Pollution Squeezing Out Rare Plants

Nitrogen, that miracle cure for ailing plants in our gardens, may actually be pushing a lot of rare and endangered plants closer to extinction, according to an exhaustive new study.

Although nitrogen is already the most common element in the atmosphere, its abundance is rising -- largely through the burning of fossil fuel -- and the study indicates that the change is upsetting the natural balance across the North American continent. That makes it harder for many plants to compete.

"Nitrogen changes a lot of things in the system, and those things are probably what's making a lot of plants endangered," says Katharine Suding, assistant professor of ecology and evolutionary biology at the University of California, Irvine. She is the lead author of a report on the research in a recent edition of the Proceedings of the National Academy of Sciences.

"It's an indirect cascade," she says, because when the level of nitrogen goes up, some plants do better than others, thus smothering, or starving, plants that may already be in a life-or-death struggle.

Noting Nitrogen Nationwide

The study was carried out by scientists from eight institutions across the country, and it consisted of 34 experiments in areas ranging from the desert of the Southwest to the Arctic tundra, and from coast to coast. It is part of a long-range study of environmental changes, sponsored by the National Science Foundation, and while the findings do not suggest that we should stop dumping nitrogen on the plants in our gardens, they underscore the wide-ranging effect of human activities.

In most areas of the country, nitrogen availability, through such things as acid rain, has more than doubled since the 1940s, the researchers say. Factories and automobiles in one area of the country can release nitrogen that is carried by the wind to far-flung places, even the tundra of the far north and the alpine meadows of the high mountains.

So if we think we are protecting some of our parks and other resources from human contamination, we better think again.

"You can't just put a fence around a wilderness area" and think it's fully protected, Suding says.

Cascading Conundrum

In their research, which extends over several years, the scientists studied the effect of nitrogen enrichment on 967 plant species. Part of what they found is not surprising. All plants need nitrogen, almost as much as they need water, so some of the plants thrived in their new atmosphere.

But as they thrived, they grew larger, demanding more and more of the available resources, and eventually towering over their neighbors. The "cascade" effect, as Suding puts it, had a major impact.

"Some plants got squeezed out," she says. And the ones that lost were the rarer plants, those that simply couldn't hold their own in the new environment.

In every case, the scientists report, biodiversity fell as nitrogen rose.

"It doesn't matter if you are in the alpine tundra or a salt marsh or in a Midwestern prairie," Suding says. When the nitrogen goes up, some of the plants go down.

Nitrogen fertilization "changes the rules of the game," she adds. "So there's winners and losers."

That's a matter of great concern to ecologists, as well as folks who simply enjoy nature, because biodiversity means a healthier planet, and it's just a lot more interesting.

The Winners and Losers

Among the examples cited by the scientists:

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.