Algal Zonation in the New England Rocky Intertidal Community: An Experimental Analysis

Zonation patterns of plants, including marine algae, have commonly been attributed solely and directly to physical factors. Experimental investigations of the factors affecting zonation of macroscopic, benthic algae in the New England rocky intertidal region demonstrated that biological factors set the lower limits of these plants. The mid zone at all but very exposed sites is usually dominated by brown fucoid algae. These plants are virtually absent in the low zone, which is dominated by the red alga Chondrus crispus (Irish moss). Total removal of Chondrus (including the prostrate holdfast) results in establishment of Fucus vesiculosus or F. distichus ssp. edendatus in the low zone. Fucus grows faster in the low than in the mid zone, appears healthy, and reproduces. Thus competition from Chondrus sets the lower limit of Fucus, not changes in light intensity or immersion time, per se, as previously assumed. If herbivores (primarily the perinwinkle snail Littorina littorea) are absent where Chondrus is removed, Fucus can settle very densely and occupy 100% of the space. If herbivores are present, Fucus colonizes, but less abundantly. Thus competition is the primary determinant of the zonation pattern (affecting presence or absense) and herbivory is of secondary importance (affecting abundance). Other experimental evidence suggests that the upper limit of Chondrus is determined by desiccation. The lower limit of Chondrus has not been investigated except where a sharp lower limit exists at the low intertidal—shallow subtidal interface. Experiments demonstrate that this is due to the grazing by sea urchins (Stronglyocentrotus droebachiensis) where they are locally abundant. Normally, Chondrus extends well into the subtidal region. These results parallel experimental studies of animal zonation in rocky intertidal regions in which biotic factors also set lower bathymetric limits. It is suggested that biogeographic ranges of some species may be similarly affected by biotic factors.