During the last ice age, huge numbers of large mammals roamed the temperate zones of North America and Eurasia that lay south of vast continental glaciers. Familiar animals such as Woolly Mammoths, Woolly Rhinoceroses, Reindeer, Musk Oxen, Steppe Bison and the wild ancestors of domesticated horses along with more exotic creatures such as Glyptodon, a car-sized relative of armadillos and Megatherium, an enormous ground-dwelling sloth were abundant. With the ending of the ice age, which began around 21,000 years ago, many of these species experienced dramatic declines or went extinct. Woolly Rhinos, Mammoths, Glyptodon, and Megatherium went completely extinct, while Bison, Reindeer, Musk Oxen and wild Horse went through serious declines and range contractions.
These population declines roughly coincided with another major event in earth’s history, the global expansion of modern humans. Because of this synchronicity, there has long been debate about whether either is the cause. Did humans fuel their global expansion by hunting these animals to extinction, were they victims of a changing climate, or was it some combination of the two?
To answer this question, we need to know two main things. First, did climate change create extremely inhospitable environments for these species? Second, did the decline of these species coincide with contact with humans?
A recent paper by Lorenzen et al. (2011) takes a new look at this issue. Lorenzen et al. apply a couple remarkable techniques to ask these questions about Woolly Rhinos, Woolly Mammoths, Bison, Reindeer, Musk Oxen and Horses. First, they use species distributional modelling (SDM) to estimate the distribution of suitable habitat for each of these species at four time periods from 42,000 years ago to 6,000 years ago. SDM uses data about climate conditions at locations where species are known to occur to create an educated guess about where else they could exist. In this case, the authors have radio-carbon dated fossil remains and they use models of historical climate to infer the extent of suitable habitat for each species at each time period.
Second, they extract, sequence, and analyze DNA from many of these fossils. Genetic diversity within a population carries the signal of that population’s demographic history. Using complex statistical analyses, we can uncover from a collection of DNA sequences whether or not a population has experienced growth, decline, or fragmentation, and when. Here the authors use these techniques to learn about the demographic history of each species over the last 50,000 years.
Finally, the authors use human archaeological sites to determine the extent of temporal and spatial overlap of humans and the study species based on 1) the SDM models and 2) the presence of faunal remains in those archaeological sites.
Lorenzen et al. find little consistency in results among any of the taxa. Musk Oxen appear to have declined purely as a result of climate change. The distribution of suitable habitat shrunk enormously after the last glacial maximum and their remains are very rare in archaeological sites. Climate change is also implicated for Woolly Rhinoceros. There was substantial overlap with early humans and their remains occurred at high frequency in archaeological sites, but this occurred during an early period of population growth. During their decline and extinction, remains are nearly absent from human settlements. Horse, by contrast, appears to have been strongly influenced by humans. It has a large predicted range at times when genetic evidence indicates it is declining and its remains are abundant in archaeological sites. Bison may be a mix of the two factors, as genetic diversity and predicted range decline simultaneously in Eurasia, but a sharp decline population decline in North America coincides with humans arrival there, and their remains are common in North American archaeological sites. Reindeer show a quite different pattern. Their range size declines drastically after the last glacial maximum and their remains are common in archaeological sites, but their genetic diversity remains stable and their populations today are in the millions.
The results suggest that the history of large mammal declines and extinctions from the Pleistocene to the present yields few generalities. While blaming these extinctions on the voracious appetite of early man may provide an attractive allegory for modern times, reality appears to have been far more complex, with each species responding quite differently to tumultuous climatic and ecological change flowing from the end of the last ice age.
Lorenzen E.D., et al. 2011. Species-specific responses of late Quaternary megafauna to climate and humans. Nature 479:359-365. doi:10.1038/nature10574