This EnviroAtlas dataset identifies rare ecosystems using base landcover data from the USGS GAP Analysis Program (Version 2, 2011) combined with landscape ecology principles. The metrics included in this data set identify total rare acres and the protected rare acres within each 12-digit Hydrologic Unit (HUC). The percentage of the terrestrial area of each HUC covered by rare ecosystems as well as the percent of rare acres that are protected are also included. This dataset was produced by the US EPA to support research and online mapping activities related to EnviroAtlas. EnviroAtlas (https://www.epa.gov/enviroatlas) allows the user to interact with a web-based, easy-to-use, mapping application to view and analyze multiple ecosystem services for the contiguous United States. The dataset is available as downloadable data (https://edg.epa.gov/data/Public/ORD/EnviroAtlas) or as an EnviroAtlas map service. Additional descriptive information about each attribute in this dataset can be found in its associated EnviroAtlas Fact Sheet (https://www.epa.gov/enviroatlas/enviroatlas-fact-sheets).