Gary Haynes talk on the megafauna extinctions of North America

Despite saying I was going to talk about the climate arguments in my next post, I came across this video which is part of the 2012 Royal Tyrrell Museum Speaker Series, given by Gary Haynes from the University of Nevada.

It's fairly long (just under 49 minutes), but highly interesting and brings up some key points about the fossil and artifact evidence surrounding the theories for human related megafauna extinction in North America. The point that struck me most was that there is evidence for humans in North America, pre-Clovis.

Haynes seems to suggest that this would weaken the position of the human extinction argument. If humans were there before 13,000 years ago, then the megafauna that died out around this time may not have had anything to do with humans, they being nothing new in terms of pressures on the megafauna populations. 

To me (a self-declared non-expert on the subject) this doesn't quite follow. There is considerable evidence that points to the megafauna of 13,000 years ago, when the Clovis people arrived, being residual populations that were already in decline and long past their heyday.

Without evidence of people in North America pre-Clovis, this clearly implicates climate. But if people had been around before, and had been hunting megafauna, then surely this furthers the human extinction argument? Even if this was not the case, maybe it was the additional growth of human populations (and the resulting growth in resource competition/ hunting/ introduction of new diseases) when the Clovis arrived that pushed the megafauna over the edge?

Comments from people wiser than me are welcome!

Clovis or Climate? The roots of the debate

The debate surrounding the roles of climate and humans in the megafauna extinctions remains contentious (Lorenzen et al, 2011). Up until the 1960’s, conventional wisdom held that the megafauna were victims of a warming climate at the end of the last Ice Age (Levy, 2011). In an age old story, rising temperatures caused changes in vegetation ranges and community compositions; thereby altering habitats. The herbivores that relied on these habitats then were thought to have died out, followed by the predators that hunted them. 

But today, many scientists believe that Stone Age hunters expanding across the globe were responsible for these extinctions. This theory was first conceived by naturalists in the 19th century (amongst competing theories of geological and slow 'natural changes' of the environment) and pioneered by Paul Martin in the 60’s (Martin and Klein, 1984). He saw a pattern of human arrival in each new continent and the rapid extinction of its native megafauna shortly after. He named this the “blitzkrieg” or overkill hypothesis. 

The overkill hypothesis works based on the assumption that these prehistoric people had the ability to kill off the megafauna, and also that these species coexisted with humans. Big names of the 19th century such as Wallace, Darwin, Cuvier, Flemming, Lyell, Buckland, Owen and Agassiz argued fiercely on the topic, but came to no consensus (Martin and Klein, 1984).

"We live in a zoologically impoverished world, from which all the largest, and fiercest, and strangest forms have recently disappeared" 

                                            -Alfred Russel Wallace (1876)

Martin argued that the megafauna (mammals exceeding 44kg) survived the last and previous glaciations, only to die out shortly after. Outside of Africa, most genera of large mammals lost became extinct in the last 100,000 years. To Martin, the extinction pattern didn't track changes in climate (Martin and Klein, 1984). Martin's focus was broadly on the Clovis people, descendants of East Asians, they met woolly mammoths and rhino in Siberia on their journey to North America, where Martin envisioned they met a land full of unsuspecting and unprepared lunch.

Source: Prehistoric Cultures - University of Minnesota Duluth

A striking example of this is provided by palaeoecologist Guy Robinson. Bits of fossilised pollen found in Shasta sloth coprolites (fossil dung) in Rampart Cave seem to prove, at least in this instance, that the habitat outlived the animal itself (Robinson et al., 2005). The dung contains globe mallow, Ephedra and cholla cactus spores, indicating these plants made up the diet of the sloth. So if the climate had changed, these plants should have gone extinct, or at the very least moved, to have prompted the death of the sloths that subsisted on them. But the same plants grow near the caves today (Robinson et al., 2005). This doesn't fit with the climate theory that relies on declining habitat as the force driving extinction (Levy, 2011).  

Recently, the climate argument has staged a comeback, and there is fierce opposition to the overkill hypothesis. The next post will discuss some of these ideas, so if overkill hasn't convinced you, then maybe climate will...

Eulogy for the Megafauna

While science blogs may not be as popular as the dedicated how-to-create-your-own-dog-pajamas or make-a-fabulous-soap-dispenser-in-12-easy-steps type blogs, I am bypassing that domestic delirium, for now, to discuss the sudden decline in large-bodied vertebrates at the end of the last Ice Age. Whoopee! you are surely thinking.  

But actually it's a topic that is both more connected to our history and embedded in our future than it might first seem. These ancient giants permeate our culture and many are familiar today through films like Ice Age, a personal favourite. Saber-tooth cats, dire wolves (yes, they exist(ed) outside the realms of Game of Thrones), camels and giant ground sloths roamed North America. Eurasia boasted woolly rhinoceros and mammoth, cave lions and wild horses, while giant kangaroos and wombats coexisted with marsupial lions in what is now Australia. Then, between fifty and ten thousand years ago, around 90 genera of large-bodied mammals disappeared (Koch & Barnosky, 2006).

So why did they disappear? Their icy world was drying out; did they fail to adapt to a warming planet?

Doomed? The Meltdown.
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Paleontologists long thought that these beasts disappeared due to a shift to a warmer interglacial period in the late Pleistocene, their habitats shrinking as the ice retreated (Levy 2011). However, evidence implicating Homo sapiens as the driver of these extinctions has been mounting in recent years. Many species had survived previous interglacials (Levy 2011), so what was different? Was it humans, climate, or both?

This blog will explore the evidence for both human and climate mediated extinctions of the Pleistocene megafauna, investigating the species themselves as well as their environments and the roles they played within them. Parallels will also be drawn with modern times: are the extant ancestors of the Pleistocene megafauna currently experiencing a second wave of extinctions?