Note that the data published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution. For the most recent data, purchase the 2008 Report from Island Press.

The Indicator

This measure would report the percentage of grasslands and shrublands in patches of different sizes. Patch sizes and percentages would be reported separately for grasslands and shrublands. The patch sizes for this indicator are as follows: less than 10 acres, 10–99 acres, 100–999 acres, 1000–9,999 acres, and 10,000 acres or greater.

Species and ecosystem processes are sensitive to spatial heterogeneity. Landscape diversity is an important component of species diversity, habitat conservation, and human health. While much research has been undertaken to determine these relationships in forested ecosystems, there is general agreement among grass/shrub experts that such spatial patterns are important in grasslands and shrublands as well. For example, in the sagebrush/grassland mosaic of western intermountain basins, fuel buildup after a period of minimal grazing and sufficient rainfall creates highly flammable conditions. When a fire does occur, the sagebrush is greatly reduced in abundance because it cannot sprout, unlike nearly all the grassland plants. Grassland expands, reducing the area of habitat for sagebrush-dependent species.

The Data Gap

High-quality satellite data provide an excellent baseline to assess future changes in patch sizes. Many of the indicators in this report are based on data from the National Land Cover Dataset, produced by a federal interagency consortium including the U.S. Geological Survey, the USDA Forest Service, the National Oceanographic and Atmospheric Administration, and the U.S. Environmental Protection Agency (see http://www.epa.gov/mrlc/nlcd.html and the technical note for the national extent indicator. It is expected that satellite data will be used for this indicator.

However, the software currently available for analyzing patch size characteristics was developed for use on relatively small landscape areas. It is not designed, and cannot be used, to process datasets as large as are required for this indicator. An alternative approach involves analyzing patch characteristics for smaller landscape areas (such as 7.5 x 7.5 km), then combining the statistics on these many individual areas to describe much larger areas, such as ecoregions. However, in this approach, any patch that crosses the boundary of one of the 7.5 by 7.5 km squares is not accurately represented, because a portion is in one square and a portion is in the adjoining square. This is referred to as a rightcensored distribution and will provide consistent underestimates of the number of larger patches.

In addition, the indicator should distinguish between lands that have been altered (e.g., cultivated and seeded for pastures) and more “natural” lands; only patches of the more natural grasslands and shrublands would be included.

References

Turner, M.G., and R.G. Gardner, eds. Quantitative methods in landscape ecology. Springer-Verlag Ecological Studies, Vol. 82. New York: Springer-Verlag.