Livestock grazing in intermountain depressional wetlands: effects on breeding waterfowl

Abstract

Livestock grazing is a prevalent land use in western North American intermountain wetlands, and physical and biotic changes related to grazing-related disturbance can potentially limit wetland habitat value for waterfowl. We evaluated breeding waterfowl use in 34 wetlands in relation to water retention, amount of wetlands on the landscape, and livestock grazing intensity. The study was conducted over 2 years in the southern intermountain region of British Columbia, Canada. For a subset of 17 wetlands, we measured aquatic invertebrate abundance over 1 year. Waterfowl breeding pairs and broods were classified into three functional groups: dabbling ducks, and two types of diving ducks, overwater and cavity nesters. We evaluated candidate models with variables considered singly and in combination using the Akaike Information Criterion. When selected, bare ground (an indicator of grazing intensity) and wetland density were negatively associated with breeding use while wetland fullness and invertebrate density were positively associated. Each factor was a significant predictor in at least one of the models, but unexpectedly, grazing intensity was the most consistent predictor of waterfowl wetland use (e.g., it was present in more ‘best models’ than wetland fullness). Grazing was associated with declines in the number of waterfowl pairs and broods, likely mediated through effects on wetland vegetation and aquatic macroinvertebrates. Models with site- and landscape-scale variables generally performed better than simpler models. Waterfowl breeding use of wetlands can be improved by reduced livestock grazing intensity adjacent to wetlands and by grazing later in the season. Wetland water retention is also an important constraint on waterfowl use of wetlands and may become more limiting with a shifting climate.