Long-term biodiversity erosion: limited co-existence of the Eurasian zebra mussel and native mussels in a North American river after 25 years

North American unionid mussel populations have experienced significant mortality due to competition and fouling by the Eurasian zebra mussel Dreissena polymorpha (Pallas, 1771). Habitats whose water chemistry is suboptimal for the zebra mussel could plausibly serve as refugia in which unionids and zebra mussels co-exist. The Richelieu River, invaded by the zebra mussel in the mid-1990s, has a mean calcium concentration (∼18 mg L −1 ) believed insufficient for supporting a zebra mussel population capable of severely damaging unionids. Using a 25-year dataset, we tested how unionid diversity and abundance have changed along with zebra mussel fouling levels in the river over time. Local unionid populations suffered major declines across sites regardless of their distance from the river’s headwaters, Lake Champlain—a constant source of zebra mussel larvae. Over the past 25 years, unionid diversity and abundance have eroded to a similar extent and followed a similar timeline of population decline as has been observed in invaded calcium-rich habitats. We hypothesize that sustained exposure to zebra mussel fouling and food competition has produced a limited co-existence in which long-term impacts of zebra mussels have accrued.