Pollution Squeezing Out Rare Plants

In the upper Midwest, the number of species declined by 50 percent as invasive European grasses took over. Many of those that were lost were low-growing plants that found themselves in the shade of the taller grass, and thus deprived of the sunlight they needed to survive.

In the Kansas prairie, more than half the legumes -- plants that bear pods -- were lost because they couldn't metabolize the added nitrogen, and other plants simply took over. The lost species included members of the pea family.

In California, grasslands flourished and California poppies and other wild flowers diminished.

And in Alaska, birch shrubs on the tundra, which are dinky little things compared to the towering birch trees to the south, grew a whopping fivefold while reducing the number of species to a mere handful.

Although these were controlled experiments, designed to measure the impact of nitrogen fertilization, similar cases have been documented in the wilds, where nitrogen enrichment is an inadvertent result of human activities.

Nitrogen may be wonderful for our gardens, Suding notes, but the research shows "it is possible to have too much of a good thing."

Lee Dye's column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.

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