Comparisons of congeners can be a useful tool for determining factors that affect invasiveness in introduces species, as it allow for a control of many other factors that are likely to differ among species.  Dreissena polymorpha (zebra mussel) and D. rostriformis bugensis (quagga mussel) are closely related, have similar dispersal potential, share a native habitat, and share dispersal vectors, D. polymorpha has invaded more lakes in Europe and North America than D. r. bugensis.  However, recently, quagga mussels are spreading rapidly and can displace zebra mussels where they co-occur.  We examined historic data to test for differences in the invasion rates of these two species at different spatial scales. 

Results/Conclusions

We found that the zebra mussel had higher invasion rates than the quagga mussel at all spatial scales (regional, local, and waterbody scales) throughout their invasion history.  We also found that the time lag between when each was first detected in a waterbody and maximum population density was reached was much shorter for the zebra mussel (2-4 years) than for the quagga mussel (6-19 years).  Although in many waterbodies D. r. bugensis has been reported to outcompete D. polymorpha, competition may be much more dependent upon local environmental conditions and will determine which species will become dominant in a given waterbody.  However, even lakes where quagga mussel dominate (e. g. lakes Erie and Ontario), zebra mussels are still abundant and thus these lakes can act as sources for the spread of zebra mussels.