Littornia and Fucus: Effects of Herbivores, Substratum Heterogeneity, and Plant Escapes During Succession

Abstract

The mid—zone rocky intertidal community at protected and semiprotected areas in New England is characterized by a few, relatively small—bodied herbivore species. The effects of gastropod herbivores, primarily Littorina littorea, on secondary succession in the community were experimentally investigated. These periwinkle snails preferentially consume early successional, ephemeral algae such as Ulva lactuca L., Enteromorpha spp., and Porphyra spp. If not grazed, these early stages inhibit the appearance of a later successional species, Fucus vesiculous L. The effect of the herbivores on succession is to speed up the early successional sequence by releasing Fucus from inhibition by ephemeral species. Although these herbivores effectively control the ephemeral algae, they are usually unable to regulate the establishment of Fuchs. The periwinkles will readily graze small Fucus germlings (<3—5 cm long), and can prevent their successful establishment on smooth rock surfaces. However, substratum microtopography created by small crevices or pits or barnacles provides spatial refuges for small germlings. These plants quickly grow to a size at which they are less susceptible to periwinkle grazing. Thus although Fucus has a grazer—vulnerable stage, it often escapes herbivore control by virtue of spatial and size—related refuges. These escapes are herbivore—density dependent, however; unusually high number of perwinkles can prevent Fucus establishment, even when spatial refuges occur. Although the periwinkles harm young Fucus, they benefit older Fucus plants by grazing epiphytes from the blades, in lieu of the plant itself. The grazers thereby increase the persistence of individual Fucus and stabilize the community that depends upon Fucus for a habitat. The perwinkles thus play a subtle but important role in the community.