Foreign Invader Hydrilla Gets Its Green Card

The first reference I could find (via Nexis) in the Washington Post to hydrilla, an asian water plant that "invaded" the Potomac River in the early 1980s, is headlined "'Monster' Hydrilla Takes Hold on River; Aquatic Plant May Grow Out of Control." That article appeared in the April 29, 1984 edition. Five days later another short Post item read:

Hydrilla, called "the killer weed" by Mount Vernon District Supervisor Sandra Duckworth, is a rapidly reproducing, oxygen-eating plant that clogs waterways, interrupts the food chain and emits a foul odor when it decays in the fall. Since it was discovered on the Potomac's shores two years ago, hydrilla's growth has gone unchecked, and aquatic experts are forecasting a lifeless and foul-smelling river if the plant's progress is not soon checked.

The hydrilla story seemed to be following the well-worn script of ecological hysteria about introduced species. However, in this case, good sense fairly quickly prevailed over incipient alarmism. A year later Maryland's Department of Natural Resources supported a bill that would limit the cutting of hydrilla. Chesapeake Bay Foundation spokesperson Will C. Baker told the Post that hydrilla was beneficial, noting:

"Hydrilla does the same things" all aquatic grasses do, said Baker… "It buffers against erosion, screens sediments, reoxygenates and clarifies the water and serves as habitat for aquatic species and as food for waterfowl."

Twenty-two years later, a Science Notebook item in today's Post reports:

But a new study by the U.S. Geological Survey has found that hydrilla is not such a bad neighbor after all. The submerged plant, which can grow to the surface and form dense mats, did not impair the reemergence of native species at a time when officials were working to reduce nitrogen concentrations in the river from sewage-treatment facilities, the study found. Cutting nitrogen levels reduces algae blooms and allows more light to get to the bottom of the river, which in turn permits aquatic plants to grow.

Hydrologists Nancy B. Rybicki and Jurate M. Landwehr of the USGS examined data between 1985 and 2001 in aquatic plant beds in the Potomac.

"As hydrilla increased, the other plants increased," Rybicki said. "In a particular year, if the water quality was good and the plants increased, not only did one species increase, the others increased as well."

The hydrilla also has been a boon to waterfowl, which eat the plant's tubers, a kind of underground stem, in the winter, Rybicki said. The research appears in the May issue of the journal Limnology and Oceanography.

Of course not all introduced species are benign--West Nile Virus comes to mind--but the war on them is a bit hysterical. This point was well made by Alan Burdick in his 2005 Discover article "The Truth About Invasive Species: How to stop worrying and learn to love ecological intruders." I will immodestly note that this is exactly the point that I made five years earlier in my reason article "Bio-Invaders: Are we under attack by 'non-native' species? Should we care?"

Addendum: It is only fair to mention that folks in Florida have a different take on hydrilla.