Factors affecting the abundance and size of Pisaster ocharceus in the rocky intertidal zone of southern British Columbia

This study investigates the abiotic and biotic factors that affect the abundance and distribution of the sea star Pisaster ocharceus in the intertidal zone of Southern British Columbia (B.C.). P. ochraceus is very important to the intertidal community as it is seen as a keystone species because when its numbers are reduced, mussels are seen to out-compete all other organisms, reducing the species richness of the community. Climate change is expected to impact strongly on intertidal habitats and there is a strong possibility that intertidal habi- tats may serve as a bellwether for change in other ecosystems. During June and July 2008, 16 sites were visited along the south coast of B.C., 12 abiotic and biotic variables were recorded and all P. ochraceus individuals were counted and measured within a 50 m transect at each site. Across all sites, 25% contained no P. ochraceus. The abundance of sea stars ranged from 0 to 255 individuals. The mean size (diameter) of individuals ranged from 4.7 to 23.5 cm. Human disturbance (P ¼ 0.001) was found to be significant, negatively linked to the size of P. ochraceus. Although no significant results were found with regard to the abundance of P. ochraceus, human disturbance (P ¼ 0.080) and exposed sites (P ¼ 0.079) just failed to reach significance. These finding indicate that anthropogenic factors such as human disturbance are having a negative impact of P. ochraceus. This study serves as a pilot for further research in this area, a more in depth large-scale study of P. ochraceus is needed to obtain greater insight into its abundance and size distribution and the factors which influ- ence them.