data are of limited use beyond this for species whose weight is 

 largely composed of nonliving material (e.g. , clams, stony ectoprocts) 



Species associations as determined by statistical differences 

 within and between the 13 preliminary associations on each side of 

 the island are shown in Figures 9 to 12. These associations may be 

 compared with the preliminary species associations of Figures C-3 to 

 C-6. Based upon statistical analysis, 4 of the 13 preliminary associ- 

 ations were combined with other associations, resulting in a total 

 of 9 distinctly different major species associations. Areas covered 

 by each of these final associations are given in Table D-3. 



The quantitative characteristics of these major species associ- 

 ations are discussed below. 



a. Barnacle- Limpet Association . This uppermost association 

 (association A in Figs. 9 to 12) was relatively uniform in composi- 

 tion on all sides of the island. Dominant biota include acorn barna- 

 cles (Chthamalus fissus, Balanus glandula, and Tetraclita squamosa, 

 in descending order of abundance) and limpets (Collisella digitalis , 

 C. scabra, and Lottia gigantea) . 



The thatched barnacle, Tetraclita squamosa, was the species with 

 the highest biomass in the aggregate samples. The only algae occur- 

 ring in the samples from this zone were small amounts of Enteromorpha 

 sp. and patches of Ralfsia sp. 



b. Mytilus/Pollicipes Association . This association (associ- 

 ation B in Figs. 9 to 12) is largely confined to a narrow band (about 

 2 meters wide) on the west side of the island. A small area of this 

 association also exists on the southwest wing, but it was not sam- 

 pled. The association is dominated in biomass by the California 

 mussel (Mytilus californianus) , which has an average biomass of 16.9 

 kilograms per square meter, and gooseneck barnacles (Pollicipes 

 polymerus) which average 1.0 kilograms per square meter. A few 

 limpets, striped shore crabs (Pachygrapsus crassipes) , and acorn 

 barnacles (Balanus spp.) are also found here. Small bay mussels 



(Mytilus edulis) were common below the surface layer of larger 

 California mussels. Both species also occur in small numbers on the 

 north and south sides, but only M. edulis was found on the east (most 

 sheltered) side. Algae occurring in this association include 

 Bossiella orbigniana and Lithothamnium complex. The Mytilus- 

 Pollicipes association is higher in biomass per unit area than any 

 other association on the island. 



c. Anthopleura spp. Association . This association (associ- 

 ation C in Figs. 9 to 12) is composed almost entirely of green 

 anemones of the genus Anthopleura. Although Anthopleura spp. occur 

 in large numbers in the macrophytic algae zone, their occurrence in 



52 



