Q&A: Extinctions and the impact of Homo sapiens
Robert M May
Correspondence: Robert M May email@example.com
Department of Zoology, University of Oxford, The Tinbergen Building, South Parks Road, Oxford OX1 3PS, UK
BMC Biology 2012, 10:106 doi:10.1186/1741-7007-10-106
Received: 13 December 2012
Accepted: 19 December 2012
Published: 20 December 2012
© 2012 May; licensee BioMed Central Ltd.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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?
As in so many areas of science, we know quite a lot, and continue to learn more. One measure of our knowledge is indicated in Table 1, by Raup, which gives the estimated average lifetime, from origination to extinction, of a variety of animal groups. Figure 1 complements this by showing numbers of families (remember the taxonomic hierarchy: species, genus, family,...) of marine animals over the sweep of geological time. The figure testifies to increasing diversity and species richness, interrupted by episodes of mass extinction. Overall, these data suggest average life-spans of animal species in the fossil record to be around 1 to 10 million years, with significant variation within and among taxonomic groups, and with the higher end of the range being more common.