Genetic change versus stasis over the time course of invasions: trajectories of two concurrent, allopatric introductions of the Eurasian ruffe.
Few investigations have examined whether population genetic changes occur over the course of nonindigenous species invasions, which would facilitate understanding their trajectories and ecological successes. The Eurasian ruffe fish Gymnocephalus cernua unintentionally was introduced from ballast water released into western Lake Superior of the North American Laurentian Great Lakes in ∼1986, likely originating from the Elbe River region where ruffe is indigenous. The ruffe spread quickly to several other areas in the upper Great Lakes, but despite early predictions, has not invaded the lower Great Lakes. In ∼1991, the ruffe was introduced to Bassenthwaite Lake in northern England through bait bucket releases from southern England, where it is indigenous (and genetically distinct from the Great Lakes invasion). The genetic trajectories of these two independent invasions offer intriguing evolutionary and ecological comparisons. This study tested two alternative hypotheses for the genetic compositions of the two invasions, with reference to two native northern European populations (Elbe River and Vistula Lagoon): whether they have (1) experienced temporal consistency or (2) changed over their respective 30-year time courses. At each invasion stage (early, middle, or later), population genetic diversity may (A) remain similar, (B) decrease due to drift, or (C) increase over time due to addition and establishment of new propagules. Analyses of variation at 10 nuclear DNA microsatellite loci and entire mitochondrial DNA control region sequencing revealed that both invasions overall have lower effective population sizes, allelic richness, and observed heterozygosity compared to the native populations, indicating founder effects. The genetic compositions of both invasions significantly changed over 30 years, supporting hypothesis 2. Diversity has remained relatively consistent overall (A), with decreased allelic richness at the middle invasion stage (B), followed by recovery (C), suggesting arrival of new propagules. Moreover, population differentiation occurred with spread in the upper Great Lakes, with no overall differences in genetic diversity levels (scenario 2A). Ruffe numbers in the Great Lakes have been declining over time, and its continued absence from the lower Great Lakes may reflect limited genetic diversity stemming from a single source population area and few new supplements. The genetic diversity reservoirs of both ruffe invasions, along with continued measures to prevent new inoculants, may constrain their populations and limit long-term ecological success.