Available at: https://digitalcommons.calpoly.edu/theses/896
Date of Award
MS in Biological Sciences
An Analysis Of Human Disturbance To Rocky Intertidal Communities Of San Luis Obispo County
Grant Tyler Waltz
The number of coastal areas open to public access in California and San Luis Obispo County is increasing due to the acquisition by California State Parks of land previously owned by private entities. For example, California State Parks acquired property from the Hearst Corporation in 2005, which included 18 miles of coastline. California State Parks is responsible for providing public access in these newly acquired areas and also for maintaining the health of the natural systems found on these properties. Part of the California State Parks’ strategic vision maintains that they seek to consider the impacts of every decision they make on the next seven generations of Californians. To balance the competing demand of providing access with long-term sustainability, State Parks managers require sound scientific data to evaluate the impacts of human access to the ecosystems they manage.
One ecosystem susceptible to human access in these new State Park areas and in other areas throughout the state is the rocky intertidal (e.g. Beauchamp and Gowing 1982, Ghazanshahi et al. 1983, Hockey and Bosman 1986, Povey and Keough 1991, Addessi 1994, Fletcher and Frid 1996, Brown and Taylor 1999, Murray et al. 1999, Van De Werfhorst and Pearse 2007). This thesis represents a collaborative effort between State Parks Managers scientists at California Polytechnic State University San Luis Obispo, and scientists at Tenera Environmental Inc. to provide sound scientific data on the impacts of visitors to rocky intertidal biological communities in San Luis Obispo County. A three-pronged approach was used to assess the effect of visitors to rocky intertidal communities: 1) an observational study to quantify visitor densities in publicly accessible rocky intertidal communities, 2) an experimental manipulation of visitor density to rocky intertidal communities based on the visitor densities observed in part 1 and used to identify organisms susceptible to foot traffic (access-indicator taxa), and 3) an observational study of publicly accessible rocky intertidal sites exposed to levels of foot traffic shown to cause declines in access-indicator taxa from part 2. I was involved with all three portions of the study and my thesis is focused on presenting and discussing parts 1 and 3 in detail.
Visitor counts and the observational access-indicator taxa study (parts 1 and 3) were conducted in Montaña de Oro State Park (MDO) in San Luis Obispo County from 2007-2009. There was abundant accessible rocky intertidal coastline in the park. Three popular rocky intertidal sites were chosen within the park to conduct visitor counts. Visitors were quantified from fixed locations on the bluff above each of the three observation sites on sixteen occasions during the course of three years. These counts were used to estimate the annual number of visitors to each site. The area of each intertidal observation site was also calculated and with the annual number of visitors, was used to calculate the annual density of visitors to the rocky intertidal at each site. This represents a novel approach to quantifying visitor numbers to rocky intertidal communities. Additionally, I examined whether there was a relationship between the number of cars entering the park and the density of rocky intertidal visitors or between the number of cars parked at each site and the density of rocky intertidal visitors.
The annual density of visitors at one of the observation sites in MDO, Hazard Reef, was shown to be approximately equal to the moderate treatment level from the experimental study (part 2). This moderate level of visitor density was shown to significantly reduce the abundance of five rocky intertidal taxa: rockweed (Silvetia compressa, Hesperophycus californicus, and Fucus gardneri), Endocladia muricata, Mastocarpus papillatus, limpets, and chitons. To assess whether long-term exposure to foot traffic could impact the abundance of access-indicator taxa in MDO, the abundance of these taxa was sampled at Hazard Reef and compared to the abundance of the same taxa at two adjacent sites with much lower annual densities of visitors. A stratified random sampling design was used to assess the abundance of the five access-indicator taxa found in the mid-intertidal zone at these three sites in the spring of 2009.
My work demonstrated that visitor densities and patterns of use were variable among the three accessed intertidal sites in MDO. Annual visitor numbers to the rocky intertidal for the three observation sites within MDO were between 3,000-5,000 people. There was no relationship between the number of cars entering the park and the annual density of visitors to the rocky intertidal. The number of parked cars was significantly related to visitor density at one study site suggesting that under specific circumstances, controlling parking lot size may be a viable approach to managing impacts to intertidal areas. Significant differences in limpet density (60 per m2) were detected in a moderately accessed intertidal site relative to adjacent and less visited sites. The abundance of combined algae and limpets were lower at the moderate use site when the lower use sites were compared together against it. Patterns of rocky intertidal habitat use and the estimated annual visitor density suggest that some areas in San Luis Obispo County may be exposed to damaging levels of visitors. The current study identified that the abundance of one out of five experimentally identified access-indicator taxa (Rockweed, Mastocarpus papillatus, Endocladia muricata, Limpets, and Chitons) had been significantly reduced at a popular rocky intertidal site, relative to adjacent and less visited sites.