Stress has emerged as an important factor affecting species distributions. In the high intertidal along the temperate central California coast, stressors include high temperature, high desiccation, and large waves in winter. This study addresses how the positive and negative factors associated with intertidal algal cover modify juvenile survival of the ubiquitous fucoid alga Pelvetiopsis limitata. Initial observations of Pelvetiopsis juveniles growing within the turf-like thallus of the alga Endocladia muricata suggested a facilitative relationship between algal cover and juvenile survival. Permanent plots were established and algal cover was manipulated to include Endocladia only, Pelvetiopsis only, or total removal of algal cover for one year. Pelvetiopsis recruits <1cm high in the different treatments were counted monthly. The effects of positive and negative factors on Pelvetiopsis recruitment varied as a function of algal cover abundance. Recruitment was enhanced at intermediate levels of algal cover, and reduced at very low and high levels of cover. The springtime appearance of the ephemeral alga Porphyra sp. in the plots, followed by subsequent high numbers of Pelvetiopsis recruits, emphasized the potential for individual species to structure the system. The relationship between algal cover and recruitment success of intertidal organisms suggests that in stressful systems, habitat modification by algae can facilitate algal recruitment by modifying the microhabitat. However, as algal cover increases and stress levels are reduced, the positive effects of algal cover are replaced by negative, competitive interactions among species.
