Matthew W.

Part of being a keystone species is having a disproportionate effect on its environment relative to its biomass. Humans continue to have a very large effect on the environment across nearly the entire planet and this effect is very disproportionate since humans have a fairly small biomass compared to the amount of surface area we inhabit. Soon, nearly 70% of the world population will be urban and most humans will have a much smaller direct effect on the environment. Even though some ecosystems will change dramatically if humans are removed, humans do not inhabit the majority of the Earth's surface, and if the oceans are included they inhabit an even much smaller percentage of the planet.
However, the most important function of a keystone species is to maintain ecological structure and species diversity within its habit. Humans have the opposite effect. Through habitat destruction with urbanization, natural resource harvesting, and agricultural clearing humans have destroyed much of the habitat that thousands of species depend on. After these habitats have been destroyed only a handful of species are able to survive. It can be estimated that nearly 40% of the Earth's surface is used for agriculture that only supports a handful of species.
Even though humans have a large effect on the environment they are not a keystone species because they do not maintain species diversity.