Researchers have long debated the consequences of introducing non-native species into ecosystems. Recently, these debates have centered upon the effects of invasive exotics, and dramatic pictures of grasslands filled with leafy spurge, water pipes clogged by zebra mussels, and forest trees killed by kudzu vines have fostered the public's understanding of the issue. But now, two Canadian scientists are suggesting that even the introduction of some less aggressive species may have far-reaching negative repercussions.
Janice Christian and Scott Wilson studied crested wheatgrass (Agropyron cristatum) to test a controversial hypothesis: that the consequences of introducing exotic species can extend far beyond the simple displacement of native plants and animals. Their work, published in the October issue of the journal Ecology, suggests that crested wheatgrass has precipitated a decline in soil quality and may have increased the amount of carbon dioxide in the atmosphere.
Crested wheatgrass (sometimes called fairway wheatgrass) has wrested the soil from native grasses on much of the northern Great Plains, but its conquest has seemed relatively harmless to the larger environment. The grass was originally brought to the Americas from Siberia during the drought-filled years of the Dust Bowl, when farms were failing and the local prairie grass was producing scant fodder for cattle. Farmers discovered that the wheatgrass was hardy, produced good hay, resisted drought and overgrazing, and had a long growing season. It was so successful that ranchers and government agencies still continue to spread it (and several close relatives) enthusiastically from the Great Basin to the Northern Plains. It currently blankets 25 million acres of prairie in North America.
To determine how crested wheatgrass affects the prairie ecosystem, Christian and Wilson compared hundreds of samples of grass and soil from Canada's Grasslands National Park in southwestern Saskatchewan. The park is a mosaic of undisturbed prairie and 50-year-old fields of the introduced wheatgrass.
After diligently cleaning, weighing and analyzing their samples, the scientists found that the soil beneath crested wheatgrass contained significantly less nutrients and organic matter than the soil under native prairie. They attribute this to the growth strategy of the different grasses. The wheatgrass devotes most of its energy to producing aboveground shoots, while maintaining only a meager root system. Conversely, the native grasses do not grow as tall, but instead develop an extensive network of roots. Previous studies have shown that roots play a more important role than do shoots in enriching the ground with nutrients and organic matter.
One of the interesting implications of the study is that the conversion of native prairie to wheatgrass may have eliminated an important storage area for atmospheric carbon. The researchers reason that if native prairie grasses put more nutrients into the soil and plant tissue, they are effectively tying up carbon that would otherwise be in the atmosphere. When carbon is in the atmosphere, it takes the form of carbon dioxide, a greenhouse gas.
"Our results suggest that soils re-vegetated with native grasses would be a much more effective sink for carbon," says Wilson. He and Christian calculate that in the long run, the introduction of wheatgrass may have left 480 million tons of carbon in the atmosphere, which could have been stored beneath rich native prairie. This atmospheric carbon might be contributing to the greenhouse effect.
To put the figure in perspective, 480 million tons is approximately the same as one month of fossil fuel burning by the entire globe. And crested wheatgrass is only one species; no-one knows yet how other introduced species might compound the effect.
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