Long-term changes in a population of an invasive bivalve and its effects

Background/Question/Methods Although the ecological and economic effects of non-native species probably often change through time, few studies have documented such effects. The zebra mussel (Dreissena polymorpha) is an important invader that has had large ecological and economic effects on the ecosystems it has invaded in North America and western Europe. We have been studying the zebra mussel and its effects on the benthic animals of the freshwater tidal Hudson River, New York, for 20 years, to look for long-term changes in the invasion. We used a stratified random design to sample populations of benthic animals, including the zebra mussel, at 8-51 sites throughout the river.

Results/Conclusions The characteristics of the zebra mussel population and its effects on other benthic animals both changed substantially through time.  Over the period of study, annual survivorship of adult zebra mussels fell >100-fold, which caused the aggregate filtration rate of the population to fall by 82%. Population size and body size of zebra mussels may also have fallen. In the early years of the invasion, densities of nearly all benthic animals in deepwater sites fell steeply (by 80-99%). After about 8 years of decline, these populations began to recover, and are approaching pre-invasion densities. The littoral zoobenthos showed neither the initial decline nor the subsequent recovery. Although the mechanisms behind these changes are not yet clear, our study shows that the effects of an invader may change considerably over time. Understanding and management of non-native species would benefit from more long-term studies of the ecological effects of invaders.