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published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution.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

Undeveloped land in urban and suburban areas was analyzed to identify patches of natural land. “Natural” is defined to include all lands that have been classified in the extent indicator as any of the following: forests, grasslands and shrublands, or wetlands. The indicator presents the size distribution of contiguous patches composed of any of these land cover types, or combinations of them, by region.

There is a generally understood “rule” among conservation ecologists that smaller patches of habitat generally provide lower quality habitat than larger patches. There is some debate as to whether this is true for wetlands. There is some evidence that the quality of the habitat remains fairly constant regardless of its size (see Gibbs 1993). On the other hand, there is also evidence that isolated wetlands habitats (i.e., those not surrounded by undeveloped upland vegetation) are compromised in their habitat value (see Calhoun and Klemens, 2002).

The Data

Data Source: Satellite data are derived from the National Land Cover Dataset (NLCD), a product of the Multi-Resolution Land Characterization (MRLC) Consortium, which is a partnership between the U.S. Geological Survey (USGS), the U.S. Forest Service, the National Oceanographic and Atmospheric Administration (NOAA), and the Environmental Protection Agency (see http://www.epa.gov/mrlc/ or http://landcover.usgs.gov/nationallandcover.html).

Data Collection Methodology: Please refer to the national extent indicator technical note for a discussion of the NLCD.

Data Manipulation: Eight of the 21 NLCD classifications were defined as “natural” for this analysis. These include three classes considered as “forest” for this report (deciduous forests, evergreen forests, mixed forests); three types considered as “grasslands/ shrublands” (shrubland, grasslands/herbaceous, bare rock/sand/clay), and two wetlands types (woody wetlands, emergent herbaceous wetlands). Patches were defined as collections of 30-meter pixels in any of these eight classifications that touched one another either on their sides or at their corners. (Patches can be as few as one or as many as hundreds of pixels.) Data were processed on a state-by-state basis, and then these data were grouped based on the four regions. For a given region, the number of patches of various sizes were counted, thereby creating a distribution.

Data Quality and Caveats: Data were processed on a state-by-state basis, which means that in some cases a patch of natural land may have been broken into two segments at the state boundary by the analysis process. In addition, natural patches may well extend beyond the boundary of urban and suburban areas, meaning that the value reported here would be an underestimate of the actual size of the patch. Also, the smallest patches cannot be characterized by these methods, so estimates of the acreage (and percentage of total urban and suburban areas) in the less-than-10-acre category are an underestimate of the true value. This occurs because it is difficult to distinguish very small patches (e.g., one to a few pixels) that are mixed in with developed land cover types.

Also, the satellite data cannot be used to distinguish between a parcel of land that has always been grassland/shrubland or wooded and one that was developed but has since reverted to this apparently natural land cover (e.g., a dump or landfill). It would be misleading to label such land as “natural.” It is expected that this mislabeling occurred infrequently; however, it is not possible to estimate how much of an effect this might have had on the data.

Note: Additional caveats are listed in the technical note for the Area of Urban and Suburban Lands indicator.

Data Access: All these analyses were conducted at the Land Cover Applications Center at the USGS’s Earth Resources Observations Systems Data Center. The data are available (http://edcwww.cr.usgs.gov/programs/lccp/mrlcreg.html) at no cost from the MRLC Consortium, but considerable computing power is necessary to manipulate them. Note: The data available at the Web site listed here are the “raw” data from which estimates of urban/suburban area, and the size of natural areas within, were prepared. The actual data presented in this report were prepared specially for The Heinz Center for this report.

References

Calhoun, A., and M.W. Klemens. 2002. Best development practices (BDPs) for conserving pool breeding amphibians in residential and commercial developments (MCA Tech. Paper 5).

Gibbs, J.P. 1993. Importance of small wetlands for the persistence of local populations of wetland-associated animals. Wetlands 13(1):25-31.