The 250,000 acres of wetlands in the Chesapeake Bay provide essential habitat for a multitude of plants and animals, including more than one million waterfowl in the winter.
But the Chesapeake Bay watershed has the fastest growing population in the United States, with over 16.5 million people currently residing there. Can people, animals, and plants coexist? What level of human disturbance would force animals out?
Scientists at the Smithsonian Migratory Bird Center wondered specifically what effect a burgeoning human population would have on the marsh- and water-bird communities of the bay.
The below maps show wetlands surrounded by heavy development (right) and one that is relatively pristine (left). Even moderate levels of development will force some kinds of birds to abandon a wetland home.
In the maps above:
P = Pristine Marsh - IBI score of 9.69
D = Developed Marsh, Curtis Bay - IBI score of 2.27
96 different wetlands around the Chesapeake Bay were monitored in the summers of 2002 and 2003. They ranged in size from one to 125 hectares (roughly 2.5 to 310 acres) and were located both near and far from human developments.
Wetlands that were located in heavily developed areas had the least diverse bird communities. The birds that specialize in inhabiting marshes were not present and the birds that were there tended to be those species that also inhabit nearby agricultural and urban habitats, such as red-winged blackbirds and common grackles.
The least bittern, pictured below, is an example of a marsh specialist that is extremely vulnerable to development.
Development covering as little as 14 percent of an area within 500 meters (less than a third of a mile) of a wetland were enough of a disturbance that marsh specialist birds would abandon the marsh.
The impact of development was even more severe for water birds, birds that used the surrounding open water or shoreline of the marsh to feed. Even urban areas covering four percent of an area within 500 meters was enough to deter some of them.
Why are birds that live in and around marshes so affected by development? There are direct effects. Birds that occupy nearby urban habitats move in and compete with the marsh birds for food. Predators that live in high densities in residential areas, such as cats, dogs, and raccoons, forage in the marsh and eat birds and their eggs and young. In addition, there are fewer safe places for birds to roost in developed areas.
There are also indirect effects of development on birds. For example, runoff from the land into the water contains pollutants that reduces the amount of food available for the birds. This effect was particularly pronounced in the year of the study, when it was very rainy.
This article summarizes the information in this scientific paper:
DeLuca, William V., Studds, Colin E., King, Ryan S. and Marra, Peter P. 2008. Coastal urbanization and the integrity of estuarine waterbird communities: Threshold responses and the importance of scale. Biological Conservation, 141(11): 2669-2678.
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