Tuesday, December 20, 2011

Is mass extinction just human nature?

The Earth has experienced five mass extinction events since multicellular life began, the last of which occurred 65 million years ago, most likely initiated by an meteor strike.  There is good evidence that we are in the midst of the planet’s sixth mass extinction.  This one initiated by human civilization.

Civilization apologists are quick to dismiss the importance of the accelerating pace of species extinction, and even quicker to point out that this isn’t the first time that human activity has been linked to mass extinction.  The fossil record, we are told, contains proof that our early ancestors were just as destructive to the ecosphere as any modern petrochemical company.  On virtually every continent, the first appearance of humans was coincident with the extinction of local megafauna that had occupied a solid niche for millions of years.  These species were hunted into oblivion by our overzealous progenitors through the use of such wasteful tactics as driving entire herds of mastodons off of cliffs.

I will return to the issue of Pleistocene megafauna shortly.  Suffice it to say at this point that the evidence points to a slightly different conclusion than the one offered by civilization’s ardent partisans.  But before I get there, I want to address briefly the reflexive source of this partisanship.  How is it that the recklessness of Pleistocene hunters—allowing for the sake of argument that such an accusation is warranted—has any bearing on the permanent obliteration of entire ecosystems caused by 21st century global industry?  What does it mean to suggest that pre-civilized humans were also environmentally destructive?  Does the claim serve a rhetorical function?  Is it a red herring?  A distraction?  Does it reflect the operation of a defense mechanism?  Or is there something else, something more deeply embedded in commonly held beliefs about what kind of a thing civilization is?     

The most straightforward interpretation is that this accusation is meant to excuse corporate ecocide by suggesting that we can’t lay the blame for recent species extinctions entirely on industrial civilization.  Destroying the environment is part of what humans have always done.  Industrial civilization allows us access to more efficient tools of death-dealing, but death-dealing is a fundamental feature of the human species.

And there is surely a sliver of truth to this.  The life of all creatures is possible only at the expense of others.   It’s called the food chain. 

But the death-dealing of civilization is not a part of the natural food chain.  The destruction caused by the earliest civilizations is a matter of record, and proportional to their technology.  So it isn’t industrial civilization that is to blame, perhaps.  Environmental destruction is a feature of all civilization, industrial or otherwise.  And the difference between subsistence hunting and gathering and civilization—of any flavor—is not just a difference of magnitude, a matter of degree.  Civilization is a distinctly different patterning of human behavior.  So for the claim that humans are naturally destructive to be true, it needs to be qualified to read: collective human activity as organized by civilization is naturally destructive.  

That’s not a trivial qualification.

Now, about the extinctions of Pleistocene megafauna: Research strongly supports the idea that climate changes (which were also likely responsible for prompting human migration, at least in North America) were the major contributing factor.  And, further, that if humans did play a direct causal role, it was simply as a result of their wedging themselves into a precariously balanced ecological niche rather than the result of an innate drive toward pathological destructiveness.   According to Ripple & Van Valkenburgh (2010): “…the large mammalian herbivores of the North American Pleistocene were primarily predator limited and at low densities, and therefore highly susceptible to extinction when humans were added to the predator guild.”  In other words, it wasn’t aboriginal human overkill or bloodthirsty and unsustainable human hunting behavior that caused the great die-off in North America even if the presence of humans—rather than climate—was the most crucial causal variable.  It was the mere addition of another predator to an already fragile system.   

Pushkina & Raia (2008) echo these sentiments: “Our study supports the idea that the late Pleistocene extinctions were environmentally driven by climatic changes that triggered habitat fragmentation, species range reduction, and population decrease, after which human interference either by direct hunting or via indirect activities probably became critical.” 

Another problem with the human bloodlust hypothesis is that it doesn’t account for the lag time between human presence and extinction. “Importantly, the dates suggest that the local decline in biological diversity was initiated ~75,000 years before the colonisation of humans on the continent [of Australia]. Collectively, the data are most parsimoniously consistent with a pre-human climate change model for local habitat change and megafauna extinction, but not with a nearly simultaneous extinction of megafauna as required by the human-induced blitzkrieg extinction hypothesis” (Price et al (2011). 

And from Barnosky & Lindsey (2010): “…on a continental scale most megafauna have last appearances after human arrival, but seem to last at least 1000years after first human presence. Some taxa apparently survived >6000years after humans entered South America and >1000years after the end-Pleistocene climatic changes. Last-appearance patterns for megafauna differ from region to region, but in Patagonia, the Argentine and Uruguayan Pampas, and Brazil, extinctions seem more common after humans arrive and during intensified climatic change between 11.2 and 13.5ka. This pattern suggests that a synergy of human impacts and rapid climate change—analogous to what is happening today—may enhance extinction probability. Nevertheless, even in these regions, some megafauna persisted for thousands of years after human arrival and after the climate warmed.”  (Note, for the record, that the rapid climate change happening today is itself a human impact, so the “synergy of human impacts and rapid climate change” happening today is not a synergy at all.)  

Humans probably had something to do with some of the extinctions in the late Pleistocene, but the evidence in favor of the human bloodlust hypothesis hardly makes for a smoking gun.  The same cannot be said for those extinctions that have occurred in the last 200 years.  Here the gun is still smoking.  And today it smells like Big Oil.


Barnosky, A. D., & Lindsey, E. L. (2010). Timing of Quaternary megafaunal extinction in South America in relation to human arrival and climate change. Quaternary International, 217(1/2), 10-29.

Ripple, W. J., & Van Valkenburgh, B. (2010). Linking Top-down Forces to the Pleistocene Megafaunal Extinctions. (Cover story). Bioscience, 60(7), 516-526.

Price, G. J., Webb, G. E., Zhao, J., Feng, Y., Murray, A. S., Cooke, B. N., & ... Sobbe, I. H. (2011). Dating megafaunal extinction on the Pleistocene Darling Downs, eastern Australia: the promise and pitfalls of dating as a test of extinction hypotheses. Quaternary Science Reviews, 30(7/8), 899-914.

Pushkina, D., & Raia, P. (2008). Human influence on distribution and extinctions of the late Pleistocene Eurasian megafauna. Journal Of Human Evolution, 54(6), 769-782.

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