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published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution.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

The size, shape, and juxtaposition of habitat patches within a landscape, in addition to the total extent of the habitat, influence the population size and viability of sensitive species (Meffe and Carroll 1994). Dozens of metrics have been developed to quantify spatial pattern within landscape mosaics. Some metrics quantify the size, shape, or juxtaposition of individual patches of a single land cover type. Others quantify the spatial relationships (e.g., juxtaposition) among patches of different land cover types.

This indicator measures the geometry, or spatial configuration, of “natural” areas in farmland landscapes. Natural areas are native habitats, including forests, grasslands, wetlands, and naturalized habitats, such as land enrolled in the Conservation Reserve Program (CRP) that formerly was cropland and is now reverting to, for example, forest or grassland. For the purposes of this indicator, open water is not included; however, wetlands are included. (Special attention should be paid to the ability of the land cover datasets, which are based largely on satellite measurements, to account for wetlands.)

This indicator will be calculated for individual patches, yet index values will be reported at the national scale. The indicator will be based on perimeter-to-area (P/A) ratios that have been normalized by area (i.e., divided by the patch’s area). Rather than directly reported P/A values, however, three size and shape classes will be determined by a statistical analysis of each region’s P/A ratios. The shape classes will be compact (e.g., a circle, which has the lowest P/A ratio for a given area), intermediate, and elongated (e.g., a long, narrow rectangle, which has a high P/A ratio for a given area). The surface area within each size and shape class will be reported nationally. This patch-based index should be area-weighted. This weighting ensures that smaller patches (with higher P/A ratios) have less influence on the aggregate index than larger patches do. The indicator will be calculated for the aggregated area of all forests, grasslands, shrublands, and wetlands in the farmland landscape, rather than being calculated for each “natural” land cover type independently.

Landscape structure—or the spatial configuration of patches, corridors, and the intervening “matrix”—influences ecosystem integrity. Yet spatial pattern is a complex phenomenon that cannot be summarized with a single index. The number, size, shape, orientation, and spatial distribution of land cover patches are important landscape attributes. Other ecologically significant aspects of landscape pattern include the proportion and spatial arrangement of different land cover types.

Agricultural activities have extensively changed many landscapes. “Fragmentation,” caused by land use changes and other disturbances, may alter landscape structure by changing land cover area and spatial configuration. Within intensively farmed landscapes, natural areas comprise a relatively small percentage of the surface area. Typically, these natural areas include relatively small and isolated remnants of formerly contiguous native vegetation, in addition to restored conservation areas (e.g., CRP land). These native and naturalized areas provide wildlife habitat, control erosion, and perform other important ecological and cultural functions. Patch size and shape influence the differentiation of patches into distinct edge and interior habitats. Small patches typically have a higher ratio of edge to interior habitat than very large patches with the same shape. Conversely, linear patches have a much higher proportion of edge to interior habitat than patches with the same area, but more compact shapes. Small or highly dissected patches may have little or no interior habitat. The functional connectivity among patches of natural areas depends not only on the distances between the patches, but on the intervening land use and land cover conditions. The land covers (e.g., built-up) and land uses (e.g., farming) that separate natural areas can significantly influence biodiversity and species abundance at landscape and regional scales.

The Data Gap

Calculating this index will require digital data and specialized software designed to analyze landscape spatial patterns. Data appropriate for calculating this index are available from the National Land Cover Dataset, which was used to define the “farmland landscape” for this report. However, the most commonly used software (Fragstats) for analyzing landscape spatial patterns is not capable of processing the very large file sizes that would be required to calculate this index for the nation. That said, there may be simpler approaches that would not have such computing demands, although these have not been fully explored. It might be possible to make use of existing remote-sensing data through a procedure involving random sampling. In such a procedure, rather than processing the entire dataset, samples would be processed, much as a field program such as the USDA National Resources Inventory (http://www.nrcs.usda.gov/technical/NRI/1997/) collects data from a representative sample of sites. However, the specific approach needed for such a sampling program was not fully explored during the development of this report.

References

Meffe, G.K., and C.R. Carroll, eds. 1994. Principles of conservation biology. Sunderland, MA: Sinauer.