Invading crayfish in a Michigan stream: Direct and indirect effects on periphyton and macroinvertebrates

Crayfish are common inhabitants of North American streams and many species are undergoing human-assisted range expansions. We studied the effects of an introduced crayfish (Orconectes rusticus) on benthic macroinvertebrates and periphyton in a northern Michigan (USA) stream by 1) conducting a 46-d enclosure-exclosure experiment and 2) sampling benthic communities along a longitudinal gradient in crayfish density. In stream enclosures, crayfish reduced total macroinvertebrate densities by 47-58% and herbivore densities by 55-72% relative to exclosures. Over the course of the experiment, periphyton chlorophyll a increased by 48-70% in enclosures compared to an increase of only 8% in exclosures. Periphyton biomass, however, did not vary across treatments. Periphyton primary productivity increased 4-7 times in the presence of crayfish, probably because crayfish reduced grazer densities (indirect effect) and removed non autotrophic components of the periphyton matrix (direct effect). The longitudinal survey supported experimental results. At sites along a crayfish density gradient occurring over 3 km of stream, periphyton chlorophyll a on rocks increased and macroinvertebrate density decreased with increasing crayfish density. These studies show that crayfish directly and indirectly affected the stream benthos, thereby producing responses at more than 1 trophic level. Some responses were consistent with a trophic cascade, but crayfish increased food web connectance by consuming periphyton. Therefore, crayfish can have complex, multi-trophic-level effects on the food webs of invaded streams.