This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
A keystone, in an arch, is the single wedge shaped block at the top that holds the whole structure together. Pull it out and the arch collapses. The National Park Service uses that image deliberately for the beaver, a keystone species that affects habitat structure and dynamics through the damming and diverting of streams, and the felling of trees and other woody vegetation, whose removal can contribute to an ecosystem collapse the same way pulling the actual keystone drops the arch. The Park Service goes further, calling the beaver more industrious than any other mammal besides humans, and larger than any other rodent in the United States. What makes a beaver different from most animals called ecosystem engineers is that it does not merely use a habitat, it manufactures one. Damming a stream floods the surrounding land, raising the water table, reconnecting and expanding floodplains, and creating the still, shallow ponds that other fish and wildlife then move into. Many species come to rely partly or entirely on beaver ponds for their own survival, some of them already threatened or endangered in their own right, which means the beaver is not just adapted to wetlands. In a real sense, across most of North America, it is the reason wetlands as habitat exist in the first place.