ABSTRACT
This chapter discusses the bias that the best possible test of hypotheses concerning the causes of natural patterns is a carefully conceived and controlled field experiment. Extensive observations of feeding asteroids revealed that this single species of sea pen is food for seven predator species. As in many subtidal systems, field experimentation proved unfeasible, However, Birkeland obtained rough estimates of the impact of these asteroids on populations of P.gurneyi by combining predator and prey densities and sizes, predator feeding rates, and prey growth and recruitment rates. The trophic organization of this system has received considerable attention all along the west coast of North America. A large body of more indirect evidence exists which suggests many asteroids on the Pacific coast of North America are also important predators on numerous species of motile organisms. On the Atlantic shores of North America, rocky intertidal habitats occur primarily north of Cape Cod.
