Independent and Interactive Effects of Crayfish and Darters on a Stream Benthic Community

A growing body of research suggests that direct and indirect interactions among consumers can have strong effects on the abundance of their prey. We examined the independent and interactive effects of 2 common freshwater consumers, crayfish (Orconectes propinquus) and rainbow darters (Etheostoma caeruleum), on benthic macroinvertebrates in a southern Michigan, USA, stream. A secondary goal was to assess the effects of crayfish and darters on the basal trophic level (periphyton) in the system. Presence and absence of crayfish and darters were manipulated in situ in wire mesh cages (0.2 m2, 6-mm mesh) using a 2 × 2 factorial, randomized block design. Invertebrates and periphyton (for chlorophyll a analysis) were sampled biweekly for 6 wk from artificial rock substrates within these cages. Neither consumer significantly affected chlorophyll a accumulation. Darters alone had no significant effects on total invertebrate abundance. Crayfish alone caused, on average, a 58% reduction in total invertebrate abundance relative to exclosures, and most common invertebrate taxa were negatively affected. In the presence of darters, however, crayfish reduced invertebrate numbers by only 29% relative to exclosures. Daters apparently modified the ability of crayfish to suppress invertebrate abundance, although the mechanism is unclear. Evaluations of community structure based on manipulations of individual consumer species may be misleading when interaction modifications such as these occur.