Wild horses on Assateague Island are threatening the environment by eating essential grasses, and officials with the National Park Service are mulling options to reduce the horse population.

There are currently 138 horses on the island–too many for the environment to sustain itself, said Jack Kumer, wildlife specialist at the Assateague Island National Seashore.

The horses eat two types of grasses that are essential to the landscape. They enjoy the beach grasses that maintain the structure of sand dunes, which prevent flooding. And their favorite food source is cord grass, or grass in the salt marshes, which is beneficial both for the Chesapeake Bay and for humans living near it, Kumer said.

Salt marsh grasses prevent excess nutrients from seeping into the water. They also provide a nursery for most of the fish and shellfish living in the bay, Kumer told The (Salisbury) Daily Times.

“The fish cannot exist well out here without healthy salt marshes to hide in while young,” he said.

Salt marsh grasses should be about 4 feet high, but most of the grasses on Assateague Island are between 6 and 12 inches because the horses have overgrazed them, Kumer said