U.S. FISH AND WILDLIFE SERVICE
Executive Summary
This study examined recent trends in wetland extent and habitat type throughout the conterminous United States between 2004 and 2009. Wetland trends were measured by the examination of remotely sensed imagery for 5,042 randomly selected sample plots. This imagery in combination with field verification provided a scientific basis for analysis of the extent of wetlands and changes that had occurred over the four and half year time span in this study.
This information provides a quantitative measure of the areal extent of all wetlands, regardless of ownership, in the conterminous United States. Wetlands were defined using biological criteria and standardized nomenclature for the classification of wetland types. Recently acquired remotely sensed imagery was used as the principle means to assess wetland change with a number of geoprocessing and quality control measures implemented to ensure data completeness and accuracy. The spatial sample design involved randomized sampling of geospatial information on 4.0 mi2 (10.4 km2) plots. This was a well-established, time-tested procedure that provided a practical, scientific approach for measuring wetland area extent (status) and change rates (trends) in the conterminous United States. Statistical estimates provided national status and change information as well as estimates by major wetland type. Field verification was completed for 898 (18 percent) of the sample plots during 2009 to 2010. Field sites were dispersed in portions of 42 States.
Enhancements to this study included augmentation to the number of sample plots along the Pacific coast of Washington, Oregon and California. This augmentation was done to provide estimates of estuarine and marine wetlands not included in the original sample design and provide a more complete estimate for these wetland types nationally.
Because of the increased area of created freshwater ponds in recent years, additional descriptive categorization for freshwater ponds was developed and implemented as part of this study. Further categorization of the physical and ecological characteristics of freshwater ponds was intended to provide information about what types of ponds have been created over time.
This report did not draw conclusions regarding trends in the quality or condition of the Nation’s wetlands, but rather it provided data regarding trends in wetland extent and type and provided baseline information to facilitate ongoing collaborative efforts to assess wetland condition. Further examination of wetland condition on the national level has been initiated by the Environmental Protection Agency in conjunction with the Fish and Wildlife Service and other Federal, State and Tribal partners.
The study indicated that there were an estimated 110.1 million acres (44.6 million ha) of wetlands in the conterminous United States in 2009 (the coefficient of variation of the national estimate was 2.7 percent). An estimated 95 percent of all wetlands were freshwater and 5 percent were in the marine or estuarine (saltwater) systems. With the exception of minor statistical adjustments to the area estimates, the overall percentage of wetland area and representation by saltwater and freshwater components remained unchanged.
Estuarine emergent (salt marsh) wetland was the most prevalent type of all estuarine and marine intertidal wetland. Salt marsh made up an estimated 66.7 percent of all estuarine and marine wetland area. Forested wetlands made up the single largest category (49.5 percent) of wetland in the freshwater system. Freshwater emergents made up an estimated 26.3 percent, shrub wetlands 17.8 percent and freshwater ponds 6.4 percent by area.
The difference in the national estimates of wetland acreage between 2004 and 2009 was not statistically significant. Wetland area declined by an estimated 62,300 acres (25,200 ha) between 2004 and 2009. The reasons for this are complex and potentially reflect economic conditions, land use trends, changing wetland regulation and enforcement measures and climatic changes. Certain types of wetland exhibited declines while others increased in area. The result of these gains and losses yielded the net change and it was possible to have losses or gains of particular wetland types that exceed the overall net change for all wetlands.
Collectively, marine and estuarine intertidal wetlands declined by an estimated 84,100 acres (34,050 ha) or an estimated 1.4 percent between 2004 and 2009. The majority of these losses (73 percent) were to deepwater bay bottoms or openocean. Losses of estuarine emergent (salt marsh) and changes in marine and estuarine non-vegetated wetlands reflected the impacts of coastal storms and relative sea level rise along the coastlines of the Atlantic and Gulf of Mexico. The majority (99 percent) of all estuarine emergent losses were associated with processes related to the marine environment such as saltwater inundation and/or coastal storm events. The effects of sea level on wetlands are subject to considerable uncertainties; however, recent changes in non-vegetated intertidal wetlands (beaches, bars and shoals) along the South Atlantic and Gulf of Mexico indicated considerable instability and change. Coastal environments continue to face a variety of stressors that can interact with climate-related processes and potentially increase the vulnerability of coastal wetlands.
Overall, freshwater wetlands realized a slight increase in area between 2004 and 2009. Freshwater ponds have continued to increase although the rate of pond development had slowed from previous reporting periods. Freshwater vegetated wetlands continued to decline albeit at a reduced rate. This most recent annual rate of loss represented a reduction in the loss rate of roughly 50 percent since 2004. Declines in freshwater forested wetland area (633,100 acres or 256,300 ha) negated area gains in freshwater emergent and shrub categories.
Forested wetlands sustained their largest losses since the 1974 to 1985 time period. Freshwater wetland losses continued in regions of the country where there has been potential for wetlands to come into conflict with competing land and resource development interests.
Between 2004 and 2009, 489,600 acres (198,230 ha) of former upland were re-classified as wetland. These increases were attributed to wetland reestablishment and creation on agricultural lands and other uplands with undetermined land use including undeveloped land, lands in conservation programs or idle lands. The rate of wetland reestablishment increased by an estimated 17 percent from the previous study period (1998 and 2004). Conversely, the estimated wetland loss rate increased 140 percent during the same time period and, as a consequence, national wetland losses have outdistanced gains.
The cumulative effects of losses in the freshwater system have had consequences for hydrologic and ecosystem connectivity. In certain regions, profound reductions in wetland extent have resulted in habitat loss, fragmentation, and limited opportunities for reestablishment and watershed rehabilitation.
Download full version (PDF): Status and Trends of Wetlands in the Conterminous United States
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