The Big Five, and a Sixth?
This is my last blog post before I conclude and I thought it would be useful to compare the late Pleistocene megafauna extinction to modern extinction rates. Indeed, the latter is so severe that there has been concern that we are causing the ‘6th mass extinction’ through habitat destruction, killing of species, changing global climate and introducing non-native species.
There have been 5 mass extinctions in the past: near the end of the Ordovician, Devonian, Permian, Triassic and Cretaceous Periods. A mass extinction is defined as ‘having extinction rates spikes higher than in any other geological interval of the last 540 million years and exhibiting a loss of over 75% of estimated species’ (Barnosky et al 2011: 51).
A discussion on the evidence surrounding the 6th mass extinction is beyond the scope of this blog; Xijia has an interesting blog here on this topic.
Modern Extinction Rates
This paper by Barnosky et al (2011) gives a bit more insight on how extinction rates are calculated. Although this paper is primarily about the 6th mass extinction, it does provide a comparison figure for the Pleistocene extinction event. A widely-used metric in this field is extinctions per million species-years (or E/MSY). The ‘natural’ background rate of extinction is 1 E/MSY, i.e. if there were 1 million species on Earth, one would go extinct per year. Rates are estimated using fossil extinctions that occurred in million-year time bins. Current rates are projected to a million years, as current extinctions have occurred within very short timespans (a few decades to hundreds of years). It is important to note that extrapolation may introduce inaccuracies, and the shorter the time intervals, maximum ESY and its variance increase.
Using a paleontology database combined with lists of recently extinct species, the most complete set of which are available for mammals, the results are as follows:
The extinction rates observed for the past 1,000 years (24 E/MSY in 1,000 year time bins – 693 E/MSY in one-year intervals) is much higher than the maximum late Pleistocene extinction rate (9 E/MSY) and definitely higher than the background rate.
Clearly we are in the midst of a very severe biodiversity crisis and losing species much faster than they can be replaced. Given the already severe loss of biodiversity that was the late Pleistocene megafauna event (with the result that our planet is already a depauperate version of its former self in terms of biodiversity), this is a major cause for concern. While some have attempted to put an economic value on biodiversity (See Ali's blog), as we have seen biodiversity is intrinsically important in maintaining the balance of ecosystems, which are highly complex and interdependent systems.
Barnosky, A. D. et al (2011) ‘Has the Earth’s sixth mass extinction already arrived?’, Nature, 571, pp. 51-57