Landscape Metrics Used to Assess Anthropogenic Influence on Wetlands in the Anacostia Watershed

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Abstract: Urbanization is increasing and wetlands are increasingly altered by this growing trend. More information is needed to assess the impact of urbanization on wetland function. This project analyzed the effects of urbanization on wetlands in the Anacostia Watershed at a broad scale. Remote sensing, geographic information system technologies, and the FRAGSTATS landscape analysis program were used to measure landscape patterns from which ecologic processes can be inferred. Urbanization was found to decrease wetland area, increase wetland fragmentation, and increase patch perimeter-to-area ratio.

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This study investigated landscape metrics in sub-watersheds, representing different levels of urbanization within the Anacostia Watershed. The objective of this research was to demonstrate how increasing urbanization affects habitat patches within the landscape. The location of this study was chosen because the Anacostia Watershed contains both highly degraded and relatively pristine sites and the health of the Watershed directly affects the condition of the Chesapeake Bay, one of our nation's largest and historically most productive ecosystems (Metropolitan Washington Council of Governments, 1994).

Urban landscapes are expected to have a smaller percent of watershed area as wetland habitats and these habitats are expected to be more fragmented. Although loss of habitat and fragmentation are closely related, they are not identical. Loss of habitat is a simple loss of area but fragmentation is the process by which once uninterrupted wetlands become wetland patches of various sizes, isolated from one another. Wetlands in more urbanized areas are therefore hypothesized to have a higher number of patches, less habitat area, a lower mean patch size, a greater perimeter-to-area ratio, and a greater mean nearest neighbor distance.

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The Anacostia Watershed drains approximately 170 mi² and includes both the coastal plain and piedmont physiographic provinces. The headwaters of the Watershed are found within Montgomery and Prince George's Counties in the state of Maryland and its endpoint and intersection with the Potomac River can be found within the District of Columbia. Both of these areas have undergone a substantial amount of urbanization since European settlement and this changing landscape has greatly affected the region's wetlands (Metropolitan Washington Council of Governments, 1994). Three sub-watersheds within the Anacostia Watershed, the Beaverdam Creek Watershed, the Northwest Branch Watershed, and the Sligo Creek Watershed, were chosen for this study. These sub-watersheds represent increasing levels of urbanization as indicated by increasing population densities and percent impervious surface, common indicators of urbanization.

Landscape Metrics were found to be correlated with indicators of urbanization, such as, population density and average imperviousness. Number of patches or the number of wetland patches within the landscape area was found to increase with urbanization until a threshold was met and at this time number of patches decreased as urbanization increased. Patch density or number of wetland patches per 100 hectares was found to decrease with increasing urbanization. Percent of landscape area or the percent of the landscape area that is comprised of wetlands was found to decrease with increasing urbanization. Mean patch size or the average size of all wetland patches was found to decrease with urbanization. Mean nearest-neighbor distance or average distance between patches of the same type was found to increase with urbanization. Mean perimeter-to-area ratio or the average perimeter to area ratio of all wetland patches within the landscape was found to increase rapidly with urbanization.

Landscape metrics demonstrate that as urbanization increases in the Anacostia Watershed, 1) wetland area is decreasing, 2) wetland patches are increasingly fragmented, and 3) wetland patch edge is increasing in relation to wetland patch interior. As wetland area decreases, remaining wetlands may be less efficient at a variety of functions including sediment and nutrient control, floodwater retention, groundwater recharge, and more. They may also not be able to meet minimum area requirements for some species and contain fewer habitat types/niches and therefore fewer species. As wetlands become increasingly fragmented, they may be less able to support viable populations of dependent flora and fauna, more susceptible to disturbance, and species interactions, such as, those between predators and prey may be altered. As wetland patch edge-to-interior ratio increases, these wetlands may become more susceptible to edge effects, such as, altered microclimate, increased disturbance, increased herbivory, and the addition of more invasive species. Inter-patch dynamics, such as, dispersal and energy flow may also be altered.

Curious about how the Paint Branch and Little Paint Branch Watersheds (which contain the University of Maryland) compare?

Citations

Metropolitan Washington Council of Governments, Department of Environmental Programs, 1994, Anacostia Watershed Water Quality Report: 1987-90. Authors: Herson-Jones, L., Warner, A., Jordan, B. and Hagan, K.

Mcgarigal, K. and Marks, B., 1994, FRAGSTATS: Spatial Pattern Analysis Program for Quantifying Landscape Structure. USDA Forest Service, Pacific Northwest Research Station, Portland, Oregon.