A keystone species is one that:
(a) Has a higher likelihood of extinction than a non keystone species
(b) Exerts a strong influence on an ecosystem
(c) Causes other species to become extinct
(d) Has a weak influence on an ecosystem
A keystone species is the one which has a very prominent effect in the ecosystem it inhabits. They play a very important role in determining the structure, number and types of several other species in the ecosystem.
For example- Jaguar is a predator which feeds on more than 80 different species of animals and thus control the balance in forest ecosystem.