Experimental studies of herbivory and algal competition in a low intertidal habitat

• Published in: Oecologia 1985-09, Vol.67 (2), p.183-191
• Main Authors: Duggins, D.O, Dethier, M.N
• Format: Article
• Language: English
• Subjects: Algae; algae and seaweeds; Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; Brown algae; Diatoms; Ecological competition; Ecological zones; Ecology; Fundamental and applied biological sciences. Psychology; Grazing; habitats; Herbivores; intertidal environment; Katherina tunicata; Marine; Marine ecology; molluscs; plant competition; Sea water ecosystems; Synecology
• ISSN: 0029-8549; 1432-1939
• DOI: 10.1007/BF00384282

Ecological factors regulating the species composition and abundance of the plant assemblage in the low intertidal zone were studied in the semiprotected San Juan Islands in Washington state. In particular, the roles of an abundant herbivore (the chiton Katharina tunicata) and competition among plants were evaluated experimentally. Densities of Katharina were manipulated in large areas to establish 3 treatments: Katharina removals (N = 2), controls (N = 2), and Katharina addition (N = 1). These sites were monitored for 4 years. Over this time, algal abundance and diversity increased in the Katharina-removal areas; algae in most functional groups proliferated, and a multi-storied intertidal kelp bed eventually developed. In the Katharina-addition, the abundance of all plants except crusts, diatoms, and surfgrass decreased, and overall diversity declined. Control sites underwent year-to-year fluctuation in the abundance of the most conspicuous alga, Hedophyllum sessile, but remained otherwise unchanged. At the end of the third year, the dominant space occupiers (the large brown algae) were removed from half of each of the Katharina removal and control areas. These kelps positively affect abundances of Katharina but are negatively correlated with limpet numbers. In the absence of competition from the kelps, the abundance and diversity of most other algal groups increased. This change was especially dramatic in the areas lacking both chitons and kelps, indicating that herbivory and competition have negative, additive effects on smaller algae. However, even in the absence of Katharina, abiotic disturbance (desiccation and storms) apparently mediates the competitive dominance of the kelps.