Roles of crayfish: consequences of predation and bioturbation for stream invertebrates

Two field experiments were performed to examine the impacts on stream invertebrates of crayfish (Paranephrops zealandicus) as predators and bioturbators. A density manipulation experiment involved three treatments with different crayfish densities in streamside channels (0, 5, and 10 crayfish/m2) in which accrual of fine sediment, leaf decomposition, and associated invertebrates were measured. During the 50-d experiment, leaf decomposition was not affected by the presence of crayfish, whereas fine sediment was reduced to similar extents in both crayfish treatments. Some invertebrates followed a similar pattern to that of sediment accrual, suggesting that removal of fine sediment by crayfish was a primary mechanism underlying differences in invertebrate densities. Medium (4-6 mm body length) and large (≥ 7 mm) Tanypodinae were rare in crayfish treatments, probably as a result of selective predation. Conversely, crayfish appeared to facilitate Deleatidium mayflies, as predicted by a trophic cascade (large Tanypodinae predator removal) or shredder-collector facilitation (fine particulate organic matter production). In many cases, the response of dependent variables was equally strong at both crayfish densities. In a sediment flush experiment, sediment removal treatments were established, along with no-crayfish control and crayfish treatments, to assess the relative impacts of predation and sediment removal on each invertebrate taxon. Crayfish had little impact on leaf decomposition, although decomposition rate was significantly faster than in sediment flush treatments, where collector-scraper invertebrates were reduced as a result of sediment removal. Despite the fact that fine sediment was similarly reduced in crayfish and flush treatments, the response of invertebrates to treatments differed according to invertebrate taxon and size class. Overall, large Tanypodinae were selectively preyed upon; Orthocladiinae, Oribatei, and Oligochaeta responded primarily to sediment removal; Deleatidium were indirectly facilitated; and Chironominae, Ostracoda, and Cyclopoida were relatively unaffected. These results are likely to be relevant at various levels of local scale in the natural environment, but not at the catchment scale. Crayfish are therefore considered to play important roles in determining local distribution of invertebrates through both trophic and nontrophic effects.