Kevin S.

Humans are certainly a keystone species. We affect environments over the entire planet, whether through clearing land for construction, or hunting, farming, and the inadvertent effects of our other activities. These effects on the environment certainly outpace our total biomass, and so we satisfy the first condition of a keystone species. Were humanity to vanish, the effect on the tentative balance that has arisen around us would be similarly profound. All of the land that humanity inhabits, that currently is deliberately void of most other life, would be reclaimed by opportunistic species, drastically affecting the environmental balance. This is just a single example of how humanity satisfies the second condition of a keystone species. Whether it can be readily said that humanity is the "greatest keystone species" is less certain. It is hard to imagine a single species whose effects would be as profound, but that could well be because we think on the wrong scale. Many smaller organisms, like mosquitoes and certain bacteria are as ubiquitous as humanity, and the full effects of their absence from the world environment are not completely known. While humanity is most likely the greatest keystone species on the planet, we have to recognize the uncertainty cast by our incomplete knowledge of our planet.