Long-term Monitoring of Rocky Intertidal Communities: Lessons and Implications from the Redwood National and State Parks, Northern California

A challenge modern-day ecologists and resource managers face is how to separate natural variations in populations from changes caused by human activities (e.g., climate change). Long-term monitoring programs provide valuable information to assist in this endeavor. This study details the initial findings of a long-term monitoring program initiated in 2004 to monitor changes in rocky intertidal communities within Redwood National and State Parks, located in northern California. Permanent plots were established at three sites using protocols developed by the Multi-Agency Rocky Intertidal Network, a consortium that monitors rocky intertidal communities along the western coast of North America. Replicate plots were established to monitor changes in abundance of key intertidal taxa, including mussels (Mytilus californianus), barnacles (Chthamalus dalli and Balanus glandula), red alga (Endocladia muricata), and rockweeds (Pelvetiopsis limitata and Fucus gardneri). Plots were sampled annually since they were established. Results from the first 15 years of this study indicate that all taxa exhibited substantial short-term (annual) variation, with barnacles and E. muricata exhibiting the most. For barnacles, such variations were correlated with measures of recruitment. Except for P. limitata, all other target taxa experienced at least one period of large-scale major change, where abundances decreased dramatically (> 50% of long term mean) and simultaneously in most plots. However, in almost all cases abundances recovered, resulting in no apparent long-term changes. For the few instances where long-term changes were detected, it is possible this result may be an artifact of the analytical methods used to assess them. The potential implications of this finding are discussed.