Patterns of exploitation by stream invertebrates of wood debris (xylophagy)

The exploitation of wood debris by aquatic invertebrates has been investigated for about 10years by our laboratory. The initial study (Armessot4 et al. 1978) developed a general scheme ofwood processing by invertebrates, based on data from seven streams of the Coast and CascadeMountains of Oregon. Of about 40 species associated with wood, the elmid beetle, Less avara Le-Comm, the calamoceratid caddisfly, Hetemplecems califirrniaan MascrnAri, and the pleuroceridsnail Oxytrema Ina) silicula' were considered to be the dominant processors associated withwood surfaces. Based on a survey in the western states, DUDLEY & ANDERSON (1982) listed 56 tam ofaquatic and semiaquatic invertebrates as "closely associated" with wood debris and 129 as faculta-tive users of wood substrates. In contrast to the earlier study, the tunnelling and burrowing speciesreceived more attention. Diptera, especially Chironomidae and Tipulidae, were shown to be domi-nant borers, both in abundance and diversity.
This paper reports on continuing studies of aquatic insect-wood interactions, especially on long-term densities and rates of colonization. The patterns of exploitation by surface associates weremonitored for five years on tethered sticks and branch wood at Berry Creek, near Corvallis,Oregon (see ANDERSON et a1.1978). Colonization by semi-aquatic and aquatic wood borers was alsostudied at Berry Creek. Alder posts were placed so that one end was submerged to the bottom ofthe stream and the other was above the waterline in contact with the bank.
We use a broad interpretation of xylophagy to include organisms whose feeding activities effectfragmentation of wood surfaces. The scrapers that utilize the aufwuchs film probably derive littlenutrient from the wood. However, their guts contain wood (Puma et al. 1982), and their feedingactivity contributes to the mineralization of woody debris.