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.

Study Sites

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.

Urban Impact

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 publication:

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.

View abstract

Estuarine ecosystems are becoming increasingly altered by the concentration of human populations near the coastline, however a robust indicator of this change is lacking. We developed an index of waterbird community integrity (IWCI) and tested its sensitivity to anthropogenic activities within 28 watersheds and associated subestuaries of Chesapeake Bay, USA. The IWCI was used as a tool to gain insight into how human land use affects estuarine ecosystem integrity. Based on Akaike’s information criteria (AIC), a single variable model including percent developed land in estuarine watersheds was thirteen (2002) and twenty-six (2003) times more likely than models including percent agriculture and forest cover to fit the IWCI data. Consequently, we examined how suburban, urban, and total development shaped IWCI scores at three spatial scales: (1) watershed; (2) inverse-distance- weighted (IDW) watershed (land cover near the coastline weighted proportionally greater than that farther away); (3) local (land cover within 500 m of the coastline). Suburban, urban, and total development were all significant predictors of IWCI scores. Relationships were stronger at the IDW and local scales than at the whole watershed scale. Nonparametric changepoint analysis revealed a >80% probability of a threshold in IWCI scores when as little as 3.7% (2002) and 3.5% (2003) of the IDW land cover within the watershed was urban. Our results indicate that, of the landscape stressors we examined, development near estuarine coastlines is the primary stressor to estuarine waterbird community integrity, and that estuarine ecosystem integrity may be impaired by even extremely low levels of coastal urbanization.

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