How does a keystone species differ fundamentally from a foundation species?

Answer

A keystone contributes structure through a specific interaction, not sheer biomass.

Foundation species contribute structure through sheer biomass (like a massive forest), while a keystone species contributes structure through a specific, critical interaction, such as providing the only food source during a lean month.

How does a keystone species differ fundamentally from a foundation species?
speciesEcologyRolekeystone species