The Role of Aquatic Invertebrates in Processing of Wood Debris in Coniferous Forest Streams

Abstract

A study of the wood-associated invertebrates was undertaken in seven streams of the Coast and Cascade Mountains of Oregon. The amount of wood debris was determined in terms of both weight and surface area. Standing crop of wood per unit area decreases with increasing stream order. Invertebrates associated with wood were functionally categorized and their biomass on wood determined. Major xylophagous species were the caddisfly (Heteroplectron californicum), the elmid beetle (Lora avara) and the snail (Oxytrema silicula). Standing crop of these species is greater on wood in the Coast Range than in the Cascades, which is attributed to species composition of available wood debris. The density of L. avara was strongly correlated with the amount of wood available irrespective of stream size within a drainage. The standing crop of invertebrates was about two orders of magnitude greater on leaf debris than on wood. A potential strategy for wood consumption, based on microbial conditioning, is presented. The data are used to develop a general scheme of wood processing by invertebrates in small stream ecosystems. Their impact is similar to that of invertebrates which process leaf litter in terrestrial and aquatic environments when the full decomposition cycle of wood debris is considered. INTRODUCTION The allochthonous inputs to streams in western coniferous forests include coniferous needles, deciduous leaves and woody material, ranging in size from small twigs and bark to large logs. The amount of fallen wood in these streams can be extremely large. Froehlich (1973) estimated that in one watershed of old-growth douglas fir (Pseudotsuga menziesii) the standing crop of wood debris (pieces larger than 10 cm diam) was more than 15 kg/m2. The standing crop of small debris in the same stream was 1.08 kg/m2 (Sedell et al., 1974). In view of the quantities of woody material in these streams, it is apparent that wood has a significant role in energy flow, nutrient dynamics, stream morphology and in shaping the biotic community of these lotic ecosystems. Although stream ecologists have emphasized the importance of allochthonous debris as the food base for stream invertebrates, most previous studies are based on leaf inputs (Hynes, 1970; Cummins et al., 1973; Boling et al., 1974). Current literature on aquatic invertebrate communities inhabiting logs or inundated trees has emphasized the exploitation of these sites as habitats for attachment or surfaces for grazing of periphyton (Claflin, 1968; Nilsen and Larimore, 1973; McLachlan, 1970) rather than as allochthonous energy and nutrient inputs to the aquatic system. The present study is a preliminary investigation of the wood component in coniferous stream ecosystems of western Oregon and of the role of invertebrates in the biological processing or degradation of wood. The objectives were to survey the fauna associated with wood in streams and to determine some of the interactions between the fauna and the wood substrate. In order to develop generalizations on the invertebrate-wood interactions, we chose to compare large and small streams in two different areas rather than to investigate one site in detail.