Friday, 22 March 2013

Mass extinctions

Ask a person to name a mass extinction event, and chances are very high they’ll come up with the event that eradicated the dinosaurs. Whether the cause was a meteorite impact or not; 65 million years ago, in what is commonly known as the K-T event, a large group of charismatic megafauna vanished. And not just them; an estimated 76% of species (at maximum) got extinct. And that number earns it a place among the “Big Five” extinction events. 

T. rex; probably the most famous victim of the K-T event

Why am I writing about mass extinctions now? About three weeks ago, mass extinction expert Richard Twitchett held his inaugural lecture at Plymouth University. He gave us some insight into how studying the predicaments of ancient ecosystems during mass extinction events can give us some insight into what the modern ecosystems will face in the future. (Do read an article that appeared in the Age on Richard’s work here.)

Biodiversity and extinction since the Cambrian

The K-T event is the most high-profile mass extinction event, but is it also the biggest? The most relevant? The first it certainly isn’t: that honour befalls what’s often called the late Permian ent, which happened ~250 million years ago. No less than 90-95% of species is estimated to have gone extinct. Why? Nobody knows for sure, but many fingers point at epic scale volcanism in Siberia. Fissure eruptions, so big the lavas covered an area the size of Europe, spit out large amounts of greenhouse gases, raising global temperatures. Then add continental weathering, melting of gas hydrates, nutrients ending up in the ocean, ocean warming, and reduced ocean circulation, and you have a maelstrom of interconnected environmental disasters, and, le voila: a mass extinction. 

 The extent of the Siberian Traps; the lavas that erupted ~250 million years ago
So does that mean the late Permian extinction even is the most relevant? That question isn’t really answerable. But I offer another candidate. What about the 6th big one? Look out of the window. Right there, an extinction event is taking place. Think of the dodo, the great auk, the Tasmanian tiger, the golden toad, the quagga, the Cape lion. And for every species of charismatic megafauna there are numerous more obscure species that have vanished, many of which have never even been festooned with a common name. Spare a thought for the Lake Pedder earthworm, the American Chestnut Moth, Blackburns weevil, the Passenger Pigeon and its companion, the Passenger Pigeon Mite; the Hawaii Chaff Flower, the arcuate pearly mussel, the Rubious Cave Amphipod… Only in hindsight will we know if this is the start of a mass extinction event.

A great auk (picture: Mike Pennington)

Comparing modern extinction rates with fossil ones is fraught with difficulty. In the fossil record, you need to fossilise to stand and be counted. Hence the name, indeed. Nobody really knows how many non-fossilising species there have been. And we don’t know how many species there are today. How many have already gone extinct before ever having been discovered? And of many species that have been described, we don’t really know if they are still there or not. And then, of course, it’s not just the rate that counts; it’s also the duration of the event. And if you would assume that sufficient knowledge on taxonomy to be able to say anything useful about extinction rates starts with Linnaeus, that leaves us with less than 300 years of data. We do know that the rate of extinctions we experience now is estimated to be several orders of magnitude higher than the background extinction rate. And we know that if we go on like this, we’ll have made ourselves our very own mass extinction event in a few centuries to a few thousand years. If things get worse we might manage in even less time. Which would make it the fastest mass extinction event in the Earth’s history. Behold man, truly the master of creation!

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