摘要：Conversion of native sod (grassland) to cropland in the Prairie Pothole Region (PPR) is threatening important breeding habitats for migratory birds. About 50 percent of North American ducks are produced in the grasslands of the PPR, even though this habitat accounts for only ten percent of duck breeding territory. Once lost, native grassland habitats are difficult to reconstruct. To protect these habitats, the U.S. Fish and Wildlife Service (FWS) holds permanent easements prohibiting grassland-to-cropland conversion or wetland drainage on more than 3.5 million acres. The USDA also holds easements against grassland conversion.We estimated the additionality of grassland easements purchased in Montana, North Dakota, and South Dakota from 1997—2010. Higher additionality implies that a larger proportion of eased grasslands would have been converted to another use in the absence of the easement. Using propensity score models, our estimate of additionality for all easements is 3.4 percent for the years 1997-2010. For relatively high quality land, which is more likely to be converted, our estimate of additionality is 6.4 percent. When measured over longer periods, additionality will higher because more of the eased land would have been converted to another use.
