The conservation status factors used by NatureServe are organized into three categories: rarity, trends, and threats. This revised publication defines the factors and lays out the conditions for whether and how each factor should be used.
There was a revised version of this publication published in 2012 that can be found and downloaded here.
- Master L, Faber-Langendoen D, Bittman R, Hammerson GA, Heidel B, Nichols J, Ramsay L, and Tomaino A. 2009. NatureServe Conservation Status Assessments: Factors for Assessing Extinction Risk. Arlington, VA: NatureServe