When lawns, roads and buildings begin moving closer to tidal marshes, it means many of the birds living in those areas will soon be moving out.
New research, which examined marshes from the mouth of the Chesapeake to the head of the Bay, revealed a clear link between even small levels of development in areas surrounding tidal wetlands and the loss of many marsh-dwelling birds.
The study suggests that, from a bird’s point of view, simply protecting a wetland is not enough—protecting a buffer zone around the marsh from intrusion is just as important.
“Buffers should be part of all land use planning that occurs along coastal areas,” said Peter Marra, a senior scientist with the Smithsonian Environment Research Center’s Avian Ecology Lab. “We need to be much more cognizant of the fact that these are important areas for wildlife, and there are things that we can do to minimize impacts.”
The idea behind the study was to use birds as an indicator of the overall health of coastal wetlands. The development of bird community indexes have been used in other landscapes to gauge the health of ecosystems.
Such indexes use a variety of data, such as the number and types of birds present, and whether they are habitat generalists (those that can live in many places) or habitat specialists (those with very specific nesting or feeding needs), which for marsh birds would include species such as seaside sparrows, clapper rails, marsh wrens and Virginia rails.
A higher index score reflects a more diverse community—suggesting a high-quality habitat—while a lower score would reflect a degraded habitat that primarily supports generalists, such as robins or crows, which can be found almost anywhere.
Birds are often considered to be particularly good species for use in community indices because they are easy to survey, and their life histories are relatively well understood. They are also considered good indicators because, being relatively high in the food web, they are sensitive to disruptions in the web below them.
“Some people may say who cares about the birds in marshes,” said Bill DeLuca, a researcher at SERC’s Avian Ecology Lab, who headed the study. “But I think the index that we developed here is a good start toward measuring the overall integrity of the entire marsh.”
Biologists used the index to evaluate 96 wetlands of various sizes distributed among 30 small tidal watersheds throughout the Bay.
The study found that when as little as 14 percent of the land within a 500-meter buffer around a marsh was developed—with anything from a lawn to a parking lot—the index significantly dropped. The same was true if 25 percent of the land was developed within 1,000 meters of the marsh.
The study showed that, for marsh birds, proximity of development was more important than the level of development throughout the watershed. The watershed, as a whole, could have intense development, but the marsh bird community would remain intact as long as the buffer existed.
In the heavily developed Patapsco River near Baltimore, the researchers thought the bird communities would be in poor shape. But marshes often had wide buffers and had healthy bird communities—even the Virginia rail, a sensitive species.
“We just kind of figured that every marsh we went to was going to have garbage in it,” DeLuca said. “But the bird community was pretty intact.”
The bird communities did not seem to be influenced by agricultural activities within the buffer zones. “That was a little bit surprising,” DeLuca said. “We expected to see something, but we didn’t have any impacts with agriculture at all.”
The exact reason for the decline in bird community integrity near development is not clear. Marra said the lack of a farming link suggests the problems for birds does not stem from nutrients or agricultural chemicals.
Possible explanations, Marra and DeLuca said, include the fragmentation of marshes by development, which might restrict marsh bird distribution.
The close proximity of development may allow access for an increased abundance of generalist birds, which compete with marsh birds for available resources. It may also open the door to predators entering the marsh, the researchers said.
Whatever the cause of the decline, they said the study showed the community index could be a “powerful tool” to assess the overall integrity of a marsh ecosystem and help land use planners better protect high-quality marshes that remain—not just for birds, but other wildlife as well. “Five hundred meters around a marsh isn’t a whole lot,” DeLuca said.