Fragmentation of forests into smaller, more isolated patches can hinder species movement, introduce predators and parasites from nonforest areas, and reduce habitat for wildlife that requires larger, connected patches. It is also possible for naturally "patchy" areas to become more uniform, which can change their habitat value.

Research on the specific mechanisms by which fragmentation changes forests and reduces their habitat value is at an early stage; there is little consensus on this point within the scientific community. Different species prefer different landscape patterns, and it is unlikely that any single measure of fragmentation is relevant in all situations. Project participants did not reach agreement on a single measure. Additional work is necessary.

The location and extent of forests intermingled with low-density housing development, as an indication of continuous human presence in forests. (Fig. 1)

Measures of the number and size of forest patches describe the intermingling of forest and nonforest areas. (A patch is area of forest separated from other forest areas by either nonforest or forest of a distinctly different age or type. See the section entitled Measuring Fragmentation below for additional discussion.)

The number of forest patches of different sizes. (Figs. 2 through 5)

How much of a region's forest is in patches of different sizes. (Figs. 2 through 5)

About 45 million acres (7 percent of the forest in the lower 48 states) have between one house per acre and one house every 20 acres. Lands with greater housing density are not generally considered forests.

A case study in the Mid-Atlantic region found almost 2 million forest patches 1.2 acres or less in size. Most of the region's forest (80 percent) was in four large patches, each with an area greater than 1 million acres. This indicates there are a few large patches of forest remaining in the Mid-Atlantic region. However, the figure showing forests and low-density housing suggests there is extensive human development within patches of all sizes in this region.

Patches of interior forest habitat distributed across the Mid-Atlantic region are illustrated by a pair of remote sensing images. Patches of interior forest habitat approximately 17 acres (left panel) are quite common outside the major agricultural and metropolitan areas. Patches about 1,500 acres (right panel) are fairly common in mountainous areas but quite rare in the Coastal Plain

Forests and Low-density Housing (1) Technical Note

Source: USDA Forest Service, Forest Health Monitoring

This map shows areas with housing density between one house per acre and one house per 20 acres. Forests are in gray.

Number of Forest Patches (Mid-Atlantic States) (2)	Forest Area in different Patch Sizes (Mid-Atlantic States) (3)
Technical Note
Source: EPA and USGS
Data on the number and area of forest patches in different size classes were developed in a case study of the Mid-Atlantic region (Pennsylvania, Delaware, Maryland, Virginia, District of Columbia). For this study, patches were defined as areas of forest surrounded by nonforests. The satellite images and data processing techniques used in this study did not recognize small breaks less than roughly 100 feet wide within forest patches.

(4) Interior Forest at Different Scales (5) Technical Note
Source: EPA and USGS
These satellite images show an index of the availability of interior forest habitat at two scales for the Mid-Atlantic area (Pennsylvania, Delaware, Maryland, Virginia, District of Columbia). Figure 4 illustrates available interior forest habitat for species that require relatively little space (roughly 17-acre sections); figure 5 shows habitat for species with larger area requirements (1,500-acre sections).

Measuring Fragmentation
	There are many different ways to measure fragmentation, because there are a variety of features that can be seen as subdividing a forest, and different scales at which the various definitions make sense. A forest crossed by a road and a power line comprises one, two, or four patches, depending on whether the road and/or power line are considered patch boundaries. Different age classes of trees within a forest, also shown here, may be considered different patches. Note that the satellite data used in Figures 2, 3, 4, and 5 do not distinguish many small gaps between forested areas, including roads, power lines, and age-class boundaries. We were not able to reach agreement on a measure of fragmentation associated with such features.
Source: Magpie Graphics