Invasive species are often considered a bane for ecosystems, but recent research shows that some exotic species of underwater grass provide needed habitat for wildlife in the Bay.
Scientists from the U.S. Geological Survey who have studied underwater grass beds in the Potomac River found that exotic species, especially hydrilla, which was once considered a harmful nuisance, have helped to restore key ecosystem functions such as habitat for waterfowl.
"The message is you can get ecosystem services, even from exotic submerged aquatic vegetation," said Nancy Rybicki, a scientist with the USGS who has long studied grass beds in the river. "Compared to periods when there was nothing at all, underwater grass beds dominated by exotics appear to enhance waterfowl numbers by providing suitable habitat for waterfowl feeding."
And when it came to grass beds, the upper reaches of the Potomac River near Washington, D.C., had nothing at all for a decade prior to 1983, when hydrilla and 11 other species appeared.
Like grass beds in much of the rest of the Bay and its tidal tributaries, beds in the Potomac were declining by the middle of the century as pollution poured into rivers, leading to murky water that blocked crucial sunlight from reaching underwater grass beds. Hurricane Agnes in 1972 drove massive amounts of nutrients and sediment into the Bay and its tidal tributaries, eradicating most of what was left.
When upgrades to wastewater plants began to improve water quality on the Potomac in the 1970s and 1980s, hydrilla was discovered in the tidal freshwater areas near Washington. But it was often considered a noxious weed. Hydrilla harvesting programs were created to keep the grass from impeding boats. Some even feared its aggressive growth would harm waterfowl by blocking the return of native grasses.
But the opposite happened. Swans, ducks and geese began coming back to the area.
Rybicki said the change was first brought to her attention by the late Jackson Abbott, a noted birder and wildlife artist, who lived in the area and walked along the Potomac nearly every day, recording his observations.
"When the hydrilla came, it was a humongous change for his waterfowl counts," Rybicki said. "He called me, and said this is phenomenal. From that day forward, he got me interested in the waterfowl aspect."
In the study, published last year in the journal Limnology and Oceanography, Rybicki and fellow USGS scientist Jurate Landwehr analyzed underwater grass and waterfowl data for the tidal freshwater portion of the Potomac river from 1959-2001.
Using techniques to quantify diversity and the amount of underwater grass species within various grass beds in the river, they found that hydrilla did not eliminate native species as originally feared.
Their research showed that as hydrilla expanded, the overall diversity of the grass beds increased. While exotics, particularly hydrilla, still dominate the beds, the study found that the proportion of area covered by exotic plants has gradually decreased over time.
The researchers also compared the grass bed results to the number of waterfowl observed along the river during the Audubon Society's annual Christmas Bird Count.
Of the 35 species of waterfowl observed, 30 showed increased counts from 1982 to 2001. Of those, counts for 29 species more than doubled, and 17 increased by more than tenfold.
Some waterfowl eat the grasses, while others eat clams and other animals found in association with the beds.
The Potomac findings could be good news for other parts of the Bay region. Much of the return of underwater grasses around the Bay since the early 1980s, when grass coverage hit its low point, has consisted of nonnative species.
"The exotics are here, and I think they are here to stay," Rybicki said. "But they still provide habitat, it appears, from our analysis of the long-term Christmas Bird Count observations."
While exotic species of aquatic grasses may provide habitat, stabilize shorelines and clarify water, Rybicki cautioned that they still can compete with native species and grow to densities that impede navigation. "Because of the complexity of ecosystems, we must be cautious in generalizing about exotic and native species and be careful about extrapolating these results from the Potomac River to other systems where these findings have not been tested," she said.