David Mellor (Biology) College of William and Mary
Crayfish, acting as consumers across several trophic levels have the capacity to dramatically alter an ecosystem. They are known to graze on plant matter as well as predate on snails and small fishes. To measure the effect of crayfish populations on algal biomass and snail populations, twelve crayfish from the genus Cambarus were transported to an enclosure in the Crim Dell stream. The hypothesis was that the addition of crayfish would lower the snail and small fish population, allowing algae biomass to increase more than in the control enclosure.
Three sets of enclosures were set up, each with two enclosures in close proximity. In each set of two, 12 crayfish were placed in the downstream enclosure. Two 4-inch square ceramic tiles and a periphytometer with 9 glass slides were placed in each enclosure to provide the surface on which algae would grow and snails could graze. One of the three enclosures did not receive crayfish because it was damaged in a storm before crayfish could be added. Further, no crayfish could be caught in mid-July, possibly due to synchronized summer molting when crayfish hide in their burrows while a new exoskeleton is formed.
Although algal biomass did not accumulate on the growing surfaces of tiles or periphytometers, chironomid larvae did colonize them to different extents when crayfish were or were not present. In the enclosure containing crayfish, an average of 39.1 chironomids settled on each glass slide, significantly greater than the average of 15.7 settling on slides in the enclosure not containing crayfish (t-test, p < 0.05). One possible explanation of this result is that the crayfish predated on the chironomid's predators, the snails. Owing to cage damage following a large storm event at the end of the study, however, no estimation of snail populations in the two enclosures could be completed.
For additional documentation David Mellor provided a PowerPoint Presentation entitled "The Effect of Cambarus Crayfish on Stream Ecology" provided here in PDF form.