“Species-specific responses of Late Quartenary megafauna to climate and humans”

E.D. Lorenzen, et al. 2011. Nature 479, pp. 359–364.
Despite decades of research, the roles of climate and humans in driving the dramatic extinctions of large-bodied mammals during the Late Quaternary period remain contentious. Here we use ancient DNA, species distribution models and the human fossil record to elucidate how climate and humans shaped the demographic history of woolly rhinoceros, woolly mammoth, wild horse, reindeer, bison and musk ox. We show that climate has been a major driver of population change over the past 50,000 years. However, each species responds differently to the effects of climatic shifts, habitat redistribution and human encroachment. Although climate change alone can explain the extinction of some species, such as Eurasian musk ox and woolly rhinoceros, a combination of climatic and anthropogenic effects appears to be responsible for the extinction of others, including Eurasian steppe bison and wild horse. We find no genetic signature or any distinctive range dynamics distinguishing extinct from surviving species, emphasizing the challenges associated with predicting future responses of extant mammals to climate and human-mediated habitat change.

Most people will agree that it is useful to look at the past to predict the future. This is explicitly the basis for all predictive models on climate change. One thing that some of these models do is predict extinctions of plants and animals based on changes in temperature. (Of course temperature is only one dimension of any organisms ecological niche, but it is relatively easy to model and we know a lot about temperature). But just as we can use this methodology to look into the future, we can also look back at known extinctions in the past and ask how they may have been influenced by changes in the environment.

We know that in the not so distant past many large animals have gone extinct or suffered enormous shrinkage of their ranges. The Woolly Mammoth would have been a common sight just 21 thousand years ago, but it was completely extinct only six thousand years ago. What drove these majestic animals, which were about the size of African Elephants, their close relatives, over the edge? And what about the other extinct mega-fauna? Woolly Rhinos, the European Cave Bear, and many more species disappeared from our planet just as modern humans ascended. Was it climate change or human impact that drove them to extinction?

According to this 2011 paper by Lorenz and his large team it was probably both. It seems that the Woolley Mammoth experienced a shrinking range and its populations broke up into smaller populations that suffered from inbreeding. Some species, which are not extinct like the Musk ox or the Reindeer, also experienced shrinking ranges, but escaped the genetic inbreeding. The authors know this from looking at thousands of specimens and close to a thousand ancient DNA sequences.

Around the same time humans flourished and hunted the Mammoth. If you come to Norman, OK and visit the Sam Noble Museum of Natural History  you can see a life-sized bronze of a Mammoth under attack by two humans and a dog. Or maybe it is the other way around–the humans look so small and vulnerable against the enormous pachyderm. Nonetheless, this hunting may have had a significant impact on the populations of these long-lived, slowly reproducing animals. Up to 40% of European and Siberian Paleolithic sites have been found to include remains of Mammoths, indicating that there was hunting pressure on this species.

But in the end the data on this species are inconclusive and it seems that clearly a combination of changes in the climate and human impact interacted to drive this species to extinction. In other species, different patterns are found. For example, it seems that Musk oxen are extremely intolerant to changes in temperature and were driven north as temperatures rose.

And this is the main point of the Lorenzen paper: responses to the unique combination of climate change and human impact are highly idiosyncratic, and not easy to predict. Without very detailed knowledge of the biology of individual species, precise models of climate change, and a much better understanding of species interactions, we will experience many surprises, good and bad. Anthropogenic climate change is certainly a major factor in the “sixth great extinction” that is associated with the Anthropocene. But as the full range of Anthropocene research shows, humans influence their environment in many other ways too.  We create countless interactions, all of which may contribute to extinctions.

D. Nogués-Bravo et al. 2008. “Climate Change, Humans, and the extinction of the Wooly Mammoth.” PLoS Biology 6(4): e79, DOI: 10.1371/journal.pbio.0060079. This paper argues that changes in climate made the Mammoth vulnerable, so that pressure from human beings could deliver the “coup de grâce” that drove it to extinction.
G. Ceballos et al. 2015. “Accelerated modern human–induced species losses: Entering the sixth mass extinction.” Science Advances, vol. 1, no. 5, e1400253 DOI: 10.1126/sciadv.1400253. This paper argues that current extinction rates are dramatically higher than the background rates between previous mass extinction events.

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