How Do You Have a Mass Extinction Without an Increase in Extinctions?

Roughly 375 million years ago, the number of species on Earth plummeted. But extinction rates remained steady. What changed was that new species failed to emerge.

Middle Devonian silica shale, pyrite fossils (jsj1771/Flickr)

Roughly 375 million years ago, the Earth underwent one of the most mysterious disasters in geological history. Over a period of several million years, the number of species on the planet dropped off by 75 percent. But that's not the weird part -- mass extinctions have wiped out similar numbers of creatures, in even shorter time frames, at least five times in the past 500 million years. What's strange about this particular mass extinction is that it doesn't seem to have been caused by a spike in the number of species dying out. Instead, new species weren't evolving. For hundreds of thousands of years, there were almost no new life forms replacing the ones that were going extinct at a typical rate.

Paleobiologists call this unusual scenario a depression in speciation. It happened at the end of the Devonian period, a geological timeframe when Earth's ecosystems were in such chaos that it was as if evolution ground to a halt. In a healthy ecosystem, you expect to see about one species in a million go extinct every year. You also expect that some species will speciate, or evolve into one or more new species, too. The constant evolution of new species keeps ecosystems diverse, with a relatively stable balance between predator and prey animals, along with many plants and other life forms. But during the late Devonian, biodiversity plummeted. Not enough species were evolving to replace the ones that died out.

So what happened? This is a point of some debate among scientists who study the period, but we have a few compelling pieces of evidence that suggest the problem was not a natural disaster like an asteroid impact or supervolcano. Instead, the proliferation of new life forms, including trees on land and invasive fish in the oceans, may have caused the evolutionary equivalent of a baby bust.

I learned about this while researching my new book, Scatter, Adapt, and Remember: How Humans Will Survive A Mass Extinction. Before we survive, however, we need to understand what destroyed the world in previous eons.

The Devonian was a time when life on Earth was evolving very rapidly. Some paleontologists have called it the "age of fishes," because the seas were filled with huge, rapidly diversifying fish -- some fully plated in armor, and many with long, sharp teeth. Enormous reefs created their own ecosystems that wound in long, sinuous curves across the ocean floors. Most of these reefs were dominated by sponges, and they might have looked a lot smoother than today's coral reef systems. The seas were a big part of continental life too. Massive inland oceans covered large parts of the continents, such as the area we now call the Great Basin in the Americas.

But ocean life wasn't the only game in town. The Devonian was also the period when trees evolved. Plants that had been relatively small and ocean-dependent developed vascular systems, roots, and seeds. This allowed them to grow into giants that survived far from the coastal regions and oceans that had once been the main provenance of Earth's flora. As trees spread all across the continents, their root systems broke up the topsoil and created unprecedented amounts of nutrient runoff into the water -- in some ways similar to runoff from factory farms today. Trees began to change the environment, and then the climate.

There were moments during the Devonian when temperatures fluctuated dramatically, causing sea levels to rise and fall. Paleobotanists believe that the trees may have enriched the atmosphere with so much oxygen that temperatures dropped, causing an ice age that locked up seawater in ice. At that point, sea levels would have fallen and killed off many coastal species. But there were also cycles of hot, greenhouse conditions in which ocean levels rose even as the waters became acidic, killing off all of the reef systems and the lifeforms that depended on them.

New species would be less likely to evolve in habitats where conditions are changing too fast for creatures to adapt to them. Indeed, we are seeing extinctions on Earth for this reason right now: habitat change is causing amphibian dieoffs in the Americas, and ocean acidification is wrecking reef life.

Still, these kinds of climate changes are not enough to explain the dramatic dropoff in species diversity we see at the end of the Devonian period.

One paleobiologist, Alycia Stigall, believes that Devonian sea-level fluctuations may have caused another problem, too: Invasive species. Many of the inland oceans were full of "specialist species," creatures and plants that thrive in very specific habitats. They may only be able to survive when temperatures are in a certain range, or they may eat only one kind of food that lives in just one place. Contained ecosystems, like an inland sea or an island, are usually packed with specialist species. Generalist species, like sharks (whose ancestors lived during the Devonian), are able to live in a wide range of environments and eat many kinds of food.

Stigall believes that it's possible the rising sea levels connected inland oceans for long periods of time, allowing generalist species to move between these contained ecosystems. Soon, every inland ocean would have been packed with sharks and other generalists, who dominated food webs -- the web of connections between predator and prey -- and pushed specialist species to the margins. There was a kind of environmental homogenization. Instead of speciating, or evolving into several new species, the generalists just moved into new environments. And the specialists died out at a natural rate, leaving no evolutionary heirs because their specialist niches had been overrun by the generalists.

The Devonian ended with a slow-motion apocalypse in which life destroyed life. Trees transformed both the landscape and the climate, and as a result invasive species froze the evolutionary processes that kept life diverse in the oceans.

There are a lot of parallels between the world of 375 million years ago and the world of today. Humans have moved many invasive species around, from rats to kudzu, and made it hard for specialist species to survive. Our food webs are unraveling. And a world of homogenous ecosystems is primed for disaster, Stigall warns. Without biodiversity, a healthy mix of specialist and generalist species, a single plant disease could wipe out all the grass in a vast region. Plant loss would be intense. That would kill off grass-eating animals, and in turn kill off the predators that feed on them. We'd be looking at a future scarred by famines, all across the planet's ecosystems.

For scientists like Stigall, problems with invasive species today could be a harbinger of planetary demise on the scale of a mass extinction. Of course, eventually Earth's biodiversity recovered. We know that it's possible to restore ecosystems from near-collapse. The question is whether humans can prevent a Devonian-style depression from hitting again.