Ecoregions (or Ecological Regions) denote areas of general similarity in ecosystems and in the type, quality, and quantity of environmental resources. There are two systems of ecoregions available to use as filters in BioData. Both are arranged in geographically hierarchical "levels." The North American is available in three levels: NA I, NA II, and NA III. The Continental United States (also know as "Omernik") is available in two levels: US III, US IV.