Competition for Space, Disturbance, and the Structure of a Benthic Stream Community

Interspecific competition for space limits the distribution and abundance of many benthic insects on stones in a western Montana stream. The sessile caddisfly larva Leucotrichia pictipes is territorial and aggressively eliminates conspecifics and other species from its foraging territories. Territoriality produces an intraspecific pattern of regular spacing and negative spatial associations between Leucotrichia and several other sessile insects: Parargyractis confusalis, Rhetanytarsus sp. and Eukiefferiella sp. Experimental removals of Leucotrichia resulted in higher densities of other sessile species and several mobile insects, including Baetis, Glossosoma, and Simulium. These insects show broad microhabitat overlap with Leucotrichia; competition with Leucotrichia limits their distributions and abundances within otherwise suitable microhabitats. Leucotrichia is the only species that monopolizes large areas of space. Physical disturbances interrupt the formation of competitive monopolies by Leucotrichia. During seasonal reduced flows, Leucotrichia is eliminated from stones and boulders in shallow water. Greater densities of a short—lived sessile species with multiple annual generations occupy this ephemeral spatial resource. On permanently submerged stones, Leucotrichia competitively affects a large part of the benthic fauna. On small stones that overturn with higher frequencies during flooding, Leucotrichia densities are reduced and species' abundance are highly equitable. Larger, more stable substrates with greater densities of Leucotrichia are characterized by lower species evenness.