Ecologists are getting a lesson in the dangers of exotic species as they watch the havoc being wreaked on New York’s Hudson River by the zebra mussel, a native of Eurasia that came to the U.S. aboard oceangoing ships in the late 1980s. The population of zebra mussels in the river is estimated at 500 billion to 600 billion in peak years. The mussels have now transformed the chemistry of the freshwater portion of the Hudson and are threatening the stability of the entire ecosystem. Zebra mussels consume large quantities of phytoplankton, microscopic plants that are the basis of the aquatic food chain, with the result being that many other river creatures aren’t getting enough to eat and are dying off. New research shows that the mussels also suck up large amounts of oxygen, causing other aquatic animals to flee or die.