Stability of Rocky Intertidal Surfgrass Beds: Persistence, Preempton, and Recovery

Experiments were used to study the mechanisms underlying the persistence of beds of the surfgrass Phyllospadix scouleri, a prominent feature on horizontal benches in rocky intertidal areas ranging from British Columbia to Baja California. The stability of P. scouleri beds was investigated by: (1) quantifying structure and persistence, (2) experimentally removing P. scouleri, and (3) measuring rates of recovery from disturbance. At two Oregon study sites P. scouleri not only occupied more space than any other species but also persisted through all seasons for 3 yr. Differences between the two sites seem related to their disturbance patterns; although new free space was rarely created at either site (0.13 and 0.04% of the area per year), P. scouleri occupies much more space at the site where it is less often disrupted. Experimental removals of P. scouleri (0.25—m 2 plots) resulted in significant increases in algal cover and in upright—plant diversity, suggesting that it preempts space from other species. These invading algal species dominated the experimental plots for the remainder of the study. During the 3—yr experiment only nine surfgrass seedlings recruited into 28 plots and occupied <1% cover. Mapped seedlings in a 28—m 2 area experienced 93% mortality in 7 mo. Rhizomes of surrounding P. scouleri plants grew into experimental plots at a maximum increase in rhizome length of 6 cm/yr. The slow recovery makes even the rare disturbances that occur in these communities important. Therefore, P. scouleri beds have high persistence stability, despite their slow recovery. They owe this persistence mainly to their high preemptive ability. These characteristics appear common to many species with escapes from herbivores.