Structural complexity of woody debris patches influences fish and macroinvertebrate species richness in a temperate floodplain-river system

Abstract

A field experiment was conducted to examine the influence of variable density (complexity) of small patches of woody debris on the abundance and taxonomic richness of macroinvertebrates and fishes in the Brazos River, a meandering lowland river in east-central Texas. Woody debris patches contained bundles of either 8 or 16 sticks of two sizes, and reference plots contained no woody debris. The experiment was conducted in the littoral zone in the river channel and a nearby oxbow lake. Organisms were collected from each patch after 14 days. Abundance and taxonomic assemblage structure of macroinvertebrates in both the river channel and oxbow were significantly and positively influenced by complexity of woody debris. For fish in the oxbow, abundance and species richness were greater in woody debris than sites lacking structure, but the opposite trend was observed for fish in the river channel. This difference could be associated with isolation from source habitats and low colonization of the constructed woody debris patches in the river by fishes with affinities for complex habitats. Small lotic-adapted minnows were captured from reference habitats in the channel, but these species were rare in woody debris patches. This was in contrast to aquatic insects in the river channel, such as caddisfly and midge larvae, that efficiently colonized the small isolated patches of woody debris. In a lotic environment, woody debris provides vertical surfaces that intercept drifting insect larvae and provides protection from the water current. We speculate that greater abundance of macroinvertebrates in woody debris patches in both habitats results from the combined influence of high food resource availability and refuge from predation provided by structurally complex habitats.