This month, the conservation spotlight falls on a megaherbivore and its diet.
Where does it live?
The Green Turtle ( Chelonia mydas) is a sea turtle that lives in the waters of the Caribbean. Its name comes from the colour of its body fat, resulting from the adults’ diet of seagrass and algae.
What’s the problem?
A seagrass from the Red Sea, introduced to the eastern Caribbean in 2002, has already spread halfway across the region. Halophila stipulacea is replacing the native seagrass, which provides food and shelter for many other organisms. Between 2011 and 2017, Halophila spread from six to 20 per cent of our monitoring sites on Bonaire Island, while native seagrass declined by 33 per cent.
Why is that bad news for turtles?
Halophila is far less nutritious than the native seagrasses, and is largely ignored by grazing green turtles. They’ll only eat it if there’s nothing else available, and then they have to put more effort into grazing to meet their nutritional needs. This leads to slower growth rates and delayed sexual maturity, which is significant in this slowdeveloping species.
Turtles are making matters worse?
Yes. By preferentially grazing on the native seagrasses, the turtles are giving Halophila a competitive edge and helping it spread to new areas. The invasion is happening faster in grazed regions.
Can it be stopped?
Halophila is mainly a problem in tropical waters. In the Mediterranean, it cannot compete with the native seagrasses adapted to cooler conditions. We can’t do much to stop it spreading. But we can create conditions in which the native seagrasses flourish by addressing local environmental problems such as pollution, dredging and water clarity.
Can the turtles adapt?
The turtles may be able to switch diets from seagrass to algae, although that’s not straightforward because they carry a specialist cast of gut microbes that help them digest seagrass in particular. Our concern is that this invasion is happening faster than they can adapt. S Blackman
DR MARJOLIJN JA CHRISTIANEN
is assistant professor at Wageningen University in the Netherlands.
FIND OUT MORE
Journal of Ecology: https://besjournals.onlinelibrary.wiley. com/doi/10.1111/1365-2745.13021
The green turtle’s native seagrass diet has given a non-native seagrass a competitive edge.