Is this a mass extinction?

Extinction is the last evolutionary process any species undergoes. Extinction is so common it has been estimated that about 99% of species to ever have evolved on our planet have been lost. This level of extinction is generally balanced by the origination of new species. However, during Earth's history there have been 5 events classified as mass extinctions; a relatively short period of time during which 75% of species are lost. It is argued that the number and rate of species losses we are experiencing currently could be so high that we are entering a 6th global mass extinction. Recently, Anthony D. Barnosky explored whether current biodiversity is trending towards a 6th global mass extinction in 'Has the Earth's 6th mass extinction already arrived?' published in Nature in 2011.

Position in time of the big 5 mass extinctions

To understand whether we are undergoing a mass extinction event already, or if we are on track towards one in the near future, it is important to first examine the definition of a mass extinction. By defining a mass extinction, we can make comparisons between past mass extinctions and our rate of extinctions today.

Jack Sepkoski defined a mass extinction as 'any substantial increase in the amount of extinction (i.e. lineage termination) suffered by more than one geographically wide-spread higher taxon during a relatively short interval of geologic time, resulting in an at least temporary decline in their standing diversity' in 'A Phanerozoic overview of mass extinctions', 1986. In more general terms, a mass extinction event involves a magnitude and a rate, and most palaeontologists classify a mass extinction as the loss of 3/4 of Earth's species in a short geological time.

Estimates for the total number of species on Earth demonstrate a wide range, from 3 million to 100 million. One of the most widely accepted estimates to fall within this range is 8.7 million species, which was proposed by Camilo Mora, et al. in 'How many species are there on Earth and in the Ocean?', 2011. This total number is falling. Stuart L. Pimm predicted a current extinction rate of 20-200 E/MSY (extinctions per million species year), which is 100-1000 times greater than the rate apparent in the fossil record (The Future of Biodiversity, 1995)

As already demonstrated in older blog posts, modern day losses of biodiversity are being worsened by humans; by hunting, clearing habitats and moving species geographically. If these losses are pushing us into a mass extinction event, this could be one of the largest effects of Man on Earth's geology, and a true signature of the Anthropocene.

When trying to compare modern and historical biodiversity, very few groups have adequate records. The fossil record for example is very inconsistent and only few groups of modern animals have been sufficiently assessed, such as mammals, amphibians and birds. Therefore theoretical predictions relating losses of habitat area to losses of species are used. We also have to take into account not only species that have recently gone extinct, but those that are threatened. Barnosky found, using models assuming that all of our threatened species inevitably die out, we could reach big 5 mass extinction levels within 3 centuries. This comes as a warning cry for conservation needs.