Northern Prairie Wildlife Research Center
Studies of aquatic invertebrates in wetlands are severely bounded by limitations of available sampling gear and techniques. Invertebrate sampling in prairie wetlands is difficult because these systems are structurally complex and invertebrate populations are dynamic and patchily distributed. Researchers have developed a variety of traps for collecting invertebrates, yet all these sampling devices impart biases, which are rarely assessed.
Conventional activity traps (ATs), usually modified after designs of Whitman (1974), Swanson (1978), Murkin et al. (1983), or Ross and Murkin (1989), are widely used for semiquantitative invertebrate sampling in prairie wetlands (Murkin et al. 1983, Ross and Murkin 1989, Bataille and Baldassare 1993, Hanson and Riggs 1995, and many others). ATs are light, portable, and inexpensive to construct. ATs may be deployed horizontally or vertically and at various depths, depending on wetland characteristics and specific study objectives. More importantly, ATs perform well in terms of number and diversity of invertebrates captured, and sample processing time is usually shorter than that associated with more quantitative gear (such as sweep nets, Gerking, or sediment core samplers) (Murkin et al. 1983, Brinkman and Duffy 1996, Turner and Trexler 1997). Additionally, they gather clean samples containing little organic material, sediment, plant fragments, etc. Clean samples result in shorter sample processing times and often allow more intensive sampling, thus reducing magnitude of within-wetlands variance estimates.
We used ATs similar to those described by Swanson (1978), Murkin et al. (1983), and Ross and Murkin (1989) in studies assessing influences of invertebrate availability on early growth and survival of young mallard ducklings (Cox et al. 1998). We also developed surface-associated ATs (SATs) to better sample invertebrates in the primary duckling feeding zone. We defined this region as a vertical stratum 25.40 cm thick, extending from 15.24 cm below to 10.16 cm above the water surface. We assessed performance of our SATs in relation to ATs by comparing non-detection rates and relative abundance estimates of capture for 13 taxa of common aquatic invertebrates. We discuss implications of these results for future investigators, especially researchers interested in sampling invertebrates available to foraging birds such as young ducklings.