Species differ greatly in how strongly they influence the rest of a community.

A keystone species has an effect on community structure that is disproportionately large relative to its abundance or biomass, the total mass of living material represented by that species. Removing a keystone species can trigger major indirect changes in other populations.

For example, if a predator strongly suppresses a dominant herbivore, removing the predator can allow the herbivore to increase and reduce plant abundance. The predator therefore affects plants indirectly through its effect on the herbivore. A chain of such indirect effects across feeding relationships is often called a trophic cascade.

A foundation species, by contrast, strongly structures a habitat largely because it is abundant or physically modifies the environment. Forest trees, reef-building corals or kelps can create habitat used by many other organisms.

These concepts should not be conflated:

• keystone effect: unusually large influence relative to abundance;
• foundation effect: strong influence often associated with dominance, biomass or habitat construction.

Community effects also need not follow simple pairwise intuition. Removing one species can alter competitors, consumers and mutualists simultaneously, so the net outcome may propagate through an interaction network.

The key lesson is that community importance is not determined by abundance alone. Indirect interactions can make a relatively uncommon species central to community composition and stability.