Habitat Suitability and Local Population Persistence of the Sea Palm Postelsia Palmaeformis

Sporophytes of the sea palm Postelsia normally occur on a variety of intertidal surfaces. An experimental technique was developed permitting spores to be sown onto natural substrata whose relative suitabilities were evaluated using three criteria. All major surface categories (bare rock, animal, plant) were equivalent if judged by the appearance the following spring of a descendant sporophyte. However, the proportion of these plants surviving to maturity and continuing into the following year was highly substratum dependent. Bare rock surfaces are the most suitable for Postelsia as judged by probability of successful occupancy, sporophyte density, and inter—annual persistence. Single fertile plants can establish or maintain a population. The probability of local extinction is a function of population size: only 36% of small populations (1—30 plants) tended to continue to the next year, whereas all those >120 individuals did. Postelsia seems incapable of persisting in the presence of the turf—like algae Corallina or Holosaccion. Thus sea palms may require the presence of competitively superior mussels: mussels out—compete algae of low stature; waves remove mussels, thereby generating Postelsia's most suitable substratum.