Why this extinction isn’t like the others


Almost a month ago I told you I’d have more to say about Novacek and Cleland’s biodiversity crisis paper.  I’m sure you’re relieved to discover that I’m not a liar.

Last time I wrote about this paper I focused on why we care about biodiversity from an evolutionary standpoint – how big extinction events prune the “tree of life.”  This time I want to talk about what Cleland and Novacek think we can – and can’t – learn from prior big extinctions.

I keep talking about big extinctions and while most of you reading this are probably familiar with the big five, I bet the person who stumbled on this blog by searching for “extinct airplanes” does not.

So, a quick tour of some bad times for life on Earth:

First was the Orodvician-Silurian extinction event about 450 million years ago.  About 49% of animal genera went extinct, probably due to the onset of a really long ice age.  About 100 million years later, 50% of all genera died off in the Late Devonian extinction.  One hypothesis for this extinction event: plant evolution changed environmental conditions way too much, way too fast.  Bet you never realized how dangerous plants are!  The Permian-Triassic extinction happened about 250 million years ago and has a cheery nickname: The Great Dying.  This is the biggest extinction event Earth had ever seen (though we might be causing an even worse one)!  83% of all genera died in this event – even the bugs didn’t escape unscathed.  The most likely explanation for the Great Dying is that the earth went to hell in a handbasket – lots of different things went wrong all at the same time.  Only 50 million years later 50% of species were wiped out in the Triassic-Jurassic extinction event.  Then DINOSAURS RULED THE EARTH until a giant asteroid killed them all in the Cretaceous-Paleogene extinction event about 150 million years later.

As I mentioned in my last post on the Novacek & Cleland paper, we’re in the midst of a major extinction event.  There’s a lot we can learn about what we’re facing from past extinctions, the big lesson being that it takes millions to tens of millions of years for life to rebound.  This is a bit longer than our current political system is set up to think about.  More interesting, and perhaps more useful on timescales people can actually grasp, are the ways this extinction event is different.

The first big difference is the speed of this extinction.  For the most part, mass extinctions are documented over really long time scales (millions of years!) in the fossil record.  But for the current extinction “a period of only 75 to 100 years may be most critical to the transformation of the present biota.”  What might be different about an extinction event that occurs over centuries as opposed to millions of years?  My guess: the difference between an inch of rain every day for a year (this sucks) and 365 inches of rain in a day (we’re fucked).

Today's Mass Extinction

Image by khteWisconsin via Flickr

In past extinctions, the global environment changed in a more or less continuous way.  We’re starting to see global scale ecological change now with climate change, but a lot of the damage (and danger) to species is in how “patchy” we’ve made the world.  Imagine you’re a flower growing in an abandoned field between a town and a big industrial complex.  You grow well and produce seeds that are dispersed by the wind.  Some fall in the field, but others blow further.  None make it past the town or the industrial complex.  The next year, the field is turned into a parking lot.  A few of your seeds sprout in the town in cracks in the sidewalks and such, but they’re so spread out that the specialized pollinators they rely on don’t find them.  And just like that, the entire genetic lineage is wiped out.  What if this was a rare species?  How does this affect the pollinators?  We’ve made the world patchy in other ways, too.  For example, it’s hotter in towns because of the heat island effect.  Some places have really toxic soil and just a few yards away is normal dirt.  Global climate change makes fragmented habitats even more of a challenge as the ideal climate for a species might be on one side of Toronto this year and the other side in 10 years.

Image by Getty Images via @daylife

Perhaps the most important difference between this extinction and past mass extinctions is that we know what’s causing it.  There’s a lot of debate about the causes of past extinctions, but we know without a doubt that humans are responsible for the current one.  Since we know who’s responsible and how it’s happening, that means it is possible for us to do something about it.

Before I talk about what the authors of the paper recommend we do, I’ll go through some of the ways they point out we are most definitely NOT making things better: polluting, overfishing, changing the climate and biogeochemical cycles, introducing invasive species everywhere, fragmenting and destroying habitat, and messing with species communities.

Novacek, M. (2001). The current biodiversity extinction event: Scenarios for mitigation and recovery Proceedings of the National Academy of Sciences, 98 (10), 5466-5470 DOI: 10.1073/pnas.091093698