 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 . 
