Differing distributions of potentially competing amphipods,copepods and gastropods among specimens of the intertidal alga Pelvetia fastigiata

Possible competitive interactions are inferred from the distributions and abundances of species within three invertebrate groups (Hyale spp., Ampithoe spp. and thallusdwelling species) among unmanipulated and experimental specimens of the brown alga Pelvetia fastigiata collected between 1974 and 1976, inclusive, at La Jolla, California, USA. During the fall, when the vagile amphipods Hyale grandicornis and H. frequens were relatively abundant, they were found mostly on specimens of P. fastigiata at upper mid-intertidal and mid-tide levels, respectively. H. grandicornis often dispersed to mid-tide levels, but the two species cooccurred on few plants. Although H. frequens infrequently occupied P. fastigiata in the spring, H. grandicornis still generally sheltered in upper mid-intertidal plants, in part, because the net reproduction rates of H. grandicornis were low when space was available. The tubiculous amphipod A. tea usually was the only Ampithoe species on P. fastigiata; however, A. lindbergii and A. pollex sometimes dispersed to and released broods on this alga, particularly during the early summer when abundances of A. tea were relatively low. Similarly, net immigration rates of other thallus-dwelling copepods were relatively high when abundances of Scutellidium lamellipes were low. Moreover, several copepods were most abundant on different specimens of small isolated plants, and thallus-dwelling gastropods on larger plants at sites with relatively low densities of S. lamellipes on P. fastigiata; such copepods and gastropods generally were not abundant on aggregated plants sheltering large numbers of S. lamellipes. Such differences suggest that competition for food or space is among the factors causing negative correlations in the densities of Ampithoe spp. and thallusdwelling species on P. fastigiata.