A keystone is a central stone at the summit of an arch that locks the other stones in place. It is crucial for the stability of the arch, as it distributes weight evenly and prevents the structure from collapsing. The term "keystone" is also used metaphorically in various fields to describe a critical element that holds together a system or structure. In ecology, a keystone species is an organism that has a disproportionately large impact on its environment relative to its abundance. The removal of a keystone species can lead to significant changes in the ecosystem, affecting many other species and the overall health of the habitat.