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Evolutionary traps for the sea turtle
11 August 2021
A few decades ago there was no such thing as plastic pollution and climate change. These phenomena appeared suddenly in evolutionary terms. Animal species are not able to adjust to the new circumstances on time and run the danger of becoming extinct.
If the environment of a species changes too fast, biologists refer to an evolutionary trap. Sea turtles face several of these traps. Two were already known and now a third is looming, as newly published research into plastic pollution and young sea turtles shows.
For thousands of years, the Mexican sea turtle had nested on the Mexican coast where very few people lived and live. Climate change is pushing their breeding grounds north. The heavily populated beaches are now seriously threatening the species. Read more in this publication.
CAUGHT IN NETS
A second evolutionary trap is that turtles seek out floating objects to hide beneath and find food. This behaviour gets their flippers caught in the mesh of (abandoned) fishing nets, and they die a slow death. In other words, the turtles have not yet learned in an evolutionary sense that ghost nets are dangerous and driftwood and seaweed are not.
YOUNG TURTLES LIVE IN THE MIDDLE OF THE OCEAN
Sea turtles bury their eggs on sandy beaches. After the baby turtles creep out of their shells, they head to sea. The juveniles live in the open sea for a few years and disperse across the ocean. Only when they are adults do they seek shallow waters. The young turtles enter areas where there is a relatively high amount of floating plastic. The plastic in the stomachs of five species of young sea turtles caught as by-catch in the middle of the ocean was now analysed. This led to the hypothesis of a new evolutionary trap.
HIGH LEVELS OF INGESTION OF PLASTIC
Of the young green sea turtles, 85% had plastic in their stomachs. The most extreme case was one animal that had 343 pieces larger than one millimetre. Of other species, more than 80% had swallowed plastic. In the Pacific Ocean, most of the ingested plastic was hard pieces, while in the Indian Ocean, plastic fibres were the most common. The ingested plastic consisted mostly of polyethylene and polypropylene that are often used for packaging.
The researchers explain the high rate of plastic ingestion among juveniles compared to adult turtles in shallower waters as being caused by the juveniles growing up in badly polluted parts of the ocean. They instinctively go to areas that were previously not polluted. What this means is that the species has not yet learned to avoid these areas that pose dangers to them.
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