Effects of human foot traffic on the standing stocks, size structures, and reproduction of southern California populations of the intertidal rockweed Silvetia compressa (O. Fucales)

Rocky intertidal habitats in southern California often receive large numbers of visitors and are exposed to high levels of human foot traffic. I hypothesized that experimental trampling treatments would decrease the canopy cover, reduce total biomass and reproductive biomass, and alter the size structure of southern California populations of Silvetia compressa, a habitat-structuring seaweed. I also hypothesized that the amount of trampling damage would vary among sites exposed to different levels of natural and visitor disturbance. To test these hypotheses, I randomly located 15 plots (0.35 m 2) in the Silvetia assemblage at three sites known to receive different levels of disturbance. At each site, 5 plots served as controls while 10 received monthly trampling treatments of either 150 or 300 steps (429 steps m−2 or 857 steps m−2) for 16 consecutive months. These treatments were much less intense than those employed in most previous trampling studies yet resulted in significant impacts to Silvetia populations at all three sites. I found that Silvetia cover and the quantity of biomass detached as broken fronds from trampled plots showed a general pattern of decline throughout the study. By the conclusion of the study, trampling had reduced Silvetia cover, total biomass, and reproductive biomass, and the size structures of trampled populations had shifted to include higher frequencies of smaller Silvetia thalli. Most damage to Silvetia thalli appeared to be inflicted by the first 150 steps of trampling. Additionally, damage to Silvetia populations varied among sites and was greatest at Monarch Bay, the site receiving the least amount of natural and visitor disturbance. My results indicate that the simple act of walking across the shore can damage and change the structure of Silvetia populations and that the amount of injury varies among sites exposed to different levels of disturbance.