Dreissenid mussels (including the zebra mussel Dreissena polymorpha and the quagga mussel D. rostriformis) are among the world's most notorious invasive species, with large and widespread ecological and economic effects. However, their long‐term population dynamics are poorly known, even though these dynamics are critical to determining impacts and effective management. We gathered and analyzed 67 long‐term (>10 yr) data sets on dreissenid populations from lakes and rivers across Europe and North America. We addressed five questions: (1) How do Dreissena populations change through time? (2) Specifically, do Dreissena populations decline substantially after an initial outbreak phase? (3) Do different measures of population performance (biomass or density of settled animals, veliger density, recruitment of young) follow the same patterns through time? (4) How do the numbers or biomass of zebra mussels or of both species combined change after the quagga mussel arrives? (5) How does body size change over time? We also considered whether current data on long‐term dynamics of Dreissena populations are adequate for science and management. Individual Dreissena populations showed a wide range of temporal dynamics, but we could detect only two general patterns that applied across many populations: (1) Populations of both species increased rapidly in the first 1–2 yr after appearance, and (2) quagga mussels appeared later than zebra mussels and usually quickly caused large declines in zebra mussel populations. We found little evidence that combined Dreissena populations declined over the long term. Different measures of population performance were not congruent; the temporal dynamics of one life stage or population attribute cannot generally be accurately inferred from the dynamics of another. We found no consistent patterns in the long‐term dynamics of body size. The long‐term dynamics of Dreissena populations probably are driven by the ecological characteristics (e.g., predation, nutrient inputs, water temperature) and their temporal changes at individual sites rather than following a generalized time course that applies across many sites. Existing long‐term data sets on dreissenid populations, although clearly valuable, are inadequate to meet research and management needs. Data sets could be improved by standardizing sampling designs and methods, routinely collecting more variables, and increasing support.
20In invasive dreissenid communities, the zebra mussel usually appears earlier and then is 21 displaced by the quagga mussel. We analysed length-weight allometric relationships, 22 attachment strength (2 days, 1 week and 1 month of exposure), shell crushing resistance and 23 glycogen content across the entire size range of both species in large shallow European lakes 24where this displacement has recently occurred. In Lake Balaton (Hungary) and Ijsselmeer 25 (The Netherlands), the soft tissue dry weight increment of zebra mussels per unit length 26 decreased after the quagga mussel invasion and became lower than that of quagga mussels. In 27 Lake Markermeer (The Netherlands), having relatively worse environmental conditions, dry 28 weight increment per unit length was always higher in quagga mussels than in zebra mussels, 29 but no negative change in dry weight increment occurred in zebra mussels during the quagga 30 mussel invasion. Small zebra mussels had more resistant shells and stronger attachment than 31 quagga mussels. These differences were reduced (shell hardness) or reversed (long-term 32 attachment) in larger individuals. Zebra mussels had lower glycogen content than quagga 33 mussels across the entire size range. Thus, the quagga mussel advantage over zebra mussel 34 likely consists in the faster dry weight increment per unit length and higher storage product 35 contents of the former, due to its lower investments in attachment strength and shell crushing 36 resistance. 37 38
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