what is keystone species?

Answer 1

A keystone species is a species that has a disproportionate effect on its environment relative to its abundance. Such an organism plays a role in its ecosystem that is analogous to the role of a keystone in an arch. While the keystone feels the least pressure of any of the stones in an arch, the arch still collapses without it. Similarly, an ecosystem may experience a dramatic shift if a keystone species is removed, even though that species was a small part of the ecosystem by measures of biomass or productivity. It has become a very popular concept in conservation biology.

There's a lot more to this article at Wikipedia: http://en.wikipedia.org/wiki/Keystone_species

Answer 2

It's the particular species in an Eco-system that the system relies on to be maintained. If you increase their numbers or decrease their numbers, the entire system will be affected. Alligators are considered a keystone species. They keep the local life in check and their waste and decay causes new life to flourish which feeds the entire food chain, including themselves.