Abstract

A survey was performed in summer to investigate whether the distribution pattern of crayfish (Paranephrops zealandicus) is reflected in the invertebrate assemblage at a local scale (individual boulders as a unit) along a headwater stream. Twenty-seven boulders associated with crayfish and nine boulders without crayfish were regarded as crayfish and control treatments, respectively. Although physicochemical variables (boulder surface area, depth, current velocity, organic sediment mass, inorganic sediment mass and particulate organic matter standing crop) were relatively homogeneous, some invertebrate taxa showed statistical differences between the treatments. The positive association between crayfish and the filter-feeding flatworm Temnocephala novaezealandiae is probably a consequence of a commensal relationship between the two invertebrates. Crayfish were also negatively associated with the large predatory caddisfly Polyplectropus puerilis, and positively associated with the nymphs of the mayfly Deleatidium spp. and the stonefly Austroperla cyrene, following the prediction of a trophic cascade. The distribution pattern of crayfish is therefore reflected by the microdistribution of invertebrates at this highly localised scale.

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