The Devonian Death of Diversity

The guy I married is an agronomist. His passion is grass--and all that threatens native grasslands. He's had a lot to worry about, from cows to climate change. But his focus has been, and remains, on the invasives that threaten his beloved bunchgrasses.

Even at age 85, Dave's passion to fight this botanic plague remains undiminished. But for many of us, fighting invasive species seems, well, so 20th century. With a tattered economy, mortgage forclosures, epochal blizzards and a winter deep freeze, who has time to fret about knapweed.

Well, science--and this time it's geology--has given us a reason to take-up the hoes and weed-wackers once again. And this news is as chilling and disquieting a message as geologists can muster.

You might remember from your Geology 201 class that there was a time period known as the Devonian. It is known for giant fish, odd slimy ferns that were the first large land plants, land-pioneering amphibians, and abundant shellfish. It's also known for a major extinction that occurred between 375 and 378 million years ago, eradicating giant fish, lots of mollusks, and making the slimy ferns and amphibians glad they had escaped to land.

But we never really understood the Devonian extinction--the second major extinction event out of only five that have gripped the planet. No big meteorite whacked us. There were no cataclysmic eruptions. Things just faded, like life on a rheostat. Species disappeared. But, unlike a healthy planetary ecosystem, nothing appeared to replace them. It was actually a biodiversity crisis, more than an extinction event, said Alycia Stigall, a scientist at Ohio University.

Stigall took a closer look at the Devonian. She found a world in which invasive species, especially marine species, dominated every niche. The invasive species, the globalized, generic organisms, (the biologic equivalents of WalMart) were so prolific, and seemingly hardy, that they obliterated the opportunities for new, adaptive, innovative species to arise when upheaval or climate change modifies the ecological landscape. (Economists, take note, too...)

"The main mode of speciation that occurs in the geological record is shut down during the Devonian," said Stigall. "It just stops in its tracks."

The consequences? When one of these globalized marine species ultimately failed, the entire marine ecosystem began to unravel, taking other widespread invasives with them, until the seas lost about half of the genera. Coral reefs vanished globally. Giant jawed fish faded from the ocean. Half or more mollusks disappeared. Life in the sea was impoverished, and hanging on by a thread.

Here in the 21st century, 375 million years later, the situation may not seem so dire. But we ARE loosing biodiversity. My bird feeder is full of English sparrows--and so is every other bird feeder in North America. Dave's grasslands harbor cheatgrass, medusa head, knapweed, and tens of other generic invasives that are the sworn enemies of native, niche-filling biodiversity. Some bio-pundits have suggested that invasives would not only fill the native's ecological niches, but ultimately, diversify and evolve into new species. But the evidence from the Devonian argues against this. And indeed, I know of no new knapweeds that we can declare North American natives.

Not that I want to be unduly alarmist in this fading holiday season. Ecosystem collapse? Well, probably not by 2012 and not in my lifetime. But given this new understanding of ecosystem collapse, I'm not putting my hoe away just yet.