The importance of beach vegetation

Female-biased sea turtle populations are reported at important sea turtle rookeries globally. This heightens concerns for the conservation of sea turtles in the long-term. For this reason researchers are measuring temperatures at nesting beaches around the world to better understand the male-to-female ratio that these rookeries are producing.

I recently collaborated on a research project looking at the incubation temperatures of turtle nests in the Chagos Archipelago (Western Indian Ocean) where both hawksbill and green turtles breed. In this study, we recorded sand temperatures on Diego Garcia, the largest island of the archipelago. Temperature loggers were placed at nest depths in the different areas where the hawksbill and green turtles nest. The results showed relatively cool temperatures. The beaches of Diego Garcia have several characteristics that make for these relatively cool nest temperatures. Firstly, the island, which is in the world’s largest marine protected area, has intact natural vegetation that provides heavy shade where some turtles nest. Together with heavy rainfall and narrow beach platforms, which require sea turtles to nest close to the sea, this provides for cool sand temperatures. Consequently, we expect that hatchling sex ratios at this site are currently fairly balanced, producing 53% and 63% male hatchlings for hawksbill and green turtles respectively. The results of this study were published in Scientific Reports this week.

Dr Jeanne A Mortimer, one of the authors of the article, has studied Western Indian Ocean sea turtles since 1981. She states that “our study helps us to better understand why different species of sea turtles choose the nesting sites that they do. Our results demonstrate that in order to produce offspring with a relatively balanced sex ratio, these hawksbill turtles need to lay their eggs amongst vegetation on the upper beach crest.  Hawksbills are relatively small sea turtles with an average nest depth of only about 30-50 cm (compared to the larger green turtles whose nest depths average some 70-85 cm). Green turtles often lay their eggs on the open beach platform. Our results tell us, however, that hawksbill nests constructed in an area of open sand are more likely to produce female-biased offspring, and in some cases might even be too warm to produce viable offspring.  This highlights the importance for habitat managers to maintain the natural vegetation on the beach crest in order to provide optimal nesting habitat for hawksbill turtles – a species currently listed by IUCN as a Critically Endangered species. Our research also helps to answer the very basic question asked by just about anyone who has watched hawksbill turtles nest: ‘How come they always seem to go up into the bushes to lay their eggs?’“

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(photos courtesy of Daniel Barker, Nicole Esteban, Kip Evans and Graeme Hays)

Male hatchling production in sea turtles from one of the world’s largest marine protected areas, the Chagos Archipelago” was published by Scientific Reports (2016). Authors: Nicole Esteban, Jacques-Olivier Laloë, Jeanne A. Mortimer, Antenor Guzman and Graeme C. Hays.