Independent and interactive effects of crayfish and darters on a stream benthic community

A growing body of research suggests that direct and indirect interactions a mong consumers can have strong effects on the abundance of their prey. We e xamined the independent and interactive effects of 2 common freshwater cons umers, crayfish (Orconectes propinquus) and rainbow darters (Etheostoma cae ruleum), 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 cra yfish and darters were manipulated in situ in wire mesh cages (0.2 m(2), 6- mm mesh) using a 2 x 2 factorial, randomized block design. Invertebrates an d periphyton (for chlorophyll a analysis) were sampled biweekly for 6 wk fr om artificial rock substrates within these cages. Neither consumer signific antly 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, a nd most common invertebrate taxa were negatively affected. In the presence of darters, however, crayfish reduced invertebrate numbers by only 29% rela tive to exclosures. Darters apparently modified the ability of crayfish to suppress invertebrate abundance, although the mechanism is unclear. Evaluat ions of community structure based on manipulations of individual consumer s pecies may be misleading when interaction modifications such as these occur .