There isn't a single "top predator" in American grasslands. The specific top predator can vary depending on the region and the specific grassland ecosystem. However, some of the most common and significant predators in American grasslands include:

* Grey Wolves: Historically, grey wolves were apex predators in many grasslands, but their populations have been significantly reduced due to human activities.

* Coyotes: Coyotes are adaptable and widespread, and they play a crucial role in controlling populations of smaller herbivores like rabbits and rodents.

* Mountain Lions: Also known as cougars, mountain lions are large, solitary predators that can be found in some grasslands, especially in the western United States.

* Black-footed Ferrets: Although a small animal, black-footed ferrets are the only known predator of prairie dogs, which are key species in many grasslands.

It's important to remember that predator-prey relationships in ecosystems are complex and constantly shifting. Other factors like habitat availability, prey abundance, and human activities can influence the dominance of different predators.