Zebra mussels have coexisted in their native European range alongside sphaeriids and other unionids on a timescale ranging from decades to millennia. They were accidentally introduced to North American in the late 1980’s and have become well known for out competing native mussels and fouling beaches. It was suggested by Ricciardi et al. in 1998 that the zebra mussel invasion would result in the extinction of ~60 species of native North American unionids. In this 2007 paper, Strayer and Malcom compare pre-invasion and post-invasion data to: 1) describe the long-term dynamics of native bivalve populations in the Hudson River after the zebra mussel invasion, and 2) evaluate evidence that native bivalve populations might be recovering after an initial period of decline.
Using a Ponar dredge, the authors were able to easily sample known areas regardless of depth and sieved out bivalves using 2.5 mm mesh for unionids and 0.5 mm mesh for sphaeriids. The authors also examined body condition of both sets of bivalves, which is defined here as dry body mass of an animal of a given shell size, incorporating height, width, length, time and measures of zebra mussel impact. Zebra mussel impact itself has two parts: filtration rate (indicating the strength of the exploitive population), and the number of zebra mussels attached to each unionid (indicating the strength of local interactions, including mechanical interference and local exploitive competition).
Zebra mussels were first detected in the Hudson River in 1991 and although the population size changes from year to year, it has remained high since 1992. The filtration and fouling rates of the zebra mussels increased with population size and has not decreased significantly since 1999. The four most commonly sampled native taxa (Elliptio complanata, Anodonta implicata, Leptodea ochracea, and Pisidium sp.) were observed to have major population declines after the zebra mussel invasion on the scale of 65-100%. The data doesn’t support a formal statistical analysis, but the authors suggest that fouling may not have been the primary mechanism of mortality because the declining rates of native unionid populations were not markedly lower during the early years of the invasion when fouling rates remained low. Exponential decay models of the entire data set from 1990-2005 suggest that populations of these four species will stabilize at 4-22% of their previous densities. Growth rates of individual unionids fell during the early invasion and recovered in later years, although growth rates stayed lower than their pre-invasion values. Body condition of unionids fell after the zebra mussel invasion and was strongly, negatively correlated with the filtration rate of zebra mussel populations. The authors found no evidence that decline or recovery rates of E. complanata differed across habitats or sections of the river. The authors compared data of the North American zebra mussel invasion to a similar invasion in European waters and while the data are similar, the selection bias of choosing sites heavily infested with zebra mussels make the two invasions difficult to compare.
Although the authors admitted to not knowing exactly why, their data suggests that the native bivalves of the Hudson River may survive the zebra mussel invasion. The authors suggest that the very strong selection pressure exerted by the invading zebra mussels could favor genotypes leading to phenotypes with physiological or behavioral characteristics to resist zebra mussel effects. One of two unidentified mechanisms could be at work here: 1) fast processes that occurred in recent decades/centuries when zebra mussels spread to new parts of Europe, or 2) slow processes that occurred during the long interglacial period. If fast mechanisms are at work now in North America, then native bivalves stand a chance at survival.
In class, we discussed whether or not we thought we agreed with the authors in that the native North American bivalves could survive. Other topics that we discussed were round gobies (which are a natural predator of zebra mussels and also are invasive species in North America) and other possible ways to control zebra mussels. Now that you’ve had time to for all of this information to sink in a bit more, what’s your opinion on this research? Do you agree with the authors that the native mussels could coexist with zebra mussels?