This post doubles as a Conservation Classic and a new take on an old concept. It’s new in the sense that it updates what we believe is an advance on a major milestone in conservation biology, even though some of the add-on concepts themselves have been around for a while.
First, the classic.
The ‘evil quartet’, or ‘four horsemen of the ecological apocalypse’, was probably the first treatment of extinction dynamics as a biological discipline in its own right. Jarod Diamond (1984) took a sweeping historical and contemporary view of extinction, then simplified the problem to four principal mechanisms:
- overhunting (or overexploitation),
- introduced species,
- habitat destruction and
- chains of linked extinctions (trophic cascades, or co-extinctions).
Far from a mere review or list of unrelated mechanisms, Diamond’s evil quartet crystallized conservation biologists’ thinking about key mechanisms and, more importantly, directed attention towards those factors likely to drive extinctions in the future. The unique combination of prehistorical through to modern examples gave conservation biologists a holistic view of extinction dynamics and helped spawn many of the papers described hereafter.It would now appear prudent to add a fifth ghoul to the team - severe anthropogenic interference with the global climate system. The response of biodiversity to past global climate change characteristically unfolded over thousands to millions of years, whereas anthropogenic global warming is now occurring at a greatly accelerated rate. If carbon emissions are not reduced rapidly, the Intergovernmental Panel on Climate Change’s Fourth Assessment Report 2007 projects a rate and magnitude of 21st Century planetary heating that is 5–9 times greater than that of the past century. This is comparable to the difference between now and the height of the last glacial maximum.
A clear lesson from the past is that the faster and more severe the rate of global change, the more devastating the biological consequences, and as I’ve covered before here on ConservationBytes.com in a separate Conservation Classic, this has seriously negative implications for biodiversity.
Now it’s time though to add a sixth rider – extinction synergies (Brook et al. 2008). For example, exacerbating the problems associated with recent climate change is that species trying to shift distribution must now contend with massively modified landscapes. Even in cases where global warming might allow species to expand their range, these benefits can be outweighed by other threats such as habitat change. The new conditions and altered communities might also allow more invasions by alien species that outcompete native species or act as predators to reduce their populations further. Harvest, habitat modification and changed fire regimes will also interact with and probably enhance the direct impacts of climate change.
In summary, we now appreciate that most extinctions involve a synergy of these factors (Brook et al. 2008), with individual causes being difficult or impossible to isolate. These synergies thus represent a situation where the combined effects are substantially more problematic for biodiversity than the mere sum of their individual effects.
We should no longer talk of the ‘evil quartet’ – it is now (at least) the ‘evil sextet’.
- Brook, B.W.; Sodhi, N.S. & Bradshaw, C.J.A. (2008). Synergies among extinction drivers under global change. Trends in Ecology and Evolution 25: 453-460
- Diamond, J.M. (1984). ‘Normal’ extinction of isolated populations In: Extinctions, M.H. Nitecki (Ed.), 191-246, Chicago University Press, Chicago, USA