles have not yet gotten to. These patches do not provide enough shelter and food for the large number of species that call the seagrass their home, and faced with declining resources, they either move away or die.
The study found that fish, especially species that were commercially valuable to local artisanal fisherfolk on the island, almost vanished from the meadows after the turtles had overgrazed and left.
The change we saw in seagrass fish communities was quite dramatic. Many of the important food fish species like thumb-print emperors, rabbitfish and goatfish either declined or completely disappeared from the meadow. The first ones to notice this decline were the fisherfolk, and they laid the blame squarely on the green turtles. Even more worrying, seagrass meadows are normally critical nurseries for many reef fish. With the meadows gone, the numbers of juvenile fish that recruited to the lagoon also dramatically reduced. We don’t know if this has flow-on consequences for the reef, but I wouldn’t be surprised if it does.Rohan Arthur, co-author
In addition to supporting biodiversity and artisanal fisheries on the islands, seagrasses store large amounts of carbon. As seagrasses and other creatures that live in these meadows die and decay, they get buried in the sediment in the form of carbon. In the oxygen-poor environment of seabeds, carbon decomposes far slower than it does on land and can remain buried undersea for hundreds of years. When we lose the ecosystems created by these plants, we also lose these historic stores of carbon.
The turtles had cleared all the blades of seagrass and had now started digging out and consuming the roots and rhizomes. Within a few years, most of the organic carbon deposited over time and held in the sediment by the network of seagrass roots, was lost.Erika D'souza, co-author
Green sea turtles have transformed meadows across the Lakshadweep islands from ones that were covered in kilometres of seagrass and teeming with marine life to bare sand in just 15 years. With most of the seagrass in meadows gone, turtles are no longer found in huge aggregations and are now diffused across the archipelago. Even the few green turtles that continue to persist, researchers of the study say, are likely to keep the meadows from recovering, and there is little chance for them to grow back to the seagrass-dense, structured, biodiversity-rich meadows of the past.
The first thing you would think of when you see these declining meadows is that—surely, there must be a way to restore them. But the truth is that active restoration of seagrasses is a virtually impossible task. We would never be able to restore meadows at large enough scales to be ecologically meaningful. Given this, perhaps the best possible option is to support the natural recovery of seagrass ecosystems. Even in completely grazed meadows, there are tiny patches that remain that the turtles have overlooked. We call these little patches ‘insurance sites’, and our hope is they will keep seagrass species from going locally extinct in each atoll. If we start by protecting these tiny patches, they could be areas from which natural seagrass recovery could take place once green turtle numbers decline.Al Badush, co-author
We don’t know enough about the how seagrasses and green sea turtles interact with each other in the Lakshadweep. Are most of the turtles from the large feeding congregations leaving Lakshadweep to find new feeding grounds elsewhere after depleting the seagrass here? Are they dying because there isn’t enough seagrass to eat? Are such events—where turtles overgraze and wipe out entire seagrass ecosystems—a part of a larger natural cycle that we aren’t aware of?
Green turtles in seagrass meadows are like elephants in a forest. In large enough numbers, they can completely transform the ecosystems they inhabit. Any sound conservation of these “ecosystem engineers” should be done carefully, with a clear understanding of their impacts on the habitat. In extreme cases, like in Lakshadweep, they can transform seagrass meadows into underwater deserts.Teresa Alcoverro, co-author
The study emphasises that conservation needs to go beyond the direct protection of endangered species. We need to think beyond bringing back endangered species from the brink and the successes that come along with these interventions.
Read the study here.
Gangal, M., Gafoor, A., D'Souza, E., Kelkar, N., Karkarey, R., Marbà, N., Arthur, R. and Alcoverro, T. (2021). Sequential overgrazing by green turtles causes archipelago-wide functional extinctions of seagrass meadows. Biological Conservation, 260, 109195. https://doi.org/10.1016/j.biocon.2021.109195.
Illustration: Rohan Arthur