The relationships among biomes, landscapes, and individual SNAs are interesting to trace and significant to SNA protection decisions. SNA site descriptions are arranged throughout this guide by the biomes within which they occur. Within each SNA are found several smaller levels of organization called plant communities. Plant Communities Plant and animal species flourish or perish, depending upon their environmental conditions. Local groupings of trees, shrubs, grasses, and forbs are called plant communities, and are characterized by the kinds and quantities of species they contain.
Communities are subject to change.
The change may be rapid, as after a disturbance, when pioneer species move into the altered environment. As the community develops, it may change the local conditions enough to favor other kinds of species, and a new community succeeds the old. Climax communities, such as the maple-basswood forest, continue to thrive under the conditions they create, and remain stable until disturbed. Communities serve as the basis for evaluating SNA priorities. A Key to Natural Communities, version 1.5, contains a description of 57 natural community types and 196 community subtypes.