Northern Prairie Wildlife Research Center
With the exception of Daphnidae counts and Corisella weights, there were no significant differences between the treatment and control plots. As in the treatment by sampling day comparison, this was unexpected, as we anticipated that the duck carcasses would attract carnivores and scavengers. The significant differences between Daphnidae counts and Corisella weights were probably artifacts of natural population fluctuations or were derived from the tendency of aquatic invertebrates to occur in nonrandom aggregations (Elliott 1977, Resh 1979, Minshall 1984).
The significant differences among sampling days for the counts of the six major taxa and the weights of three of the six major taxa were expected. We anticipated that counts and weights would vary over time in response to the presence of the carcass and other environmental variables such as water temperature, dissolved oxygen, and pH. However, due to the general absence of significant differences for the treatment by day interaction and the treatment effect, the significant differences among sampling days are probably due to the characteristics of aquatic invertebrate populations discussed above.
Our sampling distance of 0.4 m may have been too far from the carcasses to detect changes in aquatic invertebrate populations. However, this distance appeared appropriate during a pilot study to determine spacing between treatment and control plots (Hicks 1992).
The purpose of the study was to determine the effect of duck carcasses on aquatic invertebrate populations. The study also demonstrated the effect of carcass removal, a common management practice on many waterfowl refuges, on aquatic invertebrate populations. Invertebrates were sampled prior to adding carcasses and also one week after carcasses were removed. We found that carcass removal did not affect aquatic invertebrate taxonomic diversity, counts, or weights. Carcass removal by refuge managers apparently does not affect the aquatic invertebrate food supply available to surviving waterfowl and other wildlife.
Based on our results, duck carcasses contribute little to increasing aquatic invertebrate taxonomic diversity or abundance in a seasonal wetland in the fall. Little benefit to migrating waterfowl in terms of increased invertebrate food supply was realized in the study wetland. McGilvrey (1966), Sugden and Driver (1980), and others have demonstrated that invertebrates are important to fall-migrating waterfowl. Refuge managers would be advised to continue to attempt to increase invertebrate food supplies for waterfowl in seasonal wetlands by more traditional methods such as water level and vegetation manipulation (Ringelman 1990).