ARI SHAPIRO, HOST:
Scientists think that grass on the prairie is growing taller because there's now more carbon dioxide in the air. You might think that would be good for wildlife. But as NPR's Dan Charles reports, it could actually be hurting them.
DAN CHARLES, BYLINE: In northeastern Kansas, there's an open-air ecological laboratory called Konza Prairie. Scientists like Ellen Welti study the wildlife there - the plants, the insects, the big animals.
ELLEN WELTI: In the spring, it has a lot of beautiful flowers. It has bison.
CHARLES: In this landscape, grasshoppers play a really important role. They eat the grass. Birds eat them. But Welti and her colleagues noticed the number of grasshoppers has been declining over the past 20 years. It wasn't for lack of food. The amount of grass growing there has actually been increasing.
WELTI: And so we thought that was kind of interesting that as there is more plants, the grasshoppers are actually declining.
CHARLES: And then they had a thought. Maybe it's not the amount of grass, but its nutritional content.
WELTI: Grasshoppers, even compared to other insects, are more sensitive to changes in plant quality.
CHARLES: Fortunately, scientists had collected samples of grass that grew at Konza Prairie every year for the past 20 years or so.
WELTI: It was stored in a shed near Konza Prairie, and then we went back and did the chemistry on it.
CHARLES: And they found that, indeed, in more recent years, the grass contained lower levels of certain key nutrients - protein, phosphorus, potassium. Welti thinks the reason is there's more carbon dioxide in the air now, which plants need to grow. Also, the climate is warmer. And both things make plants grow faster. And growing faster, she thinks, is what led to lower amounts of those crucial nutrients. Scientists have observed this so-called nutrient dilution when they've grown plants in the lab with high levels of CO2.
WELTI: These declines in nutrients over time are really pretty striking, so it seems like a pretty logical explanation for our grasshopper declines.
CHARLES: Welti's study appeared this week in the Proceedings of the National Academy of Sciences. Other grasshopper experts say it makes sense to them, like Spencer Behmer at Texas A&M University.
SPENCER BEHMER: It's a interesting paper for sure.
CHARLES: Behmer says it shows how valuable it is to observe nature carefully over many years, collecting samples of grass without knowing what they might someday show.
BEHMER: They took advantage of samples that existed and analysis that existed from a long-term ecological study site.
CHARLES: There could be implications for people, Behmer says. Crops that humans eat also will be affected by carbon dioxide levels and climate change, although humans have more food options than grasshoppers do.
Dan Charles, NPR News.
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