[摘要] Grassland ecosystems originally dominated central North America. Tallgrass prairie in the east graded into mixed-grass prairie and, finally, shortgrass prairie in the west (
Samson et al. 1998,
Askins et al. 2007). Prairies, in particular the tallgrass prairie, are among the most extensively altered systems in North America, owing to Native American management practices and subsequent settlement and development of agriculture by Europeans (
Houston and Schmutz 1999,
Higgins et al. 2002,
Askins et al. 2007). Currently, smaller and more fragmented patches of planted grasslands dominate the remaining eastern grassland systems (
Warner 1994,
Askins et al. 2007). These surrogate grasslands are composed of Eurasian grass and forb species, are typically associated with agriculture, and include hay fields, pastures, and fallow and old fields (
Sample et al. 2003). In the Great Plains, woody encroachment and agriculture are reducing the area of grasslands, reducing patch size, and increasing edge (
Coppedge et al. 2001a,
Grant et al. 2004). More recently, even these surrogate grasslands are disappearing (
Askins et al. 2007). In the United States, between 1982 and 2003, ∼6 million ha of pasture and ∼4 million ha of rangeland were lost (
Natural Resources Conservation Service 2007). Biofuel development that results in the loss of undisturbed grassland is an emerging conservation issue (e.g.,
Fargione et al. 2008).
