EPA's Report on the Environment: External Review Draft
Extent and Distribution
Extent and DistributionWhat are the trends in the extent and distribution of the nation's ecological systems?
Importance of Extent and Distribution
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- “Extent” refers to the physical area covered by an ecological system. It can be reflected as an area or as a percentage of a baseline or of the total area. Extent indicators typically are based on physical and biological characteristics that are observable by remote sensing, but some ecosystems have indistinct boundaries. Scientific or resource management principles and procedures are used to define extent for those ecosystems with indistinct boundaries.2
- “Distribution” includes the pattern or arrangement of ecological systems. Distribution depends on the scale of analysis. For example:
- At a national scale, most of the forests are located in the eastern United States, with the distribution of forests in the West being in the mountains. However, at a regional scale, the Mid-Atlantic region has the greatest distribution of undisturbed forest.
- At a local scale, forest stands may be distributed on either side of a road that bisects the forest. However, at a regional level, this road may not even be visible—the forest may appear as a continuous stand.
- At a larger scale, coastal wetlands may appear to be uninterrupted and uniform. However, at a more refined scale, edges, patches, corridors associated with tidal creeks, and discontinuous distributions of species become evident.
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Extent and distribution can be impacted by both natural forces and human activities happening over different time and space scales. For example, extent and/or distribution of ecological systems can change over long time scales, such as those associated with climate and geological forces (like the formation of glaciers), or more quickly as a result of other activities such as:
- Direct shifts in land use (for example, filling wetlands or developing a subdivision where a forest used to be).
- Altering hydrological cycles (for example, by removing a dam).
- Introducing invasive species, such as the Asian carp.
- Exposure to pollutants, such as acid rain, which can kill certain trees in high elevations.
- Extreme weather events, such as hurricanes or tornadoes.
Fragmentation (the alteration by division of previously uninterrupted habitat) is a key stressor affecting the extent and distribution of ecological systems. Examples of fragmentation include:
- Building highways through a forest.
- Building a dam on a river that limits fish migrations.
- Developing waterfronts in a manner that splits bordering marshlands apart.
Fragmentation can have negative or positive impacts on animal or plant communities.3 For example, fragmentation, and the associated increase in the area of edge habitat:
- May force migrating species to find new transport corridors.
- May allow new species to enter areas that they were previously unable to enter, which can actually increase biodiversity in some cases.4
Brief, local changes, such as seasonal droughts or a windfall in a closed forest canopy, may not have large-scale or long-lasting effects on ecological systems. Chronic changes over large areas may impact entire ecological systems for long periods of time, especially if they affect soil formation, microclimate, or the amount of refuge available for recolonizing species.
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No indicators meeting the ROE indicator criteria are available for other types of terrestrial or aquatic systems, including grasslands, shrublands, and marine hard bottom communities such as coral reefs, or for finer-scale ecosystem classifications such as riparian zones or habitat for threatened and endangered species.
National-scale land use and land cover, forest fragmentation, and ecological connectivity indicators are derived from satellite imagery. As a result, the future availability of national-scale land use and land cover information is dependent on the U.S. satellite program.
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