This photo shows a typical marine intertidal community along coastal California. The orche sea star (Pisaster orchaceaus), a classic example of a keystone species, maintains diversity of molluscs (e.g., mussels, Mytilus californianus), crustaceans (e.g., barnacles, Semibalanus balanoides), and cnidarians (e.g., sea anemone, Anthopleura elegantissima) in coastal intertidal communities. Notice how M. californianus densities are greatest and densities of other invertebrates lowest along the higher tidal zone of the rocks that lack P. orchaceaus. In contrast, the middle tidal zone of the rocks, where P. orchaceaus is present, contains the highest diverse community of invertebrates (e.g., M. californianus, S. balanoides, A. elegantissima). The lower tidal zone is dominated by brown sea weed (Hedophyllum sessile) and bull kelp (Nereocyctis luetkeana) that are adapted to high-energy wave action. Ecologist Robert Paine studied the zonation patterns of rocky intertidal habitats to determine if species coexistence within intertidal communities is physically or biologically controlled. He observed that M. californianus migrated downwards and dominated middle tidal zones where P. orchaceaus had been manually removed. He concluded that predation by P. orchaceaus facilitates species coexistence among competitors, especially in environments where space is a limiting resource), a classic example of a keystone species, maintains diversity of molluscs (e.g., mussels), crustaceans (e.g., barnacles), and cnidarians (e.g., sea anemone) in coastal intertidal communities.