One of Pennsylvania's few remaining old growth forests has stood for over 200 years, with giant trees surviving storms, disease, fires, and logging. As forests age, they show very large trees, large standing dead trees called snags, trunks with very few lower branches, deeply furrowed bark, and healthy, undisturbed soil. These ancient trees face a serious threat from the hemlock wooly adelgid, a small exotic insect accidentally introduced from other parts of the world that weakens or kills trees and destroys forests; its egg sacs appear as small tufts of cotton on the underside of hemlock branches, and stands of hemlock in the park show its devastating effects. Old growth forest also serves as a wildlife hotel, providing important food and shelter for many migratory bird species and mammals such as fisher, bobcat, and black bear.