Wood enhances crayfish (Paranephrops planifrons) habitat in a forested stream

The mechanisms of interaction between stream life and habitat are often complex and poorly understood. We explored the response of a freshwater crayfish (Paranephrops planifrons) to the functions of large wood with a view towards possible future use of wood in stream restoration in New Zealand. We compared three sub-reach types within a native forest stream in the central North Island, New Zealand; complex wood-rich, simple (straight) woodless, and geomorphically complex woodless. Stable logs embedded across the channel created considerable habitat heterogeneity, particularly diversity of substrate, water depths and velocities, that was associated with a significantly higher abundance and biomass of crayfish compared with a straight woodless reach. Although undercut banks in woodless sub-reaches also supported abundant crayfish, wood increased the proportion of stream area used as habitat, apparently by providing surrogate edge habitat. Wood appeared to increase stream complexity by retaining streambed substrate and providing microhabitat with stable cover.