What a difference a species makes: a meta-analysis of dreissenid mussel impacts on freshwater ecosystems

We performed a meta-analysis of published studies and long-term monitoring data sets to evaluate the effects of dreissenid mussels (Dreissena polymorpha and D. rostriformis bugensis), two of the world's most problematic biological invaders, on the biogeochemistry, flora, and fauna of lakes and rivers across North America and Eurasia. Dreissenid effects were structured along two distinct energy pathways. For the pelagic-profundal pathway, large mean reductions in phytoplankton (-35% to -78%) and zooplankton (-40% to -77%) biomass occurred and were dependent on habitat type. The largest effects were found in rivers, followed by littoral and pelagic habitats in lakes. In contrast, benthic energy pathways within littoral habitats of lakes and rivers showed dramatic increases in mean benthic algal and macrophyte biomass (+170% to +180%), sediment-associated bacteria (about +2000%), non-dreissenid zoobenthic biomass (+160% to +210%), and total zoobenthic biomass, which includes dreissenid mussel soft tissues (+2000%). Our study quantifies the remarkable ability of these invasive mussels to shift aquatic food webs and energy flow from pelagic-profundal to benthic littoral energy pathways, and it provides a basis for forecasting their impacts in diverse freshwater ecosystems. Our meta-analysis approach was a powerful tool for moving beyond the idiosyncrasies of individual case studies and may be equally powerful for assessing impacts of other biological invaders.