climate change, islands, and the impending demise of tuatara?

Mitchell, N. J., Kearney, M. R., Nelson, N. J., and Porter, W. P. 2008. Predicting the fate of a living fossil: how will global warming affect sex determination and hatching phenology in tuatara? Proceedings of the Royal Society. Series B. Biological Sciences, Online version, doi: 10.1098/rspb.2008.0438.

tuatara sold separately
Originally uploaded by Mollivan Jon

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.

A glimpse of the past through kakapo poo

Horrocks, M., Salter, J., Braggins, J., Nichol, S., Moorhouse, R., and Elliott, G. 2008. Plant microfossil analysis of coprolites of the critically endangered kakapo (Strigops habroptilus) parrot from New Zealand. Review of Palaeobotany and Palynology, 149:229–245.

This paper was the subject of a recent ecology journal club at Lincoln University. In it, Horrocks et al. (2008) reconstruct the diet of New Zealand's flightless kakapo using coprolites (preserved faeces). Since New Zealand's bird fauna was decimated so recently, there are still faeces to be found in places where their makers are gone. Kakapo were once common throughout New Zealand, but have now been reduced to less than 100 individuals being intensively managed on offshore island reserves. This paper uses traditional methods of identifying plant microfossils (particularly the morphology of pollen and spores) to characterise what kakapo used to eat throughout their mainland New Zealand range. Not surprisingly, the kakapo diet was broader and more varied than what is now available in their offshore island sanctuaries. There are some good tips for what other foods these island birds could be offered.

What is difficult to obtain from this study is what foods were actively sought after by the kakapo rather than which were fed on proportional to their availability in these habitats. Comparing the plant composition of the kakapo faeces with the background levels of pollen and spores at or near these sites (with some undoubtedly tricky adjustments estimating relative plant abundance from pollen and spore counts) could be an interesting follow-on from this study.

The paper can be accessed from ScienceDirect (subscription required).

The ghosts of Pacific avifauna

Diamond, J. 2007. Voices from Bird Bones. Science, 315:941–942. www.sciencemag.org (subscription required).

Jared Diamond reviews two recent publications on the Pacific's many extinct birds.

• "Extinct Birds of New Zealand" by Alan Tennyson and Paul Martinson (Te Papa Press, Wellington, New Zealand, 2006).


• "Extinction and Biogeography of Tropical Pacific Birds" by David W. Steadman (University of Chicago Press, Chicago, 2006).

The review of Tennyson and Martinson's attractive book is brief and complementary. "Martinson's gorgeously detailed paintings bring home the tragic loss of formerly breathing real animals in a way that descriptions of bones cannot achieve" (p. 941). Diamond presents it as a worthy companion to Trevor Worthy and Richard Holdaway's (2002) book, "The Lost World of the Moa".

"The New Zealand fossil avifauna is by far the most completely sampled in the world" (p. 941). Diamond is particularly fascinated by how the extinction of many of New Zealand's endemic birds was followed by a replacement by often related Australian species. He sees there being much to learn about the processes of community assembly from both the Pacific's prehistoric avifauna and the modern bird communities that have replaced them.

David Steadman's decades of excavations have revealed the details of how the Polynesian settlement of the Pacific Islands triggered a mass extinction of birds and other vertebrates. I well remember being stunned when I read Steadman's chapter in the 1997 book "Biodiversity II", with its description of the massive scale of bird extinctions in the Pacific following human arrival. Unfortunately, Diamond's opinion of Steadman's book is less than complementary, not due to any inaccuracies in the data behind the book, but rather regret at the superficial interpretation of this data. In Diamond's eyes, Steadman is stubbornly resistant to the modern analysis of his data and all the amazing things that could be learned from it. With the publication of "Extinction and Biogeography of Tropical Pacific Birds", this modern treatment may follow.