Impact of trampling on early recovery and succession of bare rock substrate along a rocky intertidal platform in central California

Human population size and tourism are both growing substantially along the coastline of California. Accompanying this growth is an increase in human pressure on coastal biotic communities. Recent studies within intertidal areas have noted a potentially deleterious relationship between visitation level and an increase in the proportion of bare rock exposed along the coastal intertidal zones. The objective of this study was to determine how human trampling affects early succession and recovery of bare rock within the rocky intertidal zone of central California. A set of experiments was conducted at three sites along the intertidal platform at Natural Bridges State Beach (NBSB), Santa Cruz, California. At each site, four blocks of four 20x20 cm quadrats were located within the mussel beds, stratified by elevation. The flora and fauna under each quadrat was removed, and then one of four trampling treatments (100 steps, 200 steps, 300 steps and a no-trample control) was randomly assigned to each quadrat. Quadrats were then trampled every other week from October 2006 to March 2007. After 6 months, cover by sessile organisms and algae and density of mobile organisms were assessed. In untrampled treatments, bare rock exposure decreased by an average of 70% over the course of the experiment. With trampling pressure, however, bare rock only decreased by an average of 47.5%. Higher levels of trampling were also shown to impede the recovery of Coralline sp. and tar-spot-like algae. Data analysis also showed a reciprocal relationship between the density of mobile grazing gastropods (Tegula funebralis and Lottia sp.) and the percent coverage of tar-spot like algae. Despite the high levels of background disturbance in the rocky intertidal zone, even a small amount of intermittent human trampling significantly impeded resilience and succession in the algal community. Results from this research can be used by coastal resource managers.