Ecosystem engineering by crayfish in a headwater stream community

We conducted an enclosure/exclosure experiment in a forested, headwater stream to determine if crayfish acted as ecosystem engineers in this community. Three sets of channels (4 channels per set) were placed in separate pools. All channels contained cobble-filled baskets. Large crayfish (Cambarus bartonii) or a 5 g chestnut oak (Quercus prinus) leaf pack were added to some channels. The 4 treatments in the experiment were: 1) no crayfish/no leaf pack, 2) no crayfish/leaf pack, 3) crayfish/no leaf pack, and 4) crayfish/leaf pack. This design allowed us to assess the effects on the benthic community of adding crayfish and a leaf pack. It also allowed us to assess the effect of large crayfish on leaf pack breakdown. Leaf pack breakdown rate was significantly faster in the presence of crayfish. Crayfish did not have a significant effect on the abundance of any invertebrate taxon associated with leaf packs. However, few large (≥3.0 mm long) chironomids were found in leaf packs exposed to crayfish after 8 wk. Crayfish significantly reduced the abundance of fine particulate matter (FPM, a mix of fine inorganic sediment and fine particulate organic matter), chironomids, and harpacticoid copepods in the baskets. Heptageniid mayfly larvae, however, were found only in channels in which crayfish had reduced the abundance of FPM. Few large chironomids were found in baskets exposed to crayfish. Only harpacticoid copepods displayed a significant response to the addition of a leaf pack; this effect was observed only after 4 wk in channels containing crayfish. Our results demonstrate that crayfish can function as ecosystem engineers in forested, headwater streams because they can influence detrital processing rates and the distribution of FPM. In addition to acting as ecosystem engineers, crayfish may affect some invertebrate taxa directly through size-selective predation.