EFFECTS OF CRAYFISH (PARANEPHROPS PLANIFRONS, PARASTACIDAE) ON IN-STREAM PROCESSES AND BENTHIC FAUNAS - A DENSITY MANIPULATION EXPERIMENT

The effects of New Zealand freshwater crayfish or koura (Paranephrops planifrons: Parastacidae) on organic matter processing, sediment accum ulation, and benthic invertebrate communities were investigated using four replicate treatments of 0 (control), 4-5 (medium), and 8-11 (high ) similar-sized koura in 0.5 m(2) artificial stream channels colonised by benthic invertebrates from a pasture stream, Waikato, New Zealand. Wineberry (Aristotelia serrata) leafpacks were placed in each channel and after 7 weeks the leaf matter remaining was significantly lower i n both medium and high koura channels than in controls. The amount of sediment (surficial cover by fines and weight of suspendable sediment) was also significantly reduced in high koura density channels. Densit ies of invertebrates other than crayfish were not significantly differ ent among treatments; however, taxa richness and invertebrate biomass were significantly lower in high koura channels than in controls. Our results suggest that freshwater koura may play a keystone role in stru cturing benthic invertebrate communities either directly through preda tion, or indirectly by sediment bioturbation and increasing organic ma tter processing rates.