Great Barrier Reef Sea Turtles Experiencing Increased Female-only Offspring

[This was a blog post assignment that I was completing for my Biodiversity elective class and I wanted to share it here.]

Everyone knows that many species are impacted by climate change due to effects of global warming on the living conditions of these species. For example, did you know that the sex of some species is completely dependent on the temperature of the environment during the incubation period? For the sea turtles of the Great Barrier Reef (GBF), higher incubation temperatures result in the death of developing eggs and female-only offspring (Jensen et al. 2018). Because of the warmer temperatures in the northern GBF (nGBF), researchers Jensen et al. hypothesized that the northern sea turtles will be in danger of becoming an all-female population, which would lead to their extinction in the near future. This is important to note in the context of biological conservation because this case shows the negative impacts that global warming has on species and the overall ecosystem. If there are negative consequences on the sea turtle population, then what about other species? What about us humans? Will we all eventually go extinct due to global warming? Unfortunately, it is possible.

Image: Green turtle, a vulnerable species, located at the Great Barrier Reefs. Credit: http://www.gbrmpa.gov.au/the-reef/animals/marine-turtles

To test their hypothesis, Jensen et al. did a comparison of sex ratios between the nGBR population and the southern GBR (sGBR) population. If the female-to-male ratio in sea turtles is higher in the northern population, then it implies a positive relationship between increasing temperatures and percentage of females within the population. Since the temperatures in the sGBR remained relatively the same in the past years (Jensen et al. 2018), the sex ratio of the sGBR sea turtles served as the baseline measurement. Jensen et al. gathered data from the foraging grounds of the GBR since the location mainly consisted of sea turtles from the nGBR and sGBR (Jensen et al. 2018). To differentiate the natal origins of each turtle, the researchers conducted genetic analysis on the turtles (Jensen et al. 2018). To find out the sex of each turtle, the researchers examined the turtles’ gonads and hormones since sea turtles lacked external sex-based traits (Jensen et al. 2018). Then they calculated the sex ratios from each region and did a comparison. To estimate the approximate sand temperatures in the nests of the past few decades, the researchers used recorded air and sea surface temperatures (Jensen et al. 2018).

Turns out, there was an extreme bias for females in the sex ratios of the nGBR sea turtle population. According to the results, “99.1% of juveniles, 99.8% of subadults, and 86.8% of adult-sized turtles [were] female” (Jensen et al. 2018). For the sGBR turtles, there was a moderately skewed sex ratio, where females consisted of 67.8% of juveniles, 64.5% sub-adults, and 69.2% adult-sized turtles (Jensen et al. 2018). These results provided strong evidence that nGBR sea turtles were experiencing an extremely skewed female sex ratio compared to the turtles from the southern GBR where there were more cooler sand temperatures. Another important thing to note is the extreme female bias in the younger populations of the nGBR turtles, which suggests that the population became more female in the past few decades. What is interesting is that estimated sand temperatures of those past few decades showed a severe warming in the nGBR (Jensen et al. 2018). This suggests that the younger population of turtles would have hatched within that time period where the nGBR experienced severe warming.

These results are compelling because it reveals the high probability that a species will go extinct within the next century due to global warming. If this study only focuses on the nGBR sea turtles, how many other species will be driven to extinction as well? Overall, global warming has been shown to have negative impacts on many species. One study investigated the upward shift of montane species on New Guinean mountains as a response to warming temperatures, with results suggesting that global extinctions are more likely to occur amongst tropical species that are endemic to mountains (Freeman and Freeman 2014). Also, since there are less land areas at higher elevations, this would cause a decrease in the species’ population which increases the probability of them going extinct (Freeman et al. 2014). Another study found that hotspots were extremely vulnerable to global warming and that estimated global-warming-induced rates of species extinction in those hotpots exceeded estimates of deforestation-induced rates of species extinction (Malcolm et al. 2006). The global warming that many species and environments experience are a result of a human-induced climatic shift (D’Amen and Bombi 2009). The rise in global temperature will decrease the amount of biological diversity present in the world, leaving the surviving population more prone to extinction due to their vulnerability to selection pressures. Anything that harms biodiversity will also harm the well-being of humans due to the intertwining relationship between nature and humans (Kareiva and Marvier 2012). Therefore, we all should make an effort to combat the effects of global warming to protect the remaining species, and us.

References

D’Amen, M., and P. Bombi. 2009. Global warming and biodiversity: Evidence of climate-linked

amphibian declines in Italy. Biological Conservation 142: 3060-3067.

Freeman, B. G., and A. M. Freeman. 2014. Rapid upslope shifts in New Guinean birds illustrate

strong distributional responses of tropical montane species to global warming. Proceedings of the National Academy of Sciences 111: 4490-4494.

Jensen, M. P., C. D. Allen, T. Eguchi, I. P. Bell, E. L. LaCasella, W. A. Hilton, C. A. M. Hof,

and P. H. Dutton. 2018. Environmental Warming and Feminization of One of the Largest Sea Turtle Populations in the World. Current Biology 28:154-159.

Kareiva, P., and M. Marvier. 2012. What Is Conservation Science? BioScience 62: 962–969.

Malcolm, J. R., C. Liu, R. P. Neilson, L. Hansen, & L. Hannah. 2006. Global Warming and

Extinctions of Endemic Species from Biodiversity Hotspots. Conservation Biology 20: 538-548.

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