What are keystone species and how do they influence species richness in communities ?

A keystone species is one that has the major influence on the community, like alligators in southern swamps. The pools they hollow out retain water during droughts and birds tend to nest in the trees above the pool because the alligator tends to eat the predators that would otherwise climb up to eat the nestlings. Generally the result of the presence of the keystone species is increased richness.