Structure and Dynamics of Arctic-Subarctic Grazing Webs in Relation to Primary Productivity

Abstract

Current discussion on structure and dynamics of food webs includes two seemingly incompatible views on the role of energy supply for food chain length. Authors of food web reviews (Pimm, 1979, 1982, 1991; Briand and Cohen, 1987; Cohen et al., 1990) conclude that the energy constraint is trivial and that chain length is determined by other factors (population dynamics according to Pimm, dimensionality of the habitat according to Cohen and his coworkers). According to the hypothesis of exploitation ecosystems (Fretwell 1977, 1987; Oksanen, 1988; Oksanen et al., 1981), however, the grazing chain shortens along gradients of decreasing primary productivity. In relatively productive ecosystems (e.g., forests and their successional stages), the grazing chain harbors three trophic links (plants-physical resources, herbivores-plants, and carnivores-herbivores). The first and third links are strong, whereas the second represents a weak interaction, as proposed by Hairston et al. (1960). Less productive ecosystems (tundras, steppes, semideserts) are supposedly characterized by two-link dynamics, with strictly resource-limited grazers and grazing-controlled vegetation. In extremely barren areas, the only strong trophic link is between plants and physical resources.