Ask Mr. Smarty Plants

Sorghastrum nutans (Indiangrass) is a wonderful native grass, found in tallgrass prairies. It is native to your area so it's not dropping dead and walking away. In this article from Illinois Wildflowers Indian Grass we found this excerpt:

"Several species of grasshoppers feed on the foliage of Indian Grass; these grasshoppers are an important source of food to many insectivorous songbirds and upland gamebirds. Other insects that feed on this prairie grass include the leafhopper Flexamia reflexus, the planthopper Myndus fulvus, the Issid planthopper Bruchomorpha extensa, and the caterpillars of Amblyscirtes hegon (Pepper-and-Salt Skipper). The foliage is also palatable to hoofed mammalian herbivores, including bison and cattle. Because of its height and tendency to remain erect, it provides good cover for many kinds of birds and animals in prairies."

Okay, so we don't think the grasshoppers are pulling the Indian Grass out by the roots. Got any cattle walking around in DC? Is there any chance that someone else with a garden that needs some prairie grasses is shopping in your park? We also thought of deer, because as their habitats disappear, more and more of them are becoming urbanized. However, we looked at our Deer Resistant Species list, and learned that Indian Grass is highly deer resistant. In fact, most deer don't care for most grasses, and I don't think they would pull them out of the ground.

We think you need to talk to some people closer to the situation and familiar with plant predators. The University of the District of Columbia has a Cooperative   Extension Office for Washington DC. An alternative would be the Virginia State University Cooperative Extension Office. If they don't know what is ravaging your stands of Indian Grass, they may know some organization to refer you to. All we can tell you is cattle like it, deer probably don't, and grasshoppers aren't big enough to do more than chew holes.