Climate change; two words synonymous with a growing list of threats facing the continuity of our planet, and coming to the forefront as one of the most prominent problems of the 21st century. The Intergovernmental Panel on Climate Change (IPCC) describes climate change as ‘a statistically significant variation in either the mean state of the climate or in its variability, persisting for an extended period (typically decades or longer)’. While the earth has gone through many climate patterns, the succinct end of the last ice age 7000 years ago marked the beginning of the current climate era- and the beginning of human civilization. The current warming trend is particularly significant due to the fact that most of it is extremely likely human- induced and proceeding at a rate that is unprecedented in the past 1,300 years.
While many people choose to remain indifferent, trying to maintain that the effects of climate change will not come to light in their lifetime, the truth is that those effects on the planet are already being seen. Historically, atmospheric carbon dioxide levels have been subject to fluctuation, but up until the beginning of the industrial revolution they generally averaged at 280 parts per million (ppm). Since then, however, CO2 levels have increased over 30%, measuring at 403.26 ppm as of April 2015.1
So what has this got to do with reptiles and amphibians? During the developmental stage of many organisms, one of the attributes that is determined is gender through a process called sex determination – (ok, the title may have been a little facetious). Sex determination can be categorised as either genotypic sex determination (GSD) or environmental sex determination (ESD). Mammals and birds exclusively experience GSD in their gender determination, as sex is established at conception by genetic factors such as sex chromosomes. In contrast, crocodilians and sphenodontians exclusively experience ESD in the form of temperature-dependant sex determination. Other groups such as squamates and turtles exhibit both forms of sex-determination; however ESD is more prominent in turtles. In order to understand the implications of climate change on sex-determination it is important to further discuss the process of temperature-dependant sex determination (TSD).
TSD is a particular form of ESD where the temperature of a defined period, the thermosensitive period, during development determines the sex of the offspring. The thermosensitive period occurs during the middle third of incubation, and its length is species specific, lasting anywhere between 7-15 days. Two distinct patterns of TSD have been identified, namely Pattern I and Pattern II. Pattern I is divided into two categories, Pattern IA and Pattern IB. Most reptiles that experience TSD have single transition patterns (male → female) that produce females at warmer temperatures (Pattern IA). Some reptiles have a rarer single transition pattern (female → male) and produce male offspring at warmer temperatures (Pattern IB). Reptiles that experience Pattern II TSD have two transition zones (female → male → female) with males dominating at intermediate temperatures and females dominating at both extremes.
As early as 1994, theories about the effect of a global temperature change on reptilian TSD were being presented. In a study of painted turtles (Chrysemys picta), Fredric J. Janzen theorised that an increase in temperature of 4°C would effectively eliminate the production of male offspring altogether.2 Painted turtles experience Pattern IA of TSD, but Janzen noted that despite the negative implications of increased temperature on their population, those species that experience Pattern IB TSD would be far more at risk. The study found that the annual offspring sex ratio was highly correlated with mean July air temperature, and that even increases of less than 2°C were enough to drastically skew the sex-ratio. Fast forward 20 years and further studies support the initial proposal made by Janzen. Grayson et al. (2014) conducted a study of a population of tuatara (Sphenodon punctatus) on North Brother Island in the Cook Strait of New Zealand and found that current nest temperatures are more likely to result in more male than female hatchlings; unlike painted turtles, tuatara experience Pattern IB TSD.3 The study concluded that in the absence of an evolutionary response or management intervention, the tuatara population would eventually be comprised only of males and would therefore be functionally extinct. The most worrying part of this is that the findings were based on current nest temperatures rather than future projections.
Research surrounding the implications of climate change on TSD is ongoing, and as recently as March of this year it continues to suggest that increased temperatures threaten to skew the sex-ratio so much as to drive the functional extinction of many species.4 While it may be possible for an evolutionary response to develop and overcome the impending threat, the fact of the matter is that global change is occurring on too rapid a timescale to allow for these responses to come into fruition. According to the IPCC Fifth Assessment Report it is virtually certain that the upper ocean warmed from 1971 to 2010; this ocean warming accounts, with high confidence, for 90% of the energy accumulation between 1971 and 2010. The report warns that warming will continue if emissions of greenhouse gases are unabated, and that the global surface temperature increase by the end of the 21st century is likely to exceed 1.5 °C relative to the 1850 to 1900 period for most scenarios, and even 2.0 °C for many scenarios. With findings such as Janzen’s that even modest temperature increases can have significant implications for sex ratio, these projections do not suggest a bright future for many crocodilians, sphenodontians, squamates and turtles.
Of course this stark reality is not limited to reptiles alone. In a comprehensive analysis published in Science on May 1st, Dr Mark Urban revealed that 1 in 6 species will face the threat of extinction as extinction rates accelerate with future global temperatures.5 He theorised that areas with naturally small native populations, such as the tropical forests of South America, could face extinction rates of up to four times that of the USA and Canada. Urban’s research comprised a comprehensive review of 131 extinction studies, accompanied by computer models and other statistical techniques in an attempt to create one global estimate. The study concluded that limiting climate change to a 2°C increase in temperature – the threshold to which the IPCC recommends we remain below – would still nearly double the risk of global extinction. Roughly 2.8% of all species around the world already face imminent extinction; a 2-degree rise would increase that risk to 5.2%. But more frightening still, if global average temperatures are to increase by 4.3 °C—a rise projected under many climate scenarios if dramatic reductions in fossil fuel emissions are not made—it was found that extinction rates could jump to 16 percent, or one in every six species.
Citing the decline of South America’s yellow crested and blue-backed manakins, as well as New Zealand’s tuatara, Urban stressed the implications on the continuity of many species populations around the globe. While he acknowledged that this new research still has many limitations, such as limited data regarding extinctions across Asia, he concluded that this new information is the best we have now.
This year, 2015, will most likely be remembered as the year the whole world sat up and collectively took climate change seriously. Influential world leaders are making commitments, people are rethinking their environmental footprint, and the year is set to culminate with the 21st Conference of the Parties (COP21) in Paris, where many remain optimistic that binding climate agreements can be reached. What is very apparent is that this action is coming at a critical time for both the planet and all of its inhabitants. Reptiles that experience TSD are just one small category on a growing list of species that urgently require intensified conservation efforts if we are to prevent their extinction altogether. Climate change poses major implications for so many aspects of our biosphere, and addressing these implications remains key to determining whether there is still time to prevent a mass extinction; or whether it is all but too late.
By Leah Gainey
About the author: Leah is currently the Senior Environmental Analyst and REDD+ Advisor for Irish eco-business Celestial Green Ventures (CGV), developers of a REDD+ project in the Brazilian Amazon aimed at reducing deforestation while providing social and biodiversity benefits to the area. She first studied as a biologist and then obtained an MSc in Climate Change in 2014 (specifically looking at the effect of carbon sequestration by trees in urban environments on overall atmospheric CO2 levels). Since then she has combined her passion for the environment and combating climate change in her role in CGV. She also enjoys occasional freelance writing on a range on environmental topics. Follow her on Twitter @ecodreamer92
2 Janzen, F. J. (1994). Climate change and temperature-dependent sex determination in reptiles. Proceedings of the National Academy of Sciences of the United States of America, 91(August), 7487–7490. doi:VL – 91
3 Grayson, K. L., Mitchell, N. J., Monks, J. M., Keall, S. N., Wilson, J. N., & Nelson, N. J. (2014). Sex ratio bias and extinction risk in an isolated population of tuatara (Sphenodon punctatus). PLoS ONE, 9(4). doi:10.1371/journal.pone.0094214
4 Wyneken, J., & Lolavar, A. (2015). Loggerhead sea turtle environmental sex determination: Implications of moisture and temperature for climate change based predictions for species survival. Journal of Experimental Zoology Part B: Molecular and Developmental Evolution, 324(August 2014), 295–314. doi:10.1002/jez.b.22620