Q&A: Extinctions and the impact of Homo sapiens
© May; licensee BioMed Central Ltd. 2012
Received: 13 December 2012
Accepted: 19 December 2012
Published: 20 December 2012
Extinctions have happened ever since life began - is there anything different about man-made extinctions?
Looked at in the large, the history of life on Earth is one of continuous change, driven by the interplay between evolutionary processes and the altered environments that can result. Some of these environmental events have had external causes (for example, the asteroidal impact that caused the most recent of the so-called Big Five mass extinctions, which eliminated the dinosaurs), while others have arisen from changing interactions among species (for example, the early appearance of oxygen in the atmosphere, resulting essentially from biogeochemical processes in primitive ecosystems). Are the recent past and impending future extinctions, unambiguously caused by humans, different? Yes and no. No, in the sense that the explosive growth of the animal species Homo sapiens can be seen as just another evolutionary process with increasingly serious ecological consequences for other species. Yes, in the sense that - unlike earlier extinctions - the causative agent (that's us) is aware of what is happening and could act to reverse current trends. Unfortunately, we show few signs of doing so.
What are the major causes of extinctions (man-made or otherwise)?
The causes of recent, human-associated extinctions are usually listed under three headings: over-exploitation, habitat destruction, introduced aliens. But you could, with a bit of a stretch, brigade many past extinctions under one or more of these headings. The above-mentioned demise of the dinosaurs, or the massive wave of marine extinctions which mark the end of the Mesozoic, could be called 'habitat change'. The opening and closing of land bridges, as tectonic plates moved around over the past billion years and more, introduced 'invasive aliens', which restructured many ecosystems. More generally, over geological time-scales, natural evolutionary processes created changes within plant and animal populations, with new winners and new losers. In that sense, humans look like being the main agents of the Big Sixth wave of extinctions, on whose breaking tip we currently stand.
How are the man-made versions distinct?
The very big difference between past extinctions and the current human-associated ones is we understand what is happening. And we can, in principle, choose to modify our behavior to preserve the awe-inspiring diversity of plant and animal life we have inherited. Even were we to do this - and we show few signs of it - there would still, over relatively long time-scales, be changes. They would, however, be more likely to be the pseudo-extinctions technically referred to as 'relay and replacement', as in the series of differently named species along the continuum as Eohippus evolved into today's horse.
How much do we know about the rate of extinction before humans started interfering?
Estimates of species' lifespan from origination to extinction
Species' average lifespan (my)
All fossil groups
How much do we know about the rate of extinction after humans started interfering?
Do we even know enough about how many species there are today?
If the Star Ship 'Enterprise' were to land on Earth, what would be the first question the crew asked of our planet? I think it would be, how many distinct species are there here? I think they would be shocked by our ignorance. We do have very good knowledge of how many bird species there are. The International Ornithological Congress says 10,448, although some would argue plus or minus 500. The mammalian total is smaller, 5,000 give or take 10%. Plant species add up to around 300,000. There are approximately 1 million known insects, but the true number could be several times this. Adding other smaller taxons gives a total species count in the neighborhood of maybe 1.7 million, although unresolved synonyms - same species identified and named separately in different museum collections - may inflate this. Estimates of the true total, in my opinion, are in the plausible range of 3 to 8 million distinct eukaryotic species. In other words, we have documented only one half, maybe only one-fifth, of our planet's biological diversity.
Why should we be concerned about extinctions?
I would distinguish three kinds of concern.
The first might be called narrowly utilitarian: the plant and animal species that are being extinguished could represent important genetic resources for tomorrow's biotech revolution. We are burning the books before we have read them. I think this is a weak argument, because tomorrow's advances in understanding the molecular machinery of life will, I believe, see us (for example) designing new drugs from the molecules up.
The second might be called broadly utilitarian: although the services provided by ecosystems, which are many and varied, are not taken into account in conventional measures of gross domestic product (GDP) , they nevertheless are very important to us (and insofar as they can be given a value, it is estimated to be roughly of the magnitude of the more conventional global GDP). The Millennium Ecosystem Assessment classifies these services under 24 headings, and finds that 15 of these are being degraded, 4 are improving, and 5 are such that we know too little to assess. Deplorable though this is, I believe we may be smart enough to survive in a biologically impoverished world. It would, however, be an unattractive world resembling that of the cult movie Blade Runner.
Which brings me to the third argument, which is that we have an ethical responsibility not to deprive tomorrow's world of its heritage. Aldo Leopold expressed it well, mourning the death of Martha, the last passenger pigeon: 'We grieve because no living man will see again the onrushing phalanx of victorious birds sweeping a path for Spring across the March skies, chasing the defeated Winter from all the woods and prairies.... Our grandfathers, who saw the glory of the fluttering hosts, were less well-housed, well-fed, well-clothed than we are. The strivings by which they bettered our lot are also those which deprived us of pigeons. Perhaps we now grieve because we are not sure, in our hearts, that we have gained by the exchange'.
Need it be an exchange?
That's the question.
Where can I go for more information?
Lawton JH, May RM: Extinction Rates. Oxford University Press; 1995.
Raven PH: Nature and Human Society: The Quest for a Sustainable World. Washington DC: National Academy Press; 1977.
Millenium Ecosystem Assessment: Ecosystems and Human Well-being: Synthesis. Washington, DC: Island Press; 2005.
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