Jun 02 2017

The Sixth Extinction

ExtinctThere have been five major extinctions since the evolution of multicellular life on earth. These are events marked by a geologically rapid loss of biodiversity, the most dramatic of which was the end Permian extinction 245 million years ago with 90% species lost. The other four events had between 19-30% species loss. It is interesting and scary to think how close we came to total extinction in the end-Permian event.

The causes of these extinctions vary. Events 1 and 4 were due to volcanic activity. Events 2 and 3 are uncertain but perhaps causes by climate change due to tectonic plate activity. The fifth extinction, the one that wiped out the non-avian dinosaurs 65 million years ago, was mostly due to an asteroid impact.

Many scientists believe we are in the midst of a sixth great extinction event, caused by human activity. It is always difficult to tell when you are in the middle of such an event – it is easier to discern from a perspective after the event is completed. But it’s pretty clear humans are having a dramatic effect on the ecosystems of the world and other species are paying the price.

There have actually been several books titled: The Sixth Extinction. The first, which I read many years ago, is by Niles Eldridge.  Most recently, in 2014, Elizabeth Kolbert published a book: The Sixth Extinction, An Unnatural History.

Niles Eldridge relates the alarming statistic:

There is little doubt left in the minds of professional biologists that Earth is currently faced with a mounting loss of species that threatens to rival the five great mass extinctions of the geological past. As long ago as 1993, Harvard biologist E.O. Wilson estimated that Earth is currently losing something on the order of 30,000 species per year — which breaks down to the even more daunting statistic of some three species per hour. Some biologists have begun to feel that this biodiversity crisis — this “Sixth Extinction” — is even more severe, and more imminent, than Wilson had supposed.

I always like to do a deep dive to investigate any critical numbers, like number of species lost per year. Typically the real answer is complicated and there is a range of estimates based on various assumptions. The 30k species per year is not documented, but estimated, and is at the upper range of estimates. We need to estimate because we haven’t catalogued every species on Earth. We’re not even sure how many there are (another difficult number to get at, but 10 million species is an average estimate). It is also difficult to know when a species has gone extinct – how certain do we have to be that there isn’t a small population out there?

At the more conservative end of the spectrum it is estimated we are losing about 8 thousand species per year. Another way to look at this is the rate of extinction compared to the background. Species go extinct all the time as part of the natural turn over. However, scientists have estimated that currently species are going extinct at between 10 and 100 times the background rate.

However, as pointed out by Eldridge decades ago, many species that are not currently extinct will very likely go extinct over the next thousand years. With habitat loss few species go immediately extinct. Rather, they simply move to a nearby similar habitat. This means, however, we have many more species crammed into a small area, with far fewer numbers. When a population falls below 2,000 individuals, they are extremely likely to go extinct within the next thousand years. They are highly vulnerable to extinction, partly because their loss of genetic diversity will make them more vulnerable to stresses. So while existing species loss may be lower than estimated, long term species loss will likely be much higher.

A recent paper in Nature takes another look at the notion of a sixth extinction. The explore which human activities are contributing to species loss, what the impact will likely be, and what we can do about it. The causes are no mystery. Primary among them are habitat loss.

About 40% of the Earth’s land mass is now covered in farmland. We have also decimated tropical forests around the globe, including 20% of the Amazon rainforest. Urban sprawl claims more land. We are getting to the point of going from wilderness dotted with civilization to civilization dotted with wilderness.  This means that islands of natural ecosystems are increasing isolated, making it more difficult for animals to migrate or to keep away from the dangers of humanity, such as roads.

We also have introduced invasive species around the world. Invasive species, without natural predators, can decimate native species. Over hunting and fishing is also taking its toll on some species. And finally, climate change is adding more stress to ecosystems.

This much is already well known and nothing new. The authors of the Nature paper also point out that there is a huge economic benefit from natural biodiversity to human. Wood, fish, hunting, and grazing land have an economic benefit. At present, they estimate, the world derives ten times the economic benefit from natural biodiversity than we spend on conservation.

Here is the good news in all of this. If it is true that most species stressed by human activity have not yet gone extinct, even though their range and populations have been drastically reduced, this means we still have a window of opportunity to prevent much of that loss. We don’t even have to make any sacrifices. In fact, preserving biodiversity can have a net economic benefit.

We just need to use the Earth smarter. This means looking carefully at the footprint of humanity on this planet and thinking of ways to reduce it. It further means carefully tracking the effects of human activity on species and taking steps to mitigate it. Preserving migration corridors, for example, is a simple method of minimizing our impact on other species.

Optimizing the efficiency of our farmland use is also critical (see my recent article on why organic farming is bad for the environment, for example). Minimizing the introduction of invasive species is important. Improving the quality of life in developing nations (especially for women) is also helpful, since this correlates strongly with stabilizing populations.

And, of course, not rapidly changing the environment would help. And again, this doesn’t have to cost anything. Cheap, renewable, clean energy (believe it or not) is actually a good thing. Energy efficiency saves money. It’s all just a matter of priorities.

Perhaps more than any, our generation are the stewards of the sixth extinction. The decisions we collectively make now will have a dramatic impact on how extensive this extinction is.

 

 

38 responses so far