Weed species find unexploited niches, especially in human-changed environments, and take over.
The list is long: dandelions, pigeons, oxalis (common, creeping woodsorel, the bane of my garden), Norway rats, crown vetch, trees like ailanthus (or "tree of heaven"), the wild mustangs and donkeys of the American Southwest, English Ivy, German cockroaches.
As people move from continent to continent, they take plants and animals with them. Sometimes it is deliberate; sometime accidental. Some invaders are welcome: there were no earthworms in North America prior to the English settlers. They brought them over in the soil surrounding the roots of their roses and fruit trees. Some are fine in one place, and an annoyance in another.
Microstegium vimineum, commonly known as Japanese stiltgrass or Nepalese browntop, is an annual grass that is common in a wide variety of habitats and is well adapted to low light levels.The stiltgrass easily lives in forest conditions, crowding out native plants. Sadly, white-tailed deer will not eat it.
But the goats do.
If the goats can get to the stiltgrass before they set seed, they can stop next year's invasion before it begins.
The goats are part of a larger program to control the weed at the sanctuary: herbicides, hand-weeding, weed trimmers. College students move rented goats from place to place and let them munch down on the grass, penned in by portable electric fences.