Green sea turtle digging its own watery grave due to invasion of non-native seagrass

Since the opening of the Suez Canal in 1892, the seagrass Halophila stipulacea, once isolated in the Red Sea, began making its way into the Mediterranean. That did not cause any problems for existing populations of seagrass in those areas. However, the exotic seagrass reached the eastern Caribbean by ship in 2002, where it turned out to be dominant over the original seagrass, the native Thalassia testudinum. This has allowed the non-native species to form extensive meadows on the sea floor.

The green sea turtle Chelonia mydas swims in these same tropical waters. For millions of years, it has been gracefully grazing in the fields of the native seagrass species. This large grazer primarily forages in the deeper areas of the seagrass meadows. However, this began to change in 2002. That was the year that this exotic seagrass invader made its entrance from the Red Sea. This foreign seagrass spread quickly across the eastern Caribbean and formed thick meadows in the foraging areas of green sea turtles.

In 2010, the new species found its way to Lac Bay on the island of Bonaire, in the Dutch Caribbean, where Marjolijn Christianen of Wageningen University & Research designed a study into the role of sea turtles in the distribution of this exotic seagrass species. With colleagues from the University of Groningen, the University of Amsterdam, Sea Turtle Conservation Bonaire, STINAPA Bonaire and Wageningen Marine Research, she wants to research how these large grazers influence the expansion of exotic plant species in aquatic systems.

She conducted field observations, experiments in underwater cages, and gathered satellite images of grazing areas from the last 40 years in order to compare the size and boundaries of the seagrass fields with their current condition. In a time lapse of satellite images, she was able to see how the underwater terrain on which the sea turtles graze has been slowly increasing by 72 per cent (up to 65 hectares). The sharp boundary between grazed and ungrazed seagrass fields was shifting towards shallower areas with native seagrass species which previously (1970-2010) had not been grazed.

Food preferences of sea turtles

Marjolijn Christianen also outfitted the green sea turtles with GPS trackers, so that she could follow their eating path underwater. These observations confirmed the idea that the turtles are largely dedicated to the newly grazed areas, confirming that changes to the underwater landscape were driven by the turtles. She also conducted experiments involving food preferences of the turtles, because, if the invasive species were equally appealing as their established food source, the invasion might have fewer consequences. However, laboratory measurements indicated that the exotic seagrass is 2.5 times less nutritious for sea turtles, not to mention that these turn up their noses. In food preference experiments (nicknamed cafeteria experiments by the researchers), they ate the new seagrass species only one out of ten times, compared to nine out of ten for their traditional food source.

However, what was even more problematic for the researchers is that they observed the exotic seagrass spreading more quickly in grazed areas in comparison to ungrazed areas. Ultimately, in the six years between 2011 and 2017, H. stipulacea underwent an expansion of six to 20% of the permanent monitoring locations. During the same period, coverage with the native seagrass Thalassia testudinum dropped by 33%.

“These results provide the first proof of the large-scale replacement of the native seagrass by the rapidly proliferating H. stipulacea in the Caribbean as well as a mechanistic explanation for the invasiveness”, says Marjolijn Christianen. “This shows how large herbivores play a significant but previously unrecognised role in the expansion of exotic plant species in aquatic ecosystems.”

Water purification helps the sea turtle

Is there nothing that can be done? According to the researchers, it would beneficial for sea regions if they were less polluted by sewage or dredging activity, as this decreases the vitality of the old seagrass fields. “We are seeing that sea turtles are facilitating the expansion of the invasive seagrass through their grazing behaviour. If you want to protect these charismatic grazers, stress factors have to be reduced throughout the ecosystem. Therefore, water purification and removing factors that generate murky water in seagrass fields will help. For the dry Caribbean islands, this would partially be addressed by counteracting overgrazing by goats on the islands, as they eat everything until the ground is bare which encourages erosion.

Read the full article (freely available for a limited time):
Christianen MJA, Smulders FOH, Engel MS, et al. Megaherbivores may impact expansion of invasive seagrass in the Caribbean. J Ecol. DOI: 10.1111/1365-2745.13021

Media contact:
Marjolijn Christianen, Wageningen University & Research, Tel: +31 06 43 004 710, Email: marjolijn.christianen@wur.nl