Dabbling Duck and Aquatic Macroinvertebrate Responses to Manipulated Wetland Habitat

Responses of breeding dabbling ducks (Anatini) and aquatic macroinvertebrates to experimental modifications of cover:water ratio and basin surface were investigated in 1977 and 1978 within an impounded whitetop rivergrass (Scolochloa festucacea) meadow on the Delta Marsh, south-central Manitoba. Three areal percentage ratios of emergent hydrophytes to open water (30:70, 50:50, or 70:30) and 2 basin treatments (mowing of existing emergents or scarification by rototilling) were tested. Between years, pair numbers of mallards (Anas platyrhynchos) and blue-winged teal (A. discors) declined, whereas pair numbers of northern shovelers (A. clypeata), gadwalls (A. strepera), and pintails (A. acuta) were comparable. The greatest density and species diversity of dabbling duck pairs occurred on 50:50 plots in both years. Only blue-winged teal and pintail pair densities in 1978 were greater on mowed than on rototilled areas. Within years, species diversity of dabbling ducks was unaffected by mowing or rototilling. More pursuit flights arose from 50:50 plots and mowed areas compared to alternative treatments. Composition and resource levels (abundance, biomass, and number of families) of aquatic macroinvertebrate communities varied within and between years in response to basin treatments. These results imply prescriptions for wetland habitat management. J. WILDL. MANAGE. 45(1):1-15 The interaction of proximate and ultimate factors continues to intrigue ecologists studying avian habitat selection. Since the pioneering work by Beecher (1942) and Svardson (1949), significant theoretical advances in our understanding of the evolution of habitat selection have emerged (e.g., MacArthur and Levins 1964, Fretwell and Lucas 1969, Bryant 1973, Rosenzweig 1974, Southwood 1977). Generally, theory predicts that animals should select habitats that maximize fitness. Unfortunately, the complexity of such a prediction makes hypothesis testing impractical under most environmental conditions. Habitat selection in birds is seemingly guided by instinctive and experiential influences from the physical and/or social environment (Hilden 1965). Numerous attempts have been made to identify key factors associated with habitat selection within and among bird species. Most investigators have correlated bird species abundance or diversity with environmental variables, and therefore cannot distinguish between habitat selection and habitat correlation (Wiens 1976). True habitat selection occurs when individuals exercise a choice among available habitats, instead of differentially occupying them as a consequence of extrinsic factors like predation and competition (Klopfer 1969; Wiens 1976, 1977). In field investigations, however, it is not usually possible to control extrinsic factors. Only rarely have researchers studied avian habitat selection using experimental procedures (e.g., Klopfer 1963, Partridge 1974). The experiment described here was designed to investigate the effects of habitat structure and resource levels of aquatic macroinvertebrates on habitat use by breeding dabbling ducks. Most literature on this subject originates from studies conducted in prairie-potIContribution 9125, Michigan State University Agricultural Experiment Station. 2 Present address: Ducks Unlimited (Canada), 1190 Waverley Street, Winnipeg, Manitoba R3T 2E2, Canada. J. Wildl. Manage. 45(1):1981 1 2 DABBLING DUCK-AQUATIC INVERTEBRATE R SPONSES * Kaminski and Prince hole habitats of the United States and Canada. Consequently, our understanding of habitat use by breeding dabbling ducks on large marshes is inadequate. Weller and Spatcher (1965) and Weller and Fredrickson (1974), studying several marshes in Iowa, reported that avian abundance and diversity were highest during years when emergent hydrophytes and open water covered approximately equal areas in a highly interspersed pattern. They termed this stage of marsh transition the hemi-marsh phase. Inspired by their work, we conducted an experiment on the Delta Marsh, south-central Manitoba, to test the hypothesis that there would be unequal responses in density and species diversity of breeding dabbling ducks to differing combinations of emergent vegetation and open water, and to modifications of basin surface. Moreover, we tested the hypothesis that differences in abundance, biomass, and familial diversity of aquatic macroinvertebrates would result in response to these treatments. We acknowledge the support of the North American Wildlife Foundation, the Delta Waterfowl Research Station, Ducks Unlimited (Canada), the Ducks Unlimited Foundation, Michigan State University Agricultural Experiment Station, Manitoba Department of Mines, Natural Resources and Environment, and the Canadian Wildlife Service. The following individuals reviewed the manuscript and/or assisted with project implementation: B. Batt, D. Beaver, P. Caldwell, L. Gysel, R. Jones, J. Larson, A. Macaulay, H. Murkin, J. Nichols, D. Rabe, G. Richardson, S. Stephenson, P. Ward, M. Weller, and F. Willis. We are indebted to the assistants of the R. Howard Webster Fellowship program, the staff, and our colleagues of the Delta Waterfowl Research Station. STUDY AREA The study was conducted on a 33-ha tract of the Delta Marsh in south-central Manitoba (50011'N, 98'19'W), approximately 2 km east of the Delta Waterfowl Research Station. A detailed description of the local physiography was provided by Fenton (1970), and the floristic communities have been thoroughly described (Hochbaum 1944, Love and Love 1954, Olsen 1959, Walker 1965, Anderson and Jones 1976). A wooded beach ridge separating Lake Manitoba from the Delta Marsh formed the northern boundary of the study area. In March 1977, an earthen dike was constructed around the southern reach of the area so water levels could be controlled. Originally, an unbroken dense stand of emergent hydrophytes covered the area. Thus, there was no existing natural interspersion of vegetation and open water to confound experimental levels of interspersion. The predominant plant species was whitetop rivergrass; it covered 60% of the study area. Other emergents included giant reed (Phragmites australis) (30%), common cattail (Typha latifolia) (5%), and sedges (Carex spp.) (5%). The soil was a histisol (Buckman and Brady