Sea Turtles and Climate Change

"Hot chicks, cool dudes," is the saying that helps us remember that warmer temperatures are likely to produce more females in a nest of reptile eggs. Anjani Ganase, marine scientist, considers the conservation of sea turtles in the face of changing climate. (First published in Tobago Newsday, January 18, 2018)

Although my career as a marine scientist is still in its infancy, I have managed to witness substantial declines in coral reef ecosystems over the past ten years. While surveying some of these reefs frequented by other megafauna, such as manta rays and turtles, it made me wonder how these long-lived ocean residents, dependent on these ecosystems, were dealing with the rapid changes that have been occurring over the last thirty years. Sea turtles in particular, are an ancient species living in the present-day marine world. They have been around for about 100 million years (humans have only been here for about 200,000 years), having survived the last mass extinction event, 65 million years ago, and have also been through significant temperature and sea level shifts (Hamann et al 2007; Hawkes et al 2009). With future predictions of a rapidly warming climate on top of man-made stressors such as habitat loss and pollution, there is concern that these long-lived creatures may not now adapt quickly enough (Hawkes et al 2009).
 
A curious loggerhead turtle approaches the camera during a survey of the far north Great Barrier Reef. Photo by Anjani Ganase
Most of what we can anticipate for turtles in the future (given our rapidly changing climate) is based on knowledge of the nesting routines of the turtles. However, less is known about their routines between nesting seasons when they are foraging or migrating across oceans. Foraging and migratory patterns of sea turtles vary considerably among species, as well as geographically. Leatherback turtles travel the open ocean often to colder waters, feeding mainly on jellyfish; while the green turtles stay within the tropics and forage on sea grass beds and coral reefs neighbouring their nesting beaches (Hawkes et al 2009). How the food sources and ecosystems on which these turtles rely will fare under the changing climate may be a useful indicator of the health and survival of turtle populations.

Warming temperatures

If you were born after 1985, you have only ever experienced a hotter than average planet (compared to the previous 100 years; IPCC 2013; Roon 2015). The same goes for sea turtles. However, sea turtles are far more sensitive to temperature. While our gender is determined by our chromosomes/genes, the sex of most reptiles - including marine turtles - is determined by the temperature the eggs are exposed to while they develop. Higher temperatures produce female marine turtles, while eggs in cooler temperatures produce males. Eggs only successfully develop within a narrow temperature range; and indeed, may not thrive at all if the temperature is too low or too high. Sea turtle nesting is therefore largely limited to beaches in the tropics and sub-tropics where the conditions are most suitable for success. Numerous studies have predicted shifts in the sex ratio of turtle hatchlings to more females due to the predicted increasing global temperatures. A recent study has revealed that a remote tropical location in the northern Great Barrier Reef (Raine Island and Moulter Cay) has already been producing mostly (99%) female hatchlings for the past two decades because of the above average temperatures (Jensen et al 2018). This shift towards predominantly female offspring brings concern over the number of males available to mate. Along the east coast of Australia, Green turtles nest in the summer typically at locations that have incubating temperatures between 25 – 33 degrees Celsius (Hamann et al 2007). Unfortunately, a further increase in temperature by more than 2 degrees (by 2050) may push the incubating temperatures of these nests beyond the survival limits of the turtle eggs (Hamann et al 2007). However, there are other factors that influence the incubation temperatures (including specific traits of the beach, such as its grain size, sand colour, ventilation, precipitation and shading of the nests) which can result in geographical variations. While some beaches such as those in the northern Great Barrier Reef may produce predominantly female offspring, this may not be true for all beaches.

With a warming climate, there is also a potential for shifts in the timing of the nesting season or to other locations, which may allow turtles to compensate for the observed warming effect. Such shifts have been observed in the past with specific sea turtle populations (Hamann et al 2007). Warmer waters may trigger females to begin nesting earlier in the season, or to choose different nesting sites (Hamann et al 2007). However, such changes in nesting patterns may depend on the particular sea turtle species as some species tend to be more faithful to their nesting sites than others.


Sea-level rise

The rate of sea-level rise over the last century has been increasing at a rate of 3.2 mm/ year between 1993 – 2010 (IPCC 2013). This may not sound like much but a higher sea-level will affect low-lying coastal areas. This poses a problem for turtle nesting habitats that occur on sandy beaches along the coast, especially in areas where a natural inland migration of beach isn’t possible because of fixed infrastructure. As a result, the area of viable nesting locations becomes significantly reduced because of rising water tables that inundate nests with salt water, thus drowning the eggs or washing them away (Hawkes et al 2009). Currently, coastal cities globally are spending millions of dollars protecting properties by relocating sand or building bluffs and embankments to reinforce foundations. Such concrete structures further reduce the available natural sandy habitats for nesting.

Although these are trends that have occurred in specific locations and might be expected to occur elsewhere, there are numerous other factors that influence the survival and successful reproduction of a turtle. These additional factors; including the topography of the beach, its vegetation and even precipitation may buffer or exacerbate the changing conditions. The combined effects of these factors are likely to result in significant geographical variations in turtle nesting habitats.

We are now living in a time when humans have altered the climate and made substantial modifications to landscapes and ecosystems; and man and nature are experiencing unprecedented changes in our planet as a result. The survival of sea turtles is dependent on the availability of new and varied nesting sites and the adaptability of turtle behaviour, in seeking new nesting grounds within their biological framework, as well as changes in our behaviours. Trinidad and Tobago is home to many turtle species that nest on our beaches and feed on our reefs, sea grass beds and other nearshore habitat. In light of current scientific findings and the predictions for turtle populations, conservation efforts need to consider the rapid changes that may occur. How will our beaches and our local climate change with warming temperatures and rising sea levels? What measures can we take to ensure that suitable habitats – nesting sites – are available for future generations of sea turtles?

Acknowledgements: Special thanks to Dr. Michelle Cazabon-Mannette for her input and feedback.

References:
Hamann, Mark, Colin J. Limpus, and Mark A. Read. "Vulnerability of marine reptiles in the Great Barrier Reef to climate change." (2007).

Hawkes, L. A., et al. "Investigating the potential impacts of climate change on a marine turtle population." Global Change Biology 13.5 (2007): 923-932.

Jensen, M. P., Allen, C. D., Eguchi, T., Bell, I. P., LaCasella, E. L., Hilton, W. A., ... & Dutton, P. H. (2018). Environmental Warming and Feminization of One of the Largest Sea Turtle Populations in the World. Current Biology, 28(1), 154-159.
IPCC, 2013: Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

Roon, R. B. Let’s call it: 30 years of above average temperatures means the climate has changed (2015), https://theconversation.com/lets-call-it-30-years-of-above-average-temperatures-means-the-climate-has-changed-36175




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