Following on from the videos I posted last week, my post today is going to focus on a recent paper by Barnosky et al. (2011). Barnosky provides a detailed background to previous mass extinctions and also focuses on arguments surrounding the possibility of a sixth mass extinction.
What is a mass extinction?
The answer lies in the past. Palaeontologists characterise a mass extinction as a short geological period in Earth's history where more than 75% of estimated species become extinct (Jablonski, 1994). This type of extinction has only happened 5 times in the past 540 million years (Barnosky et al. 2011). The 5 mass extinctions occurred at the end of the Ordovician, Devonian, Permian, Triassic and Cretaceous periods. Will the anthropocene hold the next mass extinction?
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| Biodiversity of the planet over time, showing the five mass extinctions in history. Graph adapted from Wilson (1992) |
Why do we care about past extinctions?
Scientists are interested in past extinctions because they provide a key to the future. The most famous species extinction was the Cretaceous- Tertiary which occurred 65 million years ago. This is the extinction that is famed for the death of the dinosaurs. One hypothesis for this decline was climatic cooling causing a reduction in hospitable habitats (this is the worry for current global warming). The Permian extinction (248 million years ago) also causes scientists to quake in their boots due to being caused mainly by global environmental change, including methane release from the sea floor, increased sea level and a shift in ocean circulation (Eldredge, 2011). The main concern is that history will repeat itself? Increasingly, academics are recognising contemporary species extinctions as part of an overarching 'sixth mass extinction'. Anthropogenic influences such as habitat fragmentation, disruption by invasive species and changing global climate all directly contribute to the decline of biodiversity. Scientists are now worried that the Earth will, in a few centuries, be under threat from major species decline.
Before we can come to any sort of conclusions, it remains vital to establish the current situation within the context of previous mass extinctions. There have been many landmark studies that have told of modern extinction rates an order of magnitude higher than than previous extinction rates (Doubleday, 1992; May et al. 1995). The below chart explores the extinction magnitudes of IUCN assessed taxa in comparison to the 75% benchmark (International Union of Conservation Red List, 2010). The black icons add the species that are currently 'threatened' species to those that have been extinct for over the past 500 years. Therefore, we can see that all species are threatened in the age of the anthropocene. Groupings such as mammals, birds and amphibians, which have had historically the lowest percentages of species extinctions, nowadays have the highest numbers of endangered and threatened species. Could these species be susceptible to catastrophic extinctions? The current numbers show the sixth mass extinction is looming quite far away, however anthropogenic activities are causing more and more species to become increasingly threatened. Barnosky et al. (2011) suggests that if all 'threatened' species disappeared we would be half way towards a possible anthropocene mass extinction.
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| Extinction magnitudes of IUCN assessed taxa (2010) |
It is not just terrestrial species that are vulnerable, marine species are particularly susceptible to mass extinction. At least 830 marine species have been classified as critically endangered, endangered or vulnerable. Assessments for marine species are lagging behind those for terrestrial species. However, the IUCN aimed to have 20,000 marine species assessed by 2012. It is important to understand that the past drivers of extinction in the oceans are the same as the current threats (as can be seen in the chart below). Marine scientists that released the State of the Ocean 2013 report on 3rd October 2013, gave a warning stating 'we are entering an unknown territory of marine ecosystem change, and exposing organisms to intolerable evolutionary pressure. The next mass extinction event may have already begun'. The report explains how the ocean is the world's largest carbon sink and the increasing carbon dioxide levels are creating a 'deadly trio of impacts' including ocean acidification, ocean warming and deoxygenation all of which are threatening marine biodiversity. If you want to know more about this , check out The AnthropoSea, a blog dedicated to investigating the 'deadly trio' and its threat to marine biodiversity.
What is the situation today?
Today, the rapidly changing atmospheric conditions and warming of above average interglacial temperatures cause species to become vulnerable. With rising carbon dioxide levels, habitat destruction, pollution, overfishing, over hunting and invasive species, the world now has more ecological stressors than many species have ever experienced before.
Species extinctions rising with increasing human populations.
The worrying question is how will this affect the already threatened species? It seems quite plausible that if humanity carries on living in the same way, it will not be long before the 'sixth mass extinction' is just around the corner. It is encouraging that there is still much of Earth's biodiversity that can be saved. However, it is daunting that in order to save such species there must be rapid reversal of escalating threats. How realistic is this?
While I was researching for this blog post I came across a very interesting short film that had just been released called the 'Last Hours'. The film had been produced for presentation at COP21 (the 21st session of the Conference of the parties to the UNFCCC expected to take place in 2015). The film maker was Thom Hardmann, an American radio talk show host and best selling novelist. I watched the film and I have to say although highly dramatised, with a soundtrack that could rival 'the day after tomorrow', it provides an insight into the possibility of another mass extinction. The film has caused a media outpour, with the likes of Leonardo DiCaprio tweeting his affection for the film (thats the reason we know it is worth a watch!).
However, the film has also received criticism for its methane hydrates hypothesis. In the recent IPCC 2013 report, many of the catastrophic changes being forecast were described as likely, unlikely or exceptionally unlikely. The melting of permafrost and the subsequent release of huge reservoirs of methane is deemed exceptionally unlikely, therefore the film has to be taken with a pinch of salt. Why not have a look and make your own judgements.
Do you think we are in a sixth mass extinction, or are we very close to entering one?
To end this post here is just a snapshot of some news articles that have documented species extinctions and the catastrophic possibility of another mass extinction.
Ananthaswamy, A (2004) 'Earth faces sixth mass extinction', New Scientist.
Harvey, F (2013) 'Rate of ocean acidification due to carbon emissions is highest for 300 million years', UK Guardian.
IUNC (2009) 'Wildlife crisis worse than economic crisis', IUNC.
Pappas, S (2012) ' Earth's ecosystems nearing catastrophic "tipping point" warn scientists', The Christian Science Monitor.
Score board update.
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