When zebra mussels make the news, it's usually because they've invaded yet another water body. Not this time, though. In New York's Hudson River, zebra mussels appear to be declining as displaced native species stage a comeback.
Zebra mussels are striped, nickel-sized mollusks native to western Asia. Since they first appeared in the United States in 1988 as stowaways in ship ballast water, their habit of starving out native invertebrates and fouling equipment has made them serious aquatic pests. When they showed up in the Hudson in 1991, freshwater ecologist David Strayer and his colleagues at the Cary Institute of Ecosystem Studies in Millbrook, New York, were prepared—if not to stop them then at least to study their effects. Cary researchers had been regularly sampling plankton and water chemistry within the Hudson's 150-kilometer freshwater estuary for 5 years and had started sampling riverbed invertebrates the previous year. The data set's unparalleled length and continuity has enabled Strayer's team to detect changes in the population and effects of an invasive species that have eluded researchers elsewhere.
Zebra mussels hit the Hudson hard. They quickly gobbled up most of the river's plankton, the researchers discovered. Native mussels, clams, and other invertebrates plummeted to as little as 1% of their original populations. "It looked really, really grim," says Strayer.
Then to everyone's surprise, around 2001 the native mussels stopped declining. Strayer and his colleagues feared it was a temporary respite, but the trend persisted, and in 2007 they reported a solid, albeit incomplete, comeback. This summer, they documented native zooplankton—tiny floating animals—rebounding, too, and an increased death rate among large zebra mussels.
For the current study, published last week in the journal Oecologia, Strayer and colleagues scrutinized their long data set, estimating the survival rate of each age class of zebra mussels over time and the amount of water the mussels filter as they feed. They also looked for population trends in native invertebrates, including clams, nematodes, and flatworms.
Since zebra mussels first invaded the Hudson, the team found, their annual survival rate has fallen by 99% and their water filtration by 82%. It could be that native blue crabs or some other predator are eating more zebra mussels or their larvae, or perhaps some undetected pathogen or parasite is keeping them in check. As for the native invertebrates, they are approaching their preinvasion numbers.
Still, the Hudson's recovery is far from complete. Worryingly, tiny floating plants called phytoplankton, a key piece of the food web that zebra mussels devour, have not returned. Without knowing the reasons behind the river's apparent turnaround, Strayer and colleagues can't predict whether it will continue or whether other mussel-bound waterways might follow a similar path.
Biologist Thomas Nalepa of the National Oceanic and Atmospheric Administration in Ann Arbor, Michigan, says something similar may be afoot in at least one corner of the Great Lakes. In some places, zebra mussels are declining, mainly because they're yielding to another invader, the quickly proliferating quagga mussel. Yet Nalepa has preliminary data that show both invasive mussel species declining and lakebed invertebrates recovering in Lake Huron's Saginaw Bay. Elsewhere, however, there are no signs of recovery from the bivalve blight, Nalepa says, "but then again, there aren't very many studies out there that look at changes over the long term."