This paper seems to have really touched a nerve on the web, with more than 50 media websites and blogs reporting that climate change will kill off the Brothers Island tuatara by 2080 by causing all eggs to develop into males. People, it seems, are now well primed to fear global warming. But is climate change now the number one challenge to tuatara conservation? Far from it.
This paper by Mitchell et al. (2008) was the topic of a recent ecology & evolution journal club at Lincoln University. While we were impressed with the techniques used to model soil temperature changes in Brothers Island, we were all skeptical of the conclusion, as paraphrased by the Nature news story, that "All tuatara could be born male — and thus doomed to extinction — within decades." By 2085, to be exact.
Brothers Island tuatara (Sphenodon guntheri) is a genetically distinct population of tuatara (the more widespread species being S. punctatus). Its only natural population is on the tiny (4 ha) Brothers Island in the Cook Strait, presumably the last remnant of a once much more widespread population, as tuatara remains have been recovered from throughout both main islands of New Zealand. Both species of tuatara, like many reptiles, have a temperature-dependent sex determination system. In the case of tuatara, their eggs are buried in the soil where warmer eggs develop into males and cooler eggs develop into females (the opposite of most reptiles).
With a temperature-dependent sex determination system, Mitchell et al. (2008) ask what global warming is likely to do to the tuatara sex ratios on Brothers Island. They combine an impressively detailed model of the soil temperatures on Brothers island with the sex-temperature relationship of the closely related and similar S. punctatus, to predict sex ratio shifts under two climate change scenarios. Under the more extreme of their two climate scenarios, a warming of 4° C., the tuatara are all males by 2085.
While we were all generally impressed with the methods used, none of us were particularly alarmed by the authors' conclusion. Why? Here are our reasons.
- Tuatara have survived numerous Ice Ages and interglacial periods in New Zealand's past, including the last glacial event that ended about 10,000 years ago and a warm period before 6,000 years ago that was perhaps 1.5°–3° warmer than now. At these times, tuatara were numerous and widespread, but still, the species have survived major fluctuations in climate in the past.
- The authors note that the Brothers Island female tuatara currently lay their eggs on northern and northeastern facing open slopes on the island, i.e., the warm places. They avoid the shade, i.e., cold places. The authors note that with shading, their models suggest that even a 4ºC rise in mean temperature would still result in mixed-sex nests. So, when the climate warms, all the females need do is change their behaviour to lay eggs in cooler, shaded places. Need shade? Plant native trees. Crisis averted.
- If human-driven accelerated climate change stopped tomorrow, the Brothers Island tuatara would still be in serious trouble. Their current predicament is not the result of climate change but the result of rat predation on mainland New Zealand combined with widespread habitat loss and modification in the lowlands. Climate change is just one of the many challenges now facing this tiny, isolated population. Inbreeding depression is another. Vulnerability to extinction following disturbance is another (e.g., a severe storm or drought could wipe them out). And, if rats arrived on the island, only immediate action would save the population.
- People will move this population to other locations and expand their numbers. This is not some small, obscure insect species. This is a high profile megafauna species that New Zealanders care about.
The media and bloggers are using this study as reason for urgent action to reduce the rate of climate change. While there are a great many reasons to reduce the currently alarming rate of climate change, what these tuatara need above anything else is a greater area of New Zealand habitat free from mammalian predators.
