Sea turtles have foraged in the same habitats for more than three millennia

Environment and sustainability 12. nov 2023 4 min Assistant Professor Alberto Taurozzi Written by Kristian Sjøgren

Mediterranean green turtles have been foraging in the same habitats for at least 3,000 years. This is the conclusion of a new study in which bioarchaeologists, biologists, zoologists and marine biologists collaborated in characterising what sea turtles have been eating in the eastern Mediterranean since the Egyptians built the pyramids. The researchers say that protecting these critical coastal habitats is vital because otherwise the turtles may run out of foraging habitats and become extinct.

Endangered Mediterranean green turtles have been using the same foraging habitats for millennia.

This is the conclusion of a major new study, in which researchers used archaeological findings from ancient times, modern molecular biological techniques and satellite tracking data to map where the sea turtles have foraged from the era of the pharaohs to the era of Jesus Christ and up to the present.

The research shows that some North African seagrass meadows have supported green turtle populations for at least 3,000 years and probably longer. If these meadows disappear because of climate change or other human activities, this could spell the end of the Mediterranean green turtle.

The research thereby points to a key area for nature conservancy if future sea turtle populations are to be preserved in Europe.

“Our research shows that the green turtle has probably not only had the same foraging habitats but also the same migration routes for millennia. If seagrass meadows disappear or if humans reduce the area of the seagrass meadows in other ways, this will strongly affect the green turtles, and they may even disappear. Therefore, protecting these areas and restoring those that have been lost are absolutely essential,” explains Alberto Taurozzi, Assistant Professor, Section for Geogenetics, Globe Institute, University of Copenhagen.

The study, whose first author is Willemien de Kock, PhD Fellow at the University of Groningen, the Netherlands, has been published in the Proceedings of the National Academy of Sciences of the United States of America.

Ancient bone fragments show what turtles ate three millennia ago

The researchers combined considerable data from various sources and initially examined turtle bone fragments from archaeological sites in Turkey and Lebanon.

Between 2,700 and 4,700 years ago, people living in this part of the eastern Mediterranean often ate sea turtles, and their bones therefore turn up in archaeological excavations.

The researchers studied 124 ancient sea turtle bone fragments for two reasons. Firstly, they could determine the species based on studies of proteins in collagen in the bones.

The researchers found that the turtle bone fragments came from two species of sea turtles: the green turtle, which is a distinctive herbivore, and the loggerhead turtle, which is omnivorous and eats plants, seagrass and invertebrates such as jellyfish.

Secondly, the researchers could also determine what the turtles had eaten through isotopic analysis of the collagen in the bones. Crucial to the study is that the isotopic signature of an organism not only depends on what it eats but also where it eats. Plants incorporate isotopes from the local environment, which are then transferred to the herbivores that feed on them and further up the food chain.

“Isotope analysis is critical in determining what and where turtles eat. This confirmed that the green turtle also ate seagrass 3,000 years ago. But we also quickly realised that this was not just a story about what sea turtles had eaten in ancient times but also how it compared with today,” says Alberto Taurozzi.

Foraging in the same habitats for millennia

In another study, which provided data used by the researchers behind the new study, scientists investigated the foraging habits of today’s green turtles using satellite tracking. The researchers had been following green turtles with GPS for years and could therefore determine very precisely where the sea turtles forage and which migratory routes they use.

This modern analysis shows that the sea turtles’ foraging habitats are located off the coast of North Africa, where there are large seagrass meadows.

The researchers behind this second study also collected samples of skin from living green turtles and analysed the isotopes in them.

This enabled researchers in the new study to compare these isotope values with those obtained from the ancient sea turtle samples. Interestingly, they found that the isotopic signatures were similar.

According to Alberto Taurozzi, this indicates that ancient green turtles also foraged in exactly the same habitats as their contemporary descendants.

“So things have not changed, and sea turtles used to forage for seagrass in exactly the same small habitats in which today’s turtles forage,” he explains, adding that it was not possible in the same way to determine exactly what non-herbivorous turtles ate in the past, since these not only eat seagrass but also other animals, and since prey animals move and feed in different locations, the isotopic signature cannot easily be used to geolocate the areas in which the turtles foraged.

Protecting the foraging habitats is important

Alberto Taurozzi says that the new study is important because it indicates critically important factors that determine whether the sea turtle will survive.

Today, many places worldwide try to protect sea turtles from extinction by protecting the beaches on which they lay their eggs.

However, sea turtles spend only a small part of their lives on the beach and most of it at sea, where the seagrass meadows they require to survive have existed for millennia but are not protected.

Alberto Taurozzi is therefore convinced that the key to protecting the sea turtles is protecting their seagrass meadow habitats.

“For more than 3,000 years, sea turtles’ foraging habits have been so consistent that they have foraged in the same place as long as they could. Therefore, our study clearly indicates that protecting the green turtle requires protecting their habitats,” he says.

He also thinks that the study has opened an avenue for using interdisciplinary research methods that can shed light on the factors that affect the survival of other endangered species that have lived among us for millennia but that humans have pushed very close to the brink of extinction.

“We can connect habitats and birth locations with migration routes and foraging habits in the past and present, thereby showing what influences the survival of each individual species and what has affected its survival in the past. Only by protecting all aspects that affect the survival of a species can we ensure that they will be here in the future. It’s not simply a question of having intentions to protect endangered marine species; the factors that are critical for their protection need to be identified,” concludes Alberto Taurozzi.

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