Since the first Homo sapiens emerged in Africa roughly 300,000 years ago, grasslands have sustained humanity and thousands of other species. But today, those grasslands are shifting beneath our feet. Global change -- which includes climate change, pollution and other widespread environmental alterations -- is transforming the plant species growing in them, and not always in the ways scientists expected, a new study published Monday revealed.
Grasslands make up more than 40 percent of the world's ice-free land. In addition to providing food for human-raised cattle and sheep, grasslands are home to animals found nowhere else in the wild, such as the bison of North America's prairies or the zebras and giraffes of the African savannas. Grasslands also can hold up to 30 percent of the world's carbon, making them critical allies in the fight against climate change. However, changes in the plants that comprise grasslands could put those benefits at risk.
"Is it good rangeland for cattle, or is it good at storing carbon?" said lead author Kim Komatsu, a grassland ecologist at the Smithsonian Environmental Research Center. "It really matters what the identities of the individual species are....You might have a really invaded weedy system that would not be as beneficial for these services that humans depend on."
The new paper, a meta-analysis published in the Proceedings of the National Academy of Sciences, offers the most comprehensive evidence to date on how human activities are changing grassland plants. The team looked at 105 grassland experiments around the world. Each experiment tested at least one global change factor -- such as rising carbon dioxide, hotter temperatures, extra nutrient pollution or drought. Some experiments looked at three or more types of changes. Komatsu and the other authors wanted to know whether global change was altering the composition of those grasslands, both in the total plant species present and the kinds of species.
They discovered grasslands can be surprisingly tough -- to a point. In general, grasslands resisted the effects of global change for the first decade of exposure. But once they hit the 10-year mark, their species began to shift. Half of the experiments lasting 10 years or more found a change in the total number of plant species, and nearly three-fourths found changes in the types of species. By contrast, a mere one-fifth of the experiments that lasted under 10 years picked up any species changes at all. Experiments that examined three or more aspects of global change were also more likely to detect grassland transformation.
"I think they're very, very resilient," said Meghan Avolio, co-author and assistant professor of ecology at Johns Hopkins University. "But when conditions arrive that they do change, the change can be really important."
To the scientists' surprise, the identity of grassland species can change drastically, without altering the number of species. In half the plots where individual plant species changed, the total amount of species remained the same. In some plots, nearly all the species had changed.
"Number of species is such an easy and bite-sized way to understand a community...but what it doesn't take into account is species identity," Avolio said. "And what we're finding is there can be a turnover."
For Komatsu, it's a sign of hope that most grasslands could resist the experimentally induced global changes for at least 10 years.
"They're changing slowly enough that we can prevent catastrophic changes in the future," she said.
However, time may not be on our side. In some experiments, the current pace of global change transformed even the "control plots" that were not exposed to experimentally higher global change pressures. Eventually, many of those plots looked the same as the experimental plots.
"Global change is happening on a scale that's bigger than the experiments we're doing....The effects that we would expect through our experimental results, we're starting to see those effects occurring naturally," Komatsu said.
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