Developing Population Density Estimates for Nine Rare Willamette Valley Prairie Species

The Willamette Valley prairie habitat is one of the most imperiled ecosystems in the United States (Noss et al. 1995). Once common throughout the region, today prairie habitat is restricted, for the most part, to small, disturbed, and fragmented parcels (Altman et al. 2001). This decline in prairie habitat has inevitably resulted in the decline of prairie-associated species, including the nine rare plant species examined in this study. The Recovery Plan for Prairie Species of Western Oregon and Southwest Washington, currently in preparation by U.S. Fish and Wildlife Service and cooperators, will identify recovery objectives for all nine of these species (as well as one additional species, Castilleja levisecta, which no longer occurs in Oregon). The amount of occupied habitat needed for the development of selfsustaining populations of these species, and consequently for achieving the planned recovery objectives, can be calculated using data on the density of plants (number of individuals/m2) in currently established populations. The goal of this study is to develop current population density estimates for each of the nine rare prairie species still found in Oregon.