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Report on the Environment

Extent and Distribution

What are the trends in the extent and distribution of the nation's ecological systems?

Importance of Extent and Distribution

The extent and distribution of ecological systems influence the types of animals and plants that are present; the physical, chemical, and biological processes in the system; and the resiliency of the systems to perturbations.1

  • 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. At a regional scale, New England has the largest proportion of forest land.
    • At a local scale, forest stands may be distributed on either side of a road that bisects the forest. However, at a regional scale, 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 spatial scales. For example, extent and/or distribution of ecological systems can change over long time periods, 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, constructing a dam).
  • Introducing invasive species, such as the Asian carp.
  • Exposure to pollutants, such as acid rain, which can acidify lakes and streams and 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

Fragmentation will likely result in shifting distributions of species regardless of the nature of the impact.

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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ROE Indicators

The ROE presents seven national-scale indicators of trends relevant to the extent and distribution of ecological systems in the United States: Ecological Connectivity, Forest Extent and Type, Forest Fragmentation, Land Cover, Land Use, Urbanization and Population Change, and Wetlands. These indicators present trend information for forest and wetlands ecosystems and baseline information for the other areas covered.

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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[1] Wilson, E.O. 1992. The diversity of life. Cambridge, MA: Belknap Press.

[2] The H. John Heinz III Center for Science, Economics, and the Environment. 2005. The state of the nation's ecosystems: Measuring the lands, waters, and living resources of the United States. New York, NY: Cambridge University Press. Web update 2005.

[3] Fahrig, L. 1997. Relative effects of habitat loss and fragmentation on population extinction. J. Wildl. Manage. 61(3):603-610.

[4] Fahrig, L. 2003. Effects of habitat fragmentation on biodiversity. Annu. Rev. Ecol. Syst. 34:487-515.

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