The Effects of the Crayfish Cambarus bartoni on a Headwater Stream Community

Abstract

Recent experiments have demonstrated that crayfish can have strong direct and indirect effects on stream community structure in intermediate order, sunlit streams with abundant filamentous algae. The effect of crayfish on forested, headwater streams has received little attention. The crayfish Cambarus bartoni bartoni (Fabrici us) is abundant in headwater streams in eastern North America. However, its impacts on these stream communities has not been assessed with a field experiment. Using an 8 week enclosure/exclosure experiment, I examined the impacts of C. bartoni on detrital processing and the abundance of leaf pack and cobble associated invertebrates in a headwater stream in Frederick County, Maryland. A lab experiment was also conducted to determine leaf pack processing rates. In the field experiment, crayfish significantly reduced leaf pack dry mass in 8 weeks. Crayfish did not have a significant effect on total invertebrates associated with leaf packs. However, in leaf packs exposed to crayfish, the size structure of chironomids, the most abundant taxon in leaf packs, was affected. Fewer large chironomids (body lengths >3mm) were found in leaf packs exposed to crayfish. Crayfish also significantly reduced the volume of Fine Particuate Matter (FPM) on and among cobble substrate. Crayfish significantly reduced the densities of invertebrates in FPM and Coarse Particulate atter (CPM) associated with cobbles. The densities of the two numerically dominant taxa in baskets, chironomids and harpacticoid copepods, were positively correlated with FPM volume; both were significantly reduced in abundance in the presence of crayfish. Among cobbles exposed to crayfish, fewer chironomids exceeded 3 mm in total length. Crayfish did not appear to have an effect on the size structure of any other taxa. Of the 8 other commonly encountered taxa, only Heptageniid mayflies were affected. These mayflies were only found with crayfish. The positive association between these grazing mayflies and crayfish may have been due to crayfish keeping cobble surfaces clean of FPM. In the lab experiment, crayfish significantly reduced leaf pack dry mass in four weeks. Crayfish also produced smaller leaf fragments and feces which may be important resources for cooccurring, detritivorous, invertebrate taxa. Crayfish were found to be important processors of detritus. In this system crayfish appear to be functioning as ecosystem engineers by reducing the volume of FPM in the substrate and increasing the rate of detrital breakdown. Size selective predation on the numerically dominant taxa is another possible effect crayfish are having in this headwater stream.