Rocky-shore Community Variation Along Natural and Anthropogenic Gradients of Disturbance: implications for the design and evaluation of marine reserves
- Fiorenza Micheli
End Date: April 30, 2003
Through intensive biodiversity surveys of rocky intertidal habitats of Monterey Bay, CA, we assessed how human and natural disturbances interact to affect these coastal communities. Specifically we examined whether differences in human disturbance and wave exposure lead to differences in dominance patterns within these communities and whether disturbances differ in their effects on common and rare species.
The eight survey areas span a gradient in human disturbance, taking advantage of existing marine reserves and other areas representing different levels of access and legal restriction of human activities. Areas fell into 4 Categories: (1) open access, no restrictions; (2) open access, reserve, no enforcement; (3) closed access, no restrictions (de facto reserve); (4) closed access, reserve, restrictions enforced.
Each Area was divided into a wave-exposed and a protected site and transects were stratified by tidal elevation within each. Relative rank abundance patterns were compared among areas, sites, and categories of human disturbance. Results suggest that human disturbance through trampling, collecting, and other impacts leads to shifts in species dominance. Sites with no restriction on access or harvest exhibited decreased dominance of common species and increased abundance of rare species relative to reserves with restricted access and effective enforcement. This was most apparent among sessile invertebrates and algae, i.e. those species that compete for primary space, suggesting that patchily distributed human disturbances open up primary space and decrease competitive dominance on rocky shores.
Differences between exposed and protected sites were less striking than those for human disturbance, but sessile species did exhibit decreased dominance and increased equitability in wave-exposed sites. Relative abundances of common species were most sensitive to differences in disturbance. Rank abundance patterns among rare species were generally similar among areas and sites, though identities of these species varied. Mobile invertebrates exhibited similar patterns across human and physical disturbance gradients.
Summary to DatePreliminary analyses of patterns of diversity and community structure of intertidal algae and invertebrates (approx. 250 taxa) across 8 locations with varying levels of restrictions of human access and collecting revealed significant variation in community structure but not in species richness across the sites.
Field sites included two no-take marine reserves (the Hopkins Marine Life refuge and the Point Lobos Ecological reserve), two de facto reserves where access to the shore is prevented by the presence of fenced estates (Pescadero Point, in Pebble Beach, and a site near Yankee Point), two sites with protection for a subset of intertidal species (the Point Pinos Marine Gardens Fish Refuge and the Carmel Bay Ecological Reserve), and two sites not comprised within marine protected areas (Soberanes Point and Cannery Row)
Full analyses of this dataset are ongoing.
- Species richness did not vary significantly with level of protection of the intertidal sites
- Community structure and relative dominance of species on exposed headlands varied significantly between sites where human access is restricted (marine reserves and de facto reserves) comapred to open-access sites
- At wave-protected sites, no significant variation in community structure was observed across levels of protection from human use
Study MethodsEstimates of percent cover (for sessile species) and counts of mobile organisms within 0.25 m2 quadrats. On each sampling dates, quadrats were sampled at low, mid and high tidal elevations, at both exposed headlands and wave-protected sites (12 quadrats/zone/exposure). Twelve belt transects (10x1 m) were also surveyed for sea stars, urchins and abalone densities. Surveys were repeated 4 times (twice in late winter-early fall and twice in summer).
Figures and Images
Mussel collecting at Sobranes Pt.
Study area locations and access levels.
Sampling with quadrats along a transect line.