Trampling in the rocky intertidal of central California: a follow-up study

Two previous studies were performed 14 yrs apart on the same rocky intertidal platform off Santa Cruz, California, USA, with areas differing in the intensity of human trampling. The first study detected little impact by human trampling, and the second similarly found little evidence of impact or change. Considering the high volume of human visitors on the most trampled portion of the platform, and the known effects of trampling, these results were puzzling. A new study was performed with a different, stratified sampling regime, and trampling effects were demonstrated. Although the Shannon diversity index gave mixed results, species richness was consistently higher at the least disturbed site. Mussel bed cover, with the associated higher number of species within, decreased with increased human trampling, while the frequency of bare rock areas increased. Consistent with the previous studies, the two rockweed species that were abundant on the less trampled sites were completely absent at the most trampled. These results, in conjunction with the earlier studies, indicate that rocky intertidal assemblages can be resilient to human trampling, and sampling scale and design is important for evaluating and monitoring trampling impacts.