Can any organism be classified as a keystone species, or are there specific criteria determining whether an organism can be considered a keystone species?

A keystone species affects its ecosystem's structure and biodiversity disproportionately to its numbers, and not all species meet this criteria. For instance, the intertidal sea star and the banded tetra both have critical roles in their respective ecosystems that impact numerous other species.

Explanation:

Not every organism can be classified as a keystone species. A keystone species has a disproportionate impact on its environment relative to its abundance. Some criteria for determining if a species is keystone include: their role in maintaining the structure of an ecological community, having a significant influence on biodiversity and ecosystem function, and the fact that their absence would lead to a fundamental change in the community dynamics. For example, the intertidal sea star Pisaster ochraceus and the banded tetra are both keystone species. The removal of Pisaster ochraceus from its environment leads to an overpopulation of mussels, which reduces biodiversity. Likewise, the banded tetra supplies most of the phosphorus to aquatic ecosystems; its extinction would have severe repercussions.