Mussels in the genus Dreissena (zebra and quagga mussels) are among the world’s most troublesome invaders, and can have large ecological and economic effects. The strength of impacts varies widely among sites and through time at a single site: some invaded ecosystems have been utterly transformed by large changes, whereas others have scarcely been changed. Some of this variation is surely a result of variation in population densities of Dreissena, which range over at least 4 orders of magnitude in invaded ecosystems. In addition, some evidence exists that the traits of the invader (especially mussel body size) modulate impacts. Nevertheless, it seems doubtful that the population density and traits of invading populations of Dreissena are sufficient to explain spatial and temporal variation in impacts. I will use data from Dreissena invasions to explore the idea that invader impacts are determined not just by the invaders, but by the interaction between the invader and the invaded ecosystem, and require explicit consideration of both.
Results/Conclusions
Three examples of increasing complexity show that Dreissena’s impacts depend on the invaded ecosystem as well as the invader. The accumulation of shell material, which transforms benthic habitats and affects rates of carbon sequestration, is determined jointly by Dreissena production, water chemistry, and current speed. The increase in benthic primary production resulting from increased water clarity from Dreissena feeding depends on the filtration rate of the Dreissena population, the nature of the suspended particle pool (organic vs. inorganic, presence of ungrazable phytoplankton), and the morphometry of the body of water. Finally, the change to food webs resulting from the diversion of grazed phytoplankton from the pelagic zone to Dreissena and the sediments depends on the size and traits of the Dreissena population and on details of the food web and its other controls. Each of these impacts depends on the ecosystem as well as the invader, each differs in the ease and precision with which impacts can be predicted across time and space, and each yields different functional relationships between Dreissena density and impacts. I will try to extend these ideas more generally to illuminate spatial and temporal variation in the impacts of other invaders of freshwater ecosystems.