Effect of Wood Hardness, Condition, Texture and Substrate Type on Community Structure of Stream Invertebrates

Abstract

In a field experiment, I examined the effect of wood hardness, condition, texture and substrate type on colonization of stream invertebrates and resulting community structure. Bundles of wood differing in hardness (firm and soft), condition (conditioned and unconditioned), and texture (rough and smooth) were placed in a Michigan stream and colonized for 30, 61 and 91 days. In addition, plexiglass rods were used as an inert analog of wood. Taxa richness was significantly greater on (1) rough-conditioned wood vs. rough-unconditioned wood and (2) smooth wood vs. smooth plexiglass. Larval Chironomidae were numerically dominant on all substrates and their density was affected by the interaction between wood condition and time, as well as wood condition and wood hardness. Densities of four of the six most abundant taxa were significantly greater on rough-soft wood than rough-firm wood. Densities of two taxa were significantly greater on rough-conditioned wood than rough-unconditioned wood. Wood texture was unimportant, as no taxon was found in significantly greater densities on rough-unconditioned-firm wood than smooth-unconditioned-firm wood. Finally, total invertebrate densities, as well as densities of three taxa, were significantly greater on smooth wood than on smooth plexiglass, indicating that inorganic substrates are less suitable as sites of attachment and/or food source than similar wood substrates. The fact that wood presents various hardness states to potential colonizers appears to be an important determinant of invertebrate community structure on wood substrates. Likewise, previous stream conditioning of wood and substrate type both are important in determining numbers of invertebrate taxa and individuals that colonize wood substrates in streams.