Thomas T.

The definition of a keystone species, as is stated by Wikipedia, is a species that affects its environment in a disproportionally large manner relative to its biomass. In nature, keystone species tend to have a stabilizing and/or diversifying effect on their environment, though this is not the cause when considering humans. Humans have a relatively low biomass in comparison to primary producers and other organisms found lower on the food chain. In spite of this lower biomass, humans span the earth and affect nearly every place they (we) go, changing the environment with centers of civilization and by harvesting resources. By spanning the globe, humans transcend individual ecosystems and affect the earth's biosphere as a whole. Humans meet both of the requirements to be a keystone species: they have an effect on their environment and their biomass is relatively low for the degree of that effect. It could be argued that humans are, in fact, the greatest keystone species, meaning that they have the greatest effect per unit biomass.