An enhanced method of evaluation, assessment, and statistical inference to assist management decisions on coastal access to rocky shores
Abstract Determining the effects of public access on biological communities can be difficult, as both public access and natural influences can be highly variable over time and space. The multiple sources of variation can obscure the magnitude and significance of the changes to species and ecosystem functioning due to public access. Manipulative field experiments simulating foot traffic have been a preferred method for identifying the species most sensitive to impacts from public access. However, using the results to describe the magnitude of change in real situations can be complicated if the treatment applications were not accurate in simulating the amount of foot traffic at the location of concern. Here we describe a novel three-pronged approach that we designed and implemented to help understand the magnitude of impacts from visitor traffic in a highly variable ecological community. We focused on an intertidal rockweed community in central California, U.S.A. numerically dominated by the brown rockweed Silvetia compressa and exposed to moderate levels of foot traffic. We began by determining the numbers of visitors and time spent in popular visited rocky shorelines supporting this type of community. Second, those exposure levels were applied to species of the same community type in a manipulative field experiment in an area of no public access and with the experiment designed as a before-after-control-impact study. The treatments were numbers of people and amount of time spent in large plots, not prescribed footstep numbers in smaller test plots, which has been commonly used in other studies. Third, we completed a field observational study consisting of sampling intertidal species abundances in nearby areas experiencing foot traffic levels similar to those tested in our experiment, and we also sampled adjacent areas with lower/no public access. Our highest treatment exposure resulted in a statistically significant reduction in algal cover of approximately 20%, relative to controls. The amount of change was similar to the differences in algal cover found in the field observational study between the public access and low/no public use areas, which also had similar differences in visitor exposure levels as the manipulative experiment. The consistency of the results between the observational study and field experiment strengthens the inference that the changes were attributable to visitor traffic and not natural variation. The three components of our study can be adapted to other ecosystems for determining impacts to natural resources and for supporting decisions on whether actions are necessary to protect natural resources from public access.