An interesting article appeared in the Baltimore Sun a few days ago that is causing some buzz in the usual invasive species circles. In it, USGS scientist Nancy Rybicki talks about hydrilla (Hydrilla verticillata) and other invasive species in the Potomac River from the angle of how they benefit the ecosystem.
Back in the 1970s, the Potomac was quite polluted. With flows of sewage and phosphorus pollution cut off by the 1980s, things began to look up. It was around that time that hydrilla was first discovered in the river. Along with Eurasian water milfoil (Myriophyllum spicatum), hydrilla now provides food for birds and nesting habit for crabs and fish. Nancy notes that biodiversity levels in the Potomac are good regardless of the fact that there is a mix of native and non-native species.
The article was almost certainly inspired by a study Nancy recently published with Jurate Landwehr in the journal Limnology and Oceanography, "Long-term changes in abundance and diversity of macrophyte and waterfowl populations in an estuary with exotic macrophytes and improving water quality." (full pdf, or read the USGS press release here). Covering 17 years of data, the study found that the amount of native and non-native aquatic plants were actually increasing together over time, i.e. the invaders did not completely take over the way people were concerned they would. However, the research does point out that non-native plants were the dominant components of the habitat. That doesn't seem to be as strong of a conclusion as the newspaper article is making (hydrilla as a savior - yeesh).
If the only goal is to manage the Potomac for waterfowl habitat, then we might be able to say they're doing a good job. Looking at the health of the ecosystem as a whole, it is hard to predict what it would be like if the non-native plants, invertebrates and fish were not there. This raises some interesting questions that no one can easily answer: Could native plants have better filled the niche to provide habitat for fish and fowl? Will the ability of the native plants to grow eventually be impeded by hydrilla? What is the impact on aquatic insect communities in a habitat dominated by non-native plants? And perhaps most importantly, what is the impact on neighboring bodies of water to let hydrilla thrive in the Potomac? A plant so easily spread by boaters and waterfowl surely isn't staying put in a single river.