Cape Verdean Loggerhead Turtles More Climate Change Resilient than Expected

by Wendy Noreña

Following growing concerns about the potential effects of climate change, scientists have begun to study declining levels of biodiversity in the natural world. Of especially large concern are cheloniid reptiles, or turtles, which are ectothermic organisms that rely heavily on atmospheric temperatures and regular seasons to regulate internal temperatures, metabolic rates, and, for turtles, male-to-female sex ratios during egg incubation. Marine turtles are of particular interest to conservation work as there are only seven species, there already exists a large amount of research about them, and, most importantly, they are even more susceptible to climate change than other turtle species because of their beach-dependent nesting habits. Though much has been done to form quantitative analyses of marine turtles’ current state in the face of recent climate fluctuations, Perez et. al. seek to create a qualitative ranking system with which to gauge the resilience, or potential to withstand environmental change, of a reptile now and in the future.

The study samples 71 loggerhead turtle nests and related environmental conditions from 2005 to 2008 on Boa Vista, a Cape Verde island off the coast of western Africa, ranking seven different criteria. A major component of the study was to measure the temperature of the nests during incubation and reference that against an accepted, “pivotal temperature,” which is the point at which an egg will go from developing a male to a female hatchling. Rising global atmospheric temperatures are of biggest concern here—an increase of two degrees Celsius would make 99.86% of turtle hatchlings female according to projections made in the study. Overall, the study finds that the loggerhead population of Boa Vista comes away with a score of 75 on their resilience ranking scale, a number higher than expected but still susceptible to multiple environmental uncertainties pertaining to the loggerheads’ ability to quickly change certain nesting habits, possible future island development, and general unknowns about climate change itself.

The criteria on which this study’s ranking system is based are male-to-female hatchling ratios, percent of overall eggs that hatched, “availability of spatial climate refugia,” “availability of temporal climate refugia,” beach habitat vulnerability to sea-level rise, dietary flexibility of juvenile to adult turtles, and other general threats. Both criteria referencing the availability of different, “refugia,” were based on the idea that the turtles could eventually adapt to nest during a cooler season (temporal criteria) or in a cooler area (spatial criteria), which would balance out an increasingly skewed hatchling sex ratio. The, “other threats,” category was based on general, often human-based hazards such as illegal harvesting, by-catch incidents, commercial beach development, pollution, chemical ocean changes, and more. The most important considerations of the effects of sea level rise revolved around, “coastal squeeze,” or the possibility that the increased construction of buildings or walls on the beaches, coupled with sea level rise, would greatly reduce the beach space available for nesting turtles. All of these categories were rated on a 0—100 scale in 5-point intervals, with 0 being the worst.

Perez et. al. found that, on average, 79.15% of the hatchlings are likely female, earning sex ratios a 75 on the resiliency scale. The percent of eggs hatched averaged out at 38.1%, giving these criteria a low 25, while, “temporal and spatial refugia,” were both given rankings of 100 since the turtles were already nesting during the warmest times and in the warmest places. Concerns about coastal squeeze were ranked at 50 given uncertainties about how much of the island’s small coast will be developed, while the turtles’ flexible diets earned a rank of 75. Finally, because of the considerable prevalence of illegal harvests, turtle disease, long-line fishing, and increased tourism in the area, the “other threats” category garnered a rating of 25. Though some of the results seem bleak, the overall score of 75 suggests that there is still a possibility for conservation efforts to successfully contribute to the protection of this marine species.

However, some of the results of the study could be compromised by lack of research in some areas such as how many males are necessary to ensure successful breeding rates, whether a correlation found between increased temperatures and hatchling success could benefit future nesting generations, and whether or not these turtles can actually change their nesting characteristics in time to evade climate-caused repercussions. Nevertheless, this qualitative framework is applicable to many different species and could benefit research on species-specific climate resilience in the future.

Abella Perez, E., Marco, A., Martins, S., & Hawkes, L. A., 2016. Is this what a climate change-resilient population of marine turtles looks like? Biological Conservation, 193, 124–132.



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