The sixth mass extinction
By: Ragnhild Gya
Whether we are going into the sixth mass extinction or not has been heavily debated. In 2015 a paper was published by Gerardo Ceballos and his colleagues were they state that the sixth mass extinction is happening, and that we are causing it. I am now going to try and explain their arguments the best I can, you can also read the article yourself (reference in the end of this blog entry).
Extinctions happen all the time. Did you know that over 90 % (and maybe closer to 98%) of all species that has lived on Earth is now extinct? Whether we have a net gain or a net loss of biodiversity and species depends on the rate of occurring new species and the rate of extinctions. This “normal” amount of species going extinct is called the background rate of extinctions. This background rate stands for most of the extinctions in the history of the earth; actually the mass extinctions can only be blamed for 4 % of all extinctions in the last 600 million years.
When the extinction rate is much higher than the background rate, it is classified as a mass extinction. The most famous one is the one that killed the dinosaurs, 65 million years ago, which is called the Cretaceous-Paleogene extinction event. The one that killed the most species is the Permian-Triassic extinction event, which made 95 % of marine species and 70 % of species on land go extinct. Are we also now in a period similar to these two?
To figure this out scientist estimate the normal background rate of extinctions from fossil records, written observations etc. and compare that to a number that they have calculated for our current extinction rate. A lot of scientists have published papers where they conclude that we are in a mass extinction because of the results of these calculations. Ceballos and his colleagues discussed the fact that the background rates were calculated from marine invertebrates and that the background rate would probably be higher if we considered other groups as well. So instead of using the most used background rates which are between 0.1 and 1 extinction per 10 000 species per 100 years (0.1-1 E/MSY), they used 2 E/MSY. They more than doubled the previous number for the background rate, which should make it harder to get a result that says that we are in the sixth mass extinction if we actually are not.
To find the modern extinction rate, the used the IUCNs (International Union of Conservation of Nature) list over threatened and extinct species. They have three “extinct” categories: “extinct” (EX), “extinct in the wild” (EW) and “possibly extinct” (PE). They calculated using two different approaches: the highly conservative one with only using numbers from the category extinct, and the conservative one using all three categories.
You can see the results of these calculations in the table underneath. The numbers are how much higher the current extinction rate is compared to the background rate. As you can see, it varies from 8 to 100, and this is using the conservative method to calculate it. As a conclusion we are now no longer in doubt about this period – it is the sixth mass extinction.
Table: Ceballos et al. (2015)
Just to remind you what we are losing, here are some species that have gone extinct in the past 100 years:
Extinction 1: The last Thylacine in captivity, pictured above, died in 1936.
Extinction 2: Wester Black Rhino. Once widespread in central west Africa, in 2011 it was declared extinct.
Extinction 3: Golden Toad. Only described to science in 1966, and once abundant in a 30 square mile area of the cloud forest above Monteverde, Costa Rica, none of the toads have been sighted since the 15th of May 1989
References
Kevin J. Gaston and John I. Spicer (2004). Biodiversity – an introduction. 2nd ed. : Blackwell Publishing. p19-48.
Gerardo Ceballos, P. R. Ehrlichm A. D. Barnosky, A. García, R. M. Pringle, T. M. Palmer. (2015). Accelerated modern human-induced species losses: Entering the sixth mass extinction. Science. 1 (5), .
https://en.wikipedia.org/wiki/Extinction_event (24/02/2016)
Pictures:
http://www.treehugger.com/slideshows/natural-sciences/glimpse-what-we-lost-10-extinct-animals-photos/ (24/02/2016)
https://www.flickr.com/photos/martijnmunneke/6521931525/sizes/l/in/photostream/ (24/02/2016)
https://en.wikipedia.org/wiki/File:Bufo_periglenes2.jpg (24/02/2016)
It is unfortunate to lose any species. Our species conservation works are far behind the exploitation of them. We already extinct more than 400 species after establishment of the first national park (Yellow Stone National Park in 1872).