A Sixth Mass Extinction?

There is much debate as to whether a sixth mass extinction event is currently underway. A recent paper by Barnosky et al (2011) argues we are now irrevocably in the midst of species loss which could rival that of the ‘big 5’. Papers by Rockstrom et al (2009), Raup and Sepkoski (1986) and Wakeand Vredenberg (2008) also discuss this issue, and will be used to detail some of the complexities of the ‘sixth’ mass extinction.

·       Comparison with ‘normal’ extinctions. Under normal background extinction conditions, those taxa that go extinct tend to be from small populations in restricted geographic ranges. Hence if certain species can be seen to decline rapidly in number, large creatures in particular, then extinction selectivity may be changing to enter a mass extinction state. Although 99% of all species that have ever lived on earth are now extinct, this species loss is usually balanced by speciation. Given the current rates of species loss, however, teamed with the fact that evolution of new species takes many thousands of years and revival from mass extinction spans millions of years, no meaningful biodiversity recovery is likely to take place in our lifetime (Barnosky et al 2011).

·       Estimations of species loss. Species-area relationships can be used to relate species losses to habitat area losses, these suggest that future species extinctions will be between 21 and 52% of all current species. Major problems can arise because most species have not yet been formally described; fossil remains are biased and incomplete, not all species fossilize well, if at all, and fossil analysis is often carried out at genus rather than species level which can lead to species being lumped together. Thus, estimations are likely to be under-representative as if one species in a genus becomes extinct, the genus as a whole will remain relatively untouched.

·  Approaches to reconstructions. Using an E/MSY, extinctions per million species years, approach, Barnosky et al (2011) observe that current extinction rates are notably higher than both background rates and those of the previous half-millennia. Alarmingly, comparison of historical and recent extinction rates using a 500-year rate approach has shown that if all threatened species went extinct within the next hundred years, bird and mammal extinction would take somewhere between 240 and 500 years to match the level of the big five extinctions, and in 2,265 years 75% of species would be lost. Hitherto, there have been discrepancies in the assessment of species loss through the use of rate and magnitude, and questions have been raised as to effectiveness of extrapolation of extinction rates of well-studied taxa. These rates are highly dependent on the length of time for which they are measured, so current short-term extinction measurements are not accurately comparable with long-term data. A solution to this is the estimation of species extinction rates by maximising fossil background rates and minimising current extinction rates, and the use of combined rate-magnitude comparisons.

Figure 1. Current extinction magnitudes, expressed as percentage of species. This illustrates the severity of extinctions within taxonomic classes, and suggests that extinction levels comparable to those of the big 5 are not far off. Source: Barnosky et al (2011).

·       Climate models. Climate change has progressed to the extent that some Earth systems have now exceeded their stable Holocene state. This has resulted in retreat of summer sea ice in the Arctic Ocean, retreat of mountain glaciers, loss of mass from Greenland and Antarctic ice sheets and rapid rates of sea level rise. Climate models don’t take long-term feedbacks into account, however, so may underestimate the severity of long-term climate change by ignoring considerable threats to ecological life support systems.

Conclusion

Although extinction is a necessary part of evolution, species losses are occurring at a much higher rate than is considered normal. This topic requires further discussion, and the focus of the next post will be the extent to which humans are to blame for the species loss that is arguably underway.

References

Rockstrom et al (2009) ‘A safe operating space for humanity’, Nature, 461, 472-475.

Barnosky A.D., Matzke N., Tomiya S., Wogan G.O.U., Swartz B., Quentel T.B., Marshall C., McGuire J.L., Lindsey E.L.,    Maguire K.C., Mersey B., Ferrer E.A. (2011), ‘Has the Earth’s sixth mass      extinction already arrived?’, Nature, 471, 51-57

Wake D.B. and Vredenberg V.T. (2008) ‘Are we in the midst of the sixth mass extinction? A view from the world of amphibians’, Proceedings of the National Academy of Science of the United States of America, 105, 1, 11466-11473.

Raup D.M. and Sepkoski J.J. (1986) ‘Periodic extinction of families and genera’, Science, 231, 4740, 833-836.