The Ecology and Management of Wood in World Rivers

Influence of Wood on Invertebrate Communities in Streams and Rivers

Arthur C. Benke and J. Bruce Wallace


Abstract.—Wood plays a major role in creating multiple invertebrate habitats in small streams and large rivers. In small streams, wood debris dams are instrumental in creating a step and pool profile of habitats, enhancing habitat heterogeneity, retaining organic matter, and changing current velocity. Beavers can convert sections of free-flowing streams into ponds and wetlands by killing trees and building dams. In low-gradient rivers, undercut trees that fall into the main channel (snags) are often the only stable habitat for invertebrates and provide a refuge and food resource for fishes as well. Invertebrates may use or require wood as food, but many species simply occupy wood as habitat. Although some species adapt to the woody environment by gouging and tunneling into the wood surface, others obtain their food from allochthonous and autochthonous resources that accumulate on the wood surfaces or are directly filtered from the water column. In streams of all sizes, accumulations of wood are often the hot spots of invertebrate diversity, and snags in Coastal Plain rivers of the southeastern United States support invertebrate production that is among the highest in lotic systems. The distribution of biomass and production among functional groups on wood varies greatly depending on the type of system. Loose streambed wood is colonized especially by shredders (gougers), and stable snags in larger streams and rivers are dominated by filterers and gatherers. High diversity of snag predators can result in complex food-web pathways, and snag taxa can be the major components of invertebrate drift in low-gradient rivers. Snag sampling is becoming a standard part of bioassessment, particularly in low-gradient systems, because snags are recognized as a major site of invertebrate diversity and production. Re-introduction of wood in streams and rivers is becoming an important aspect of restoration and management strategies around the world, as attempts are made to increase biodiversity and refuges both for fishes and their invertebrate prey.