Do Mass Extinctions Increase Diversity, and if so, how?

blackbeauty

Over the past half-billion years, the Earth has been hit again and again by mass extinctions, wiping out most of the species on the planet. And yet every time, life recovered, and ultimately went on to increase in diversity. Is life just incredibly resilient, or is something else going on: could extinctions actually have helped life diversify and succeed? And if so, how?

Mass extinction is probably the most striking pattern seen in the fossil record. Vast numbers of species disappear, rapidly, simultaneously, and around the world. Driving extinction on this scale generally requires some kind of global environmental change, both so severe and so rapid that species disappear, rather than adapt to it. Most extinctions seem to be driven by catastrophes. Massive volcanic eruptions drove the extinctions at the end of the Devonian, Permian and Triassic. Severe glaciation drove the Ordivician-Silurian extinctions. One, the end-Cretaceous extinction of the dinosaurs, was driven by an asteroid. These “Big Five” extinctions get the most attention because, well, they’re the biggest, but other minor, but still civilization-ending events occurred as well, like the pulse of extinction that preceded the end-Permian event, and the Cenomanian-Turonian event that wiped out spinosaurs and carcharodontosaurs among the dinosaurs.

Sepkoski curve.jpg

These events were indescribably destructive. The Chicxulub asteroid impact that ended the Cretaceous shut down photosynthesis for years and led to decades of global cooling. Anything that couldn’t shelter from the cold, or find food in the darkness- which was most species- perished. Dinosaurs and pterosaurs went extinct, as well as most birds, mammals, lizards, snakes, plants, even many insects. Extinctions were equally severe in the sea, with plesiosaurs, mosasaurs, and ammonites disappearing; many invertebrates and even many plankton and other single-celled organisms, like foraminifera, were hit hard. Perhaps 90% or more of all species disappeared in just a few years.

But life bounced back, and the recoveries were rapid. Over 90% of mammal species were eliminated by the asteroid, but they recovered, and then some, within 300,00 years, then went on to evolve into horses, whales, bats, and our primate ancestors. Similarly rapid recoveries and radiations were seen in birds, lizards and fish. Much as the first finches to reach the Galapagos Islands diversified to occupy the vacant niches, the survivors of the extinction found a largely empty world, and diversified to fill it.

This pattern of recovery and diversification occurs after every mass extinction. The end-Permian extinction saw mammal-like species take a hit, but reptiles flourished afterward. When the reptiles were hit during the end-Triassic event, the surviving dinosaurs diversified and took over the planet. Although a mass extinction ended the dinosaurs, they only evolved because of mass extinction in the first place.

Despite this seeming chaos, life has slowly diversified over the past 500 million years. In fact, several observations hint that extinction might actually drive diversity. For one, the most rapid periods of diversity increase occur immediately after a mass extinction. But the recovery isn’t just driven by an increase in the number of species. During a recovery, animals innovate, finding new ways of making a living.

They exploit new habitats, new foods, new modes of locomotion. Our fishlike forebears first crawled onto land after the end-Devonian extinction. Giant dinosaurs, larger than anything that had existed before, appeared after the end-Triassic extinction. And when those dinosaurs then disappeared, whales took to the sea, bats to the sky, horses started eating plants, and our primate ancestors became specialized for life in the trees. Extinction doesn’t only drive speciation, it drives innovation. It’s not a coincidence that the biggest pulse of innovation in life’s history- the appearance of complex animals in Cambrian Explosion- occurs in the wake of the extinction of the Ediacaran animals that went before them.

Innovation may increase the number of species that can coexist because it allows species to move into new niches, instead of simply fighting over the old ones. Fish crawling onto land didn’t compete with fish in the seas; bats hunting in the dark with sonar didn’t compete with day-active birds. Innovation means diversity isn’t a zero-sum game; species can diversify without driving others extinct. So why does extinction drive innovation?

It may be that stable ecosystems hold back innovation. A modern wolf is probably a far more dangerous predator than a Velociraptor, but a tiny mammal couldn’t evolve into a wolf in the Cretaceous because there were velociraptors. Any experiments in carnivory would have ended badly, with unspecialized mammals competing with- or simply eaten by- the well-adapted Velociraptor.

But the extinction of that Velociraptor gives the mammal the freedom to experiment with new niches. Initially, mammals were poorly adapted to a predatory lifestyle, but without dinosaurs to compete with or eat them, they didn’t have to be very good to survive. They flourished in an ecological vacuum- ultimately becaming large, fast, intelligent pack hunters. In the lull after an extinction, evolution may be able to experiment with designs that are initially poorly adapted, but which have long-term potential. With the show’s stars gone, evolution’s understudies finally get their chance to prove themselves.

Life will even recover from the current wave of human-induced extinctions, perhaps reach even higher diversity given time. That’s not to justify complacency- it will take millions of years.

Economists talk about creative destruction, the idea that creating a new order drives the destruction of the old one. But evolution hints there’s another kind of creative destruction, where the breakdown of the old system creates a vacuum, and destruction actually precedes and drives the creation of something new, and often better.

This idea has relevance to human history. The extinction of the Pleistocene megafauna must have decimated hunter-gatherer bands, but it may have given farming a chance to develop. The Black Death produced untold human suffering, but the shakeup of political and economic systems may have led to the Renaissance.

That’s perhaps worth bearing in mind given our current political and social upheavals. The loss of something old creates opportunity for  new things to take root. Every cloud has a silver lining, even the debris cloud from an asteroid impact. It’s not just that there are opportunities when things are bad. It’s that when things are at their worst, is precisely when the opportunity is the greatest.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s