Rocky intertidal community shift over 30 years: 1990–2020 rocky intertidal long term trend report

Abstract

Cabrillo National Monument (CABR) is a unit of the National Park System located on the Point Loma peninsula in San Diego, CA, USA. Despite its small size (0.65 terrestrial km2), the monument attracts 851,000 annual visitors (IRMA SRSS Reports 2011-2020), and acts as an “urban island”, providing habitat for unique algal, plant and animal species in an area of increasing development and urbanization. The coastal area of the park also leads to the rocky intertidal zone, which is regarded by many as the best conserved shorelines in mainland southern California. Due to the high-quality habitat and proximity to an urban area, it is critical to monitor community composition and visitor use of the rocky intertidal area. We leveraged over 30 years of long-term monitoring data of both rocky intertidal communities and visitation to investigate: (1) whether visitation has increased over time, (2) if community composition has changed over time across intertidal management zones. We found that visitation to management Zone I has increased over time. Additionally, we found that visitation doesn’t scale linearly across management zones: 73% of people were observed in Zone I, 19% of people in Zone II, and 4% of people in Zone III. PERMANOVA analyses indicated that community composition differed over time and across management zones for all plot types. Documenting community shifts, rather than changes in populations of single species, allows NPS staff to capture, and respond to, ecological transformation. Using the Resist-Accept-Direct (RAD) framework, we recommend that CABR either accept or direct changes to the rocky intertidal (e.g., directing changes in higher visitation areas by implementing a timed entry or shuttle system, and accepting changes in closed or very low visitation areas). Looking forward, long-term rocky intertidal monitoring will inform the effectiveness of resisting, accepting, or directing actions the park takes to uphold the NPS mission at CABR and other NPS units with rocky intertidal habitat, which span the Pacific coast.