The Stability of Boundary Regions between Kelp Beds and Deforested Areas

Two distinct organizational states of kelp forest communities, foliose algal assemblages and deforested barren areas, typically display sharp discontinuities. Mechanisms responsible for maintaining these state differences were studied by manipulating various features of their boundary regions. Urchins in the barren areas had significantly smaller gonads than those in adjacent kelp stands, implying that food was a limiting resource for urchins in the barrens. The abundance of drift algae and living foliose algae varied abruptly across the boundary between kelp beds and barren areas. These observations raise the question of why urchins from barrens do not invade kelp stands to improve their fitness. By manipulating kelp and urchin densities at boundary regions and within kelp beds, we tested the hypothesis that kelp stands inhibit invasion of urchins. Urchins that were experimentally added to kelp beds persisted and reduced kelp abundance until winter storms either swept the urchins away or caused them to seek refuge within crevices. Urchins invaded kelp bed margins when foliose algae were removed but were prevented from doing so when kelps were replaced with physical models. The sweeping motion of kelps over the seafloor apparently inhibits urchins from crossing the boundary between kelp stands and barren areas, thus maintaining these alternate stable states. Our findings suggest that kelp stands are able to defend themselves from their most important herbivores by combining their flexible morphology with the energy of wave-generated surge. The inhibitory influence of this interaction may be an important mechanism maintaining the patchwork mosaics of barren areas and kelp beds that characterize many kelp forest ecosystems.