New Zealand stream crayfish: functional omnivores but trophic predators?

1. The feeding ecology of the crayfish Paranephrops planifrons in streams d raining catchments in native forest and pastoral land was investigated usin g analyses of both stomach contents and stable isotopes of carbon and nitro gen. We aimed to (1) identify the energy sources of crayfish, (2) determine whether these were affected by ontogeny or land use change, and (3) assess the functional and trophic roles of crayfish in New Zealand hill-country s treams. 2. In native forest streams, crayfish stomach contents were dominated volum etrically by leaf detritus (> 60%), but in pasture streams aquatic inverteb rates constituted more than 40% of stomach volumes and leaf detritus < 30%. Stable isotope analyses revealed that crayfish from both native forest and pasture streams incorporated energy from aquatic invertebrates into their body tissue but did not appear to utilize detritus for growth. Therefore, d eforestation had little impact on crayfish energy sources. 3. In native forest streams, adult crayfish (greater than or equal to 20 mm orbit-carapace length (OCL)) consumed greater amounts of leaf detritus tha n juvenile crayfish, but a corresponding change in stable isotope signature s was not detected. Ontogenetic shifts in diet were not consistent between land use suggesting that change in local habitat and food resources, as a r esult of land use change, affect crayfish food choice more than factors rel ated solely to age or size. 4. Crayfish appear to occupy the trophic position of a predator, but by fun ctioning as omnivores, they have dual roles as both predators and processor s of organic matter. The use of gut content analysis in conjunction with st able isotope analyses revealed that the functional and trophic roles of P. planifrons differed, with implications for the interpretation of diet studi es and understanding of the role of omnivores in food webs.