Herbivory and Between‐Habitat Diversity: The Differential Effectiveness of Defenses in a Marine Plant

The effects of consumers on prey populations are traditionally expressed as a function of the intensity or frequency of consumption. Interactions among the intertidal alga Iridaea cordata and five common herbivores of the Oregon coast emphasize that the variety of consumers can be as important as the magnitude of feeding. The five herbivores in this system divide into two functional groups with respect to feeding on Iridaea. (1) Katharina tunicata (chiton), Strongylocentrotus purpuratus (sea urchin), and Collisella pelta (limpet) readily feed on Iridaea in all reproductive conditions. In contrast, (2) Lacuna marmorata (coiled snail) and Idotea wosnesenskii (isopod) feed on reproductively mature plants, but avoid nonfertile individuals. Idotea is deterred from eating immature plants by the alga's cuticle. Lacuna, while impartial to the presence of the cuticle, avoids nonfertile plants for an unidentified reason. At several locations along the Oregon coast these herbivores are abundant and spatially segregated. In these areas Iridaea has a restricted distribution (isolated on vertical walls) that correlates with the qualitative feeding differences among the five herbivores. Iridaea persists with Idotea and Lacuna (although reproductively mature blades are heavily damaged), but is rare or absent when either Katharina or Strongylocentrotus is common. When Katharina is removed from isolated rocky benches, Iridaea establishes in areas where it was previously absent. Qualitative variation in herbivory as a result of feeding by different species enhances between—habitat algal diversity by restricting the distribution of Iridaea. When the effects of herbivores are experimentally reduced, Iridacea shows a more uniform distribution.