Habitat Area Requirements of Prairie Wetland Birds in Eastern South Dakota

The influence of area and vegetation structure on bird use of 830 semipermanent and seasonal wetlands (0.2-217.5 ha) was studied to evaluate vegetative needs and habitat area requirements of 20 breeding wetland bird species in eastern South Dakota in 1995-1996. Vegetative preferences of species varied, but most waterfowl and nongame species occur e4i more frequently in wetlands with intermediate cover-to-water ratios whereas five over-water nesting or secretive nongame species were found in wetlands with greater coverage of emergent vegetation. The occurrence of four nongame species that depend on vegetative structure to support the weight of their over-water nests also was positively associated with semipermanent wetlands dominated by thick-stemmed emergent vegetation (e.g., cattail [Typha spp]). Upland land use influenced waterfowl habitat use as the percentage of untilled uplands was 10.4% higher near semipermanent wetlands in which dabbling duck species richness was at a maximum compared to wetlands without dabbling ducks. Despite the importance of wetland and upland vegetation, multiple and logistic regression analyses indicated that wetland area was the best single predictor of species richness and of habitat use by individual species. Fifty-five to 100% of explained variation was attributed to area (partial R2 = 0.10-.048) while the occurrences of 95 and 79% of species in semipermanent and seasonal wetlands, respectively, were positively associated with wetland area. To further investigate the importance of wetland area, I used logistic regression and probability theory to estimate area requirements of individual species, investigate the importance of small wetlands, evaluate potential effects of landscape type on species occurrence, and determine minimum wetland areas needed to preserve multiple area-dependent species. Smaller semipermanent wetlands were predominantly occupied by area-independent species and by species whose area-dependency (50% probability of occurrence) was low (0.2-4.9 ha), whereas larger wetlands generally had a higher diversity composed of species whose area requirements ranged from 0.2-164.8 ha. Lower minimum area requirements and higher average occupancy rates in seasonal compared to semipermanent wetlands for five dabbling duck species indicated that small seasonal wetlands provided extensive breeding habitat for upland nesting waterfowl. Small seasonal wetlands also provided habitat comparable to that of larger semipermanent wetlands for three species of breeding nongame birds. Occurrences of seven bird species that were positively associated with total area of semipermanent and seasonal wetlands indicated that those species were more likely to be found in wetlands that were near other wetlands compared to isolated wetlands. Occurrences of black terns (Chlidonias niger) and American coots (Fulica americana), which fluctuated in response to landscape structure, indicated that low wetland density landscapes composed primarily of small wetlands did not provide suitable breeding habitat compared to high wetland density landscapes that contained a mixture of large and small wetlands. Over the range of semipermanent wetlands surveyed, the probability of finding at least eight area-dependent species in an 8-ha semipermanent wetland exceeded 0.50 when area was used as the sole management criterion. Despite the importance of small wetlands to dabbling ducks and area-independent nongame birds, it was unlikely that any number of small wetlands, no matter how well placed within landscapes, could provide suitable habitat for species that only use large wetlands. My results indicated that attributes within wetlands (e.g., percent vegetated wetland area, stem structure of emergent vegetation) as well as landscape-level attributes surrounding particular wetlands (e.g., proximity of nesting wetlands to other wetlands, proportion of upland grasslands near nesting wetlands) influence bird use of prairie wetlands. To my knowledge, this study also provides the only empirical information concerning minimum habitat area requirements for wetland birds, a parameter which estimates the likelihood of providing for even minimum populations of individual species and is an important component in developing prescriptive management recommendations for wetlands conservation. The identification of five area-independent nongame species in this study indicated that every seasonal wetland, regardless of its area, represents valuable breeding habitat for some wetland avifauna. Large semipermanent wetlands (\u3e95 ha) embedded in contiguous tracts of upland grasslands were the only wetlands large enough to support breeding populations of species requiring the greatest wetland area. In parts of eastern South Dakota that are largely devoid of semipermanent wetlands, smaller seasonal wetlands provided extensive habitat for dabbling ducks and for 10 species of nongame birds with minimum wetland area requirements of \u3c3.3 ha