Fishers, medium-sized weasels, were reintroduced to Pine Creek Gorge as part of an effort to establish a healthy population of fishers in Pennsylvania. Prior to the lumber era, fishers were numerous throughout the forests of Pennsylvania. They are generalized predators and will hunt any smaller creatures in their territory, including porcupines. Elk have been reintroduced west of the gorge in Clinton County and occasionally wander near the west rim of the canyon. Coyotes have come back on their own. Invasive insect species in the gorge include gypsy moth larvae, which eat all the leaves off trees, especially oaks, and hemlock woolly adelgids, which weaken and kill hemlocks. Invasive plant species include purple loosestrife and Japanese knotweed.