James M.

       The keystone species hypothesis implies that there is normally an almost balanced proportion of a species' effect on an ecosystem vs. its presence. Any species that produces a larger effect than its relative population is considered a keystone species. In this sense, humans, without question, are a keystone species since the changes we have caused are much greater and more apparent than those of any other single species. But what the keystone species experiment highlighted was mainly the freeing of a niche. When the starfish were removed, a niche was opened in which another organism was able to thrive. If that niche weren't initially filled by the starfish, their removal wouldn't have caused any substantial change. This means that the niche is more important than the species which currently occupies it. Humans' ability to externally adapt to a wide variety of environments makes our species' niche requirement largely irrelevant. Our ability to affect ecosystems stems less from our filling of a niche but more from our actions to change or destroy the niche itself. Unlike the starfish, a sudden absence of humans would not reopen a former niche. From this perspective, humans are not a keystone species but rather a larger force with a far greater ability to effect change in ecosystems than other species.