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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—m2 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—m2 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.

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A variety of simple models have been proposed to describe ecological succession (e.g., Connell and Slatyer 1977), but these models do not address some agents that may increase complexity. To determine the complexity of a natural sequence, four null hypotheses were tested: (1) seasonality of growth, recruitment, and mortality does not influence succession; (2) the interspecific interactions that produce successional change are the same throughout the sequence; (3) consumers have no influence on succession, and (4) small scale spatial variation in establishment and mortality does not occur. These hypotheses were tested in a low zone rocky intertidal community normally dominated by the surfgrass Phyllospadix scouleri on the Oregon coast.Succession experiments initiated in different seasons and an herbivore exclusion experiment falsified each of these hypotheses. At two sites (Boiler Bay and Squaw Island) experimental plots that were cleared in the spring were first colonized by Ulva sp., but those cleared in other seasons were first colonized by Phaeostrophion irregulare. Ulva appears adapted to colonize space made available by winter storms, whereas Phaeostrophion takes advantage of space made available by sand movement in the fall. Another seasonal pattern was the sharp decline in total algal cover in the fall, when wave action increased. At Squaw Island, the presence of Phaeostrophion established in fall and winter significantly inhibited the summer establishment of Ulva, though Ulva occupied some space epiphytically. In contrast, at Boiler Bay, a different, stronger type of interspecific interaction occurred: Phaeostrophion totally inhibited the establishment of filamentous diatoms. The cover of Ulva in the summer was also influenced by herbivores, but Ulva cover declined in the fall in both herbivore exclusions and controls, suggesting that herbivores were not solely responsible for its replacement. Local variation was demonstrated, because after three years of succession, replicate plots often differed. The early colonists, Phaeostrophion and Ulva, continued to dominate some plots; in other plots they had been replaced by middle successional species, including Cryptosiphonia woodii and Odonthalia floccosa. In still other plots Rhodomela larix had replaced other species. Similarly complex successional sequences occur in many natural communities. Thus, the features that simple models do not address may add complexity to succession, and for some communities different approaches must be developed.