A close-up picture of a grasshopper nymph. More carbon dioxide in the atmosphere is changing how wild grasses grow in Kansas, and lowering their nutritional value to insects. That could upset the balance of life on the prairie. MANHATTAN, Kansas — Ellen Welti has a Ph.D. in, essentially, grasshoppers. And yet she was still mystified about why the number of grasshoppers in a long-protected and much-studied patch of Kansas prairie was dropping. Steadily. For 25 years. After all, the grass that the springy bugs feast on had actually grown more robustly as it absorbed mounting levels of carbon dioxide in the atmosphere.