Here's why:

* Keystone species have a disproportionately large impact on the structure and function of an ecosystem relative to their abundance.

* They play critical roles in maintaining the balance of the community, often by controlling populations of other species.

Example:

* Sea otters are keystone species in kelp forests. They prey on sea urchins, which in turn eat kelp. Without sea otters, sea urchins would overgraze kelp forests, destroying the habitat for many other species.