Expansion of a central California kelp forest following the mass mortality of sea urchins

The mass mortality by disease of a localized population of sea urchins, Strongylocencrotus franciscanus, on the seaward side of a kelp forest was followed by the rapid seaward expansion of 4 species of brown algae, Macrocystis pyrifera, Laminaria dentigera, Pterygophora california, and to a lesser extent, Nereocystis leutkeana. One other brown alga, Cystoseira osmundacea, failed to become established in the newly available area. Competition among M. pyrifera, L. dentigera, P. californica, and N. Leutkeana apparently was severe, and within 1 year after the demise of the sea urchins, M. pyrifera formed a dense, nearly monospecific stand. Experimental removal of M. pyrifera demonstrated that the canopy of these plants limited light penetration to levels below that necessary for the growth and survival of other brown and red algae.