Which of the following statements about keystone species is true?

1. Keystone species occur only in intertidal zones.
2. Keystone species play an unusually important role in determining the species composition in a habitat.
3. Keystone species can be removed from a habitat without any impact on the remaining species in the community.
4. Keystone species are primary producers and therefore usually are plants.
5. Keystone species are more expendable than commensal species, from a conservation perspective.