Why wouldn’t the clam be considered a keystone species?

Keystone species, in ecology, a species that has a disproportionately large effect on the communities in which it occurs. Such species help to maintain local biodiversity within a community either by controlling populations of other species that would otherwise dominate the community or by providing critical resources for a wide range of species. The name keystone species, coined by American zoologist Robert T. Paine in 1969, was derived from the practice of using a wedge-shaped stone to support the top of an arch in a bridge or other construction. Just as other stones in the construction depend on the keystone for support, other species in a biological community depend on the presence of a keystone species to maintain the community’s structure.

The starfish Piaster ochraceus is a keystone species in the rocky marine inter tidal communities off the northwest coast of North America. This predatory starfish feeds on the mussel Mustily Californians and is responsible for maintaining much of the local diversity of species within certain communities. When the starfish have been removed experimentally, the mussel populations have expanded rapidly and covered the rocky inter-tidal shores so exclusively that other species cannot establish themselves. Consequently, the interaction between Piasters and Mustily supports the structure and species diversity of these communities. In other communities in which Piasters occurs, however, the starfish has little overall effect on the structure of the community. Therefore, a species can be a keystone species in some communities but not in others.

weathering is the answer.