Invasive seagrass species disrupts green turtle grazing in the Caribbean

Green turtle, Bonaire. Image: Dan Hershman.
Biodiversity

Recently published research from Bonaire indicates that green turtle grazing patterns are contributing to the spread of an invasive seagrass species, which in turn reduces the turtles’ grazing areas:

A seagrass species from the Red Sea is outcompeting the native seagrass species in the Caribbean, where the green sea turtle lives. These iconic turtles are seeing their grazing areas decline, because they have little interest in the foreign seagrass. Wageningen researchers and colleagues from other research institutions discovered how these large underwater grazers seem to dig their watery grave with their own eating behaviour. The Journal of Ecology for this week reports on the topic.

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.

Marjolijn Christianen and her colleagues conducted experiments involving the 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 the sea turtle, not to mention that the green sea turtle turns its nose up at the new food. In food preference experiments (nicknamed cafeteria experiments by the researchers), they only ate the new seagrass species 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 twenty per cent of the permanent monitoring locations. During the same period, coverage with the native seagrass Thalassia testudinum dropped by 33 per cent.

“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.”

Read more at Phys.org, as well as in the original article (open access) from the Journal of Ecology: Megaherbivores may impact expansion of invasive seagrass in the Caribbean.

[Image: Dan Hershman]

 

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