Persistence and transport of fauna on drifting kelp (Macrocystis pyrifera (L.) C. Agardh) rafts in the Southern California Bight

Drifting rafts of Macrocystis pyrifera may connect isolated kelp forests in the Southern California Bight. To determine which species might utilize this dispersal mechanism, faunal samples from natural drifting rafts and attached M. pyrifera plants were collected during five cruises between March 1995 and December 1997. These rafts, which can be considered as floating islands, were aged and the macroinvertebrate assemblage enumerated. There was no significant relationship between raft age and species richness, or between species richness and distance offshore, which contrasts with predictions based on island biogeography. Species richness, however, was related to raft weight. Patterns of species presence and density were investigated relative to raft age for the species most frequently associated with rafts. Only one species, the isopod Idotea resecata, was found on all sampled rafts. Some species increased in frequency with raft age and others decreased, but only one relationship, a decline in the frequency of the anemone Epiactis prolifera with raft age was significant. When species density was examined over all cruises, only I. resecata had a significant change in density (an increase) with raft age, but additional significant relationships were found when species density patterns were considered by cruise. The results of all the tests were combined to provide a measure of "raft success". Nine of the most frequent 19 species had a positive score, indicating a favorable response to rafting, seven were unaffected, and two species had negative responses to rafting. Extinction times were calculated using species abundance and raft age relationships. Two species (E. prolifera and Paracerceis cordata), were predicted to persist on rafts for only about 100 days, which is the maximum estimated raft lifetime. All other species were predicted to persist for longer periods if the rafts floated longer. Kelp fauna that begin rafting appear to be largely unaffected by rafting, and hence dispersal on kelp rafts is possible for many members of the kelp forest community.