Time scales of ecosystem impacts and recovery under individual and serial invasions

Introductions of keystone or engineering species have complex and long-term impacts on multiple ecosystem features. Anticipating these consequences requires knowledge of the magnitude and pace of a species’ impacts and whether they subside as resident species and communities adapt. Managing for the effects of invasive species is particularly complicated by a paucity of long-term ecosystem studies, the fact that invaders can represent novel ecological types, and as serial invasions by new species mediate the impacts of an original invader. We resolve the impacts of quagga (Dreissena rostriformis bugensis) and zebra mussels (D. polymorpha), two of the most widespread species that increasingly co-invade and re-engineer energy flows in freshwater ecosystems. Following many-decade time series and seven ecosystem features, we find remarkably similar ecosystem responses to mussel invasion across seven lakes in Europe and North America. Lakes invaded by zebra mussels only experienced the most severe changes within 5-10 years of invasion, followed by a partial recovery of several key ecosystem features. However, recovery disappeared, and initial impacts amplified in lakes where subsequent, serial invasions by quagga mussels competitively displaced zebra mussels. Our results suggest that the impact of quagga is stronger per unit biomass due to their greater effects on phytoplankton during spring and fall. Thus, we show that the effects of species introductions on many lake ecosystem features can manifest quickly and partially subside, except when multiple species invade a system.