Extinctions could cut plant productivity in half

by Eve Savory, CBCNews.ca

In a study that has startled even its authors, scientists have concluded that the extinction of plant species can reduce an ecosystem’s ability to support life by up to fifty percent.

It’s called "productivity" and what a plant produces is essential to life on earth: oxygen. That’s not all. Plants produce food in crops, fibre in trees, clean water, and biofuels. And they suck the greenhouse gas carbon dioxide out of the atmosphere.

"Nature does things that support humanity that allow us to exist", says Bradley Cardinale, the lead author. "The erosion of all those services is going to come back to bite us."

In the study, published this week in the Proceedings of the National Academy of Sciences, the international group of researchers looked at 44 studies that had simulated species extinction in grasslands, estuaries and the tundra. What they found was that even if the dominant species survived, the ecosystem as a whole did its’ job less efficiently.
Michel Loreau of McGill University said "diverse communities are more productive because plants are ‘complementary’ in how they use biological resources. In other words, different plant species play unique roles." The scientists say it’s like soccer. To succeed, the star needs her supporting players. Translate that into an ecosystem, and it means the plant species that survive produce less biomass, or plant matter.

"We’re talking about looking out into a forest or a grassland or any sort of natural habitat and there being fifty percent less plant matter - you know, half. Half of all the plant life." says Cardinale, a biologist at the University of California at San Diego.

The unhappy equation: Fewer distinct species = Less biomass = Less oxygen, fibre, and food. And less carbon dioxide being scoured from the atmosphere.

The rate of extinction is so fast, researchers estimate half of all species will be gone by the end of this century. But until now many scientists, including Cardinale, have argued that as long as the dominant species – the great fish, the Douglas fir – can be managed, the ecosystems would thrive and be productive. Oxygen would not decline, and there would be no increase in carbon dioxide in the atmosphere.

No longer. What’s more, the group is convinced a fifty percent loss of productivity is not a threshold. "It’s going to prove much, much greater than that when we let these studies run out their full course", says Cardinale. While a few years is fine to understand a grassland, it could take a century to see the full impact of species loss in a forest.

He believes the findings represent a general rule: that as the supporting species in any ecosystem from ocean to forest decline, so will the productivity of the planet. "This can’t be ignored", he says. "We are in a period of catastrophic change. Whether or not that matters for humanity, I think is still a question. Our study is suggesting that maybe it does."