Graduate Thesis Or Dissertation
 

Diverse consumer guilds in intertidal communities of Oregon and the Republic of Panama and their effects on prey assemblages

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  • Low intertidal communities of Oregon and Panama have many predator and herbivore species. This diversity of consumers is important to the organization of both communities because of qualitative differences among the consumer species in their ability to circumvent specific prey defenses. In Oregon, the red alga Iridaea cordata is protected by its cuticle from damage by the isopod Idotea wosnesenskii; however, the cuticle does not prevent feeding by three other common herbivores - Strongylocentrotus purpuratus (sea urchin), Katharina tunicata (chiton), and Collisella pelta (limpet). In Panama, limpets are protected from predators by their shells, but different shell forms deter different predators. Flat, thin shells are difficult for large, shell-crushing predators (Diodon hystrix [fish], Ozius verreauxii [crab]) to seize, yet they are easily drilled by gastropods (Thais melones, Acanthina brevidentata). Thicker, more conical limpet shells pose a barrier to drilling, but are easily grasped and crushed by fishes and crabs. In both systems variability in defense effectiveness, in conjunction with high feeding intensities, greatly influences prey distribution and diversity. Two mechanisms are important. (1) When discrete habitats. harbor dissimilar consumers, feeding enhances between-habitat prey diversity (= spatial variance in species composition). Enhancement of diversity occurs when different prey species fall victim to consumption in different habitats. Via this mechanism, the Rhodophyte Iridaea is frequently restricted to vertical rock surfaces when adjacent horizontal areas harbor many cuticle-impartial herbivores. Similarly, among Panamanian limpets the opposing effects of drilling and crushing predators restrict flat limpets to smooth surfaces where predaceous gastropods are rare, and conical limpets to crevices and holes where larger predators cannot penetrate. (2) When dissimilar consumers cooccur in the same habitat, their feeding can depress within-habitat prey diversity by reducing the chance of prey escapes. A reduction in diversity occurs through complementary effects when weaknesses of one consumer are partly or wholly counteracted by strengths of another. This mechanism is especially important in Panama in specific habitats exposed to both crushing and drilling predators. Under such conditions all limpet species are rare since no shell shape deters both predator groups.
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