The 6th Mass Extinction

In the week of the 28^th July, the New Scientist ran a very interesting article on whether life on Earth is really in crisis. It was called ‘Life on the Brink’ and explored why biodiversity is important and if we are currently in a sixth mass extinction event.

The previous five mass extinctions stretch back 439 million years and it is worth considering each of these to consider the current status. We have fairly good data on previous extinctions in the fossil record. Marine invertebrate are particularly sensitive to extinction level changes and are consistent in the fossil record and so make for a good benchmark on which to measure changes.

The first mass extinction was the Ordovician-Silurian one peak occurring at the end of the Ordovician period and another in the Silurian. It was typified by the massive reduction in sea creatures such as Trilobites, Brachiopods and Graptolites. A reduction of about 85% of all species was thought to have occurred.

The Devonian extinction occurred 367 million years ago in the late Devonian period with 83% of species going extinct. It saw the loss of the armoured fish and the agnathan’s with them being replaced by modern fish.

The late-Permian mass extinction came in 245 million years ago and saw 96% of all species die out. It took place during a time of great continental upheaval. From this, the basal tetrapods were replaced by the Amniotes and allowed seed-bearing plants to become dominant over mosses, ferns and liverworts.

208 million years ago an increase in seismic activity led to extreme volcanic eruptions that had global effects on climate resulting in what we call the Triassic-Jurassic Mass Extinction. This series of extinction events over 18 million years resulted in 80% of all species dying out and led to the diversification of dinosaurs.

The final, most famous and most recent of the big five mass extinctions occurred 65 million years ago called the Cretaceous-Tertiary or K/T event that saw the end of the dinosaurs and pterosaurs along with a total of 76% species going extinct. The winners of this event were us, the mammals that diversified and proliferated greatly.

This brief history of extinction brings us to now and claims that we are living in the sixth extinction event. Extinction is a perfectly normal part of life evolution on earth. It happens regularly even outside of the main events this natural extinction rate is the background rate. It is calculated that if there are 10 million species and the average lifespan of a species is -10 million years then the extinction rate would be about 1 or 2 species every year.  During Mass Extinctions this rate is much higher, to put this in perspective with a high background extinction rate we would expect to see an extinction level in mammals of less than 1 species over 400 years but in fact we are saw 69 species going extinct. This is a strong indicator of mass extinction. Work by Jose Montoya states that the extinction rate is currently 1000x that of the background rate with many more species on the brink.

The problem with measuring extinction is that we do not have an accurate number for the total number of species on the planet, there are 91,000 on the IUCN list but this is a tiny proportion of all species extant on Earth. It is likely that 100’s of species have gone extinct before they have been recorded by science. We are all aware species number and population sizes on the whole are falling, although some species are bucking the trend, these will be the survivors of the extinction level event. A key issue in resolving the debate is the lack of data. Vertebrate taxa are pretty well documented but invertebrates, with the possible exception of the molluscs, is less well documented.

We lack rigid and robust data upon which to tie our theories and this is making it harder to push the agenda. Anecdotal evidence is abound to the reduction in insect splats of car windows in our youth to the lower numbers of butterflies seen. Work is ongoing but progress is slow, should funds be spent on studying the decline or working to combat it?

Perhaps the scariest thought is the cause of the extinction, most scientists agree it is happening, but instead of in previous causes, the cause was a meteor, major climatic change or volcanic eruption this extinction seems to be on us. The extinction rate seemed to accelerate from 1500 as the human population increased in number and spread across the whole planet. Many species have been hunted to extinction and with global warming altering the natural climatic cycle it is likely that many more will follow suit as the rate of evolution cannot keep pace with the selection pressures we are putting on species.

Don’t get me wrong, I m not trying to be all doom and gloom. Life on earth will go on. There will be survivors from the extinction event like last time, and in previous extinction events, recovery tales between 10 and 100 million years as the survivors radiate into all the newly vacated niches. The question is will human be one of the species to survive? The answer is probably yes, we are a resourceful and adaptable species but do we want to have the distinction of being the first species to extinguish the life of others? Given that the Dinosaurs thrived for millions of years it is not an auspicious start to a species as young as ours.