Reply from C.E. Lee and M.A. Bell

Strayer states that we erred in the dates we gave for the initial colonizations of fresh water1xLee, C.E. and Bell, M.A. Trends Ecol. Evol. 1999; 14: 284–288Abstract | Full Text | Full Text PDF | PubMed | Scopus (153)See all References1. However, our goal was not to document initial dates of freshwater invasions, but rather to discuss cases where there is evidence for recent habitat shifts by particular populations, independent of prior invasions by other populations. What we emphasize in our paper are the remarkable facts that (1) colonizations from salt water (brackish or marine) can occur on decadal rather than on millennial time scales, and (2) that recent immigrants from salt water can spread rapidly and persist in fresh water, through facilitation by humans.Table 1 of our perspective article1xLee, C.E. and Bell, M.A. Trends Ecol. Evol. 1999; 14: 284–288Abstract | Full Text | Full Text PDF | PubMed | Scopus (153)See all References1 lists examples for which timing and direction of invasions can be inferred from concrete evidence (the criteria for the examples we use are given in Box 1), rather than examples of first appearances in fresh water. For instance, for the copepod Eurytemora affinis, we list several dates, from the 1930s to the 1980s. These are not dates this species first entered fresh water but inferred dates of repeated and independent invasions from saltwater sources2xEvolution. Lee, C.E. See all References2.Strayer highlights the problem of using species distributions to infer pathways of freshwater invasions, especially when systematic relationships among populations are uncertain. For instance, in the case of the zebra mussel Dreissena polymorpha, the extent to which postglacial freshwater populations survived to the present day is inconclusive3xKinzelbach, R. : 5–17See all References3. In addition, populations from southwestern Europe, such as those in Lakes Ohrid and Prespa, are thought to represent ancient invasions of fresh water, and are considered sibling species of populations from the Ponto-Caspian basin3xKinzelbach, R. : 5–17See all References, 4xKinzelbach, R. Zool. Middle East. 1986; 1: 132–138Crossref | Scopus (5)See all References, the likely source of recent invasions5xTrait de Geographie Physique. De Martonne, E. See all References, 6xRosenberg, G. and Ludyanskiy, M.L. Can. J. Fish. Aquat. Sci. 1994; 51: 1474–1484CrossrefSee all References. As stated in our paper, genetic markers would be required to determine pathways and frequency of invasions, and in the case of D. polymorpha, clarify systematic relationships among populations and sibling species.The impact of humans in the past 200 years should not be underestimated. The direct transfer of organisms (including those that Strayer mentions) from salt water to fresh water has occurred on massive scales, and many of these cases are documented1xLee, C.E. and Bell, M.A. Trends Ecol. Evol. 1999; 14: 284–288Abstract | Full Text | Full Text PDF | PubMed | Scopus (153)See all References1. For example, in Eastern Europe and Russia large-scale acclimatization of numerous Ponto-Caspian species into freshwater reservoirs was undertaken for aquaculture purposes7xMordukhai-Boltovskoi, P.D. Int. Rev. Gesamten Hydrobiol. 1979; 64: 1–38CrossrefSee all References, 8xJazdzewski, K. Crustaceana (Suppl.). 1980; 6: 84–107See all References. In addition, impoundment of entire bays and lagoons resulted in the large-scale trapping and acclimatization of populations9xSee all References, 10xMiller, R.C. J. Mar. Res. 1958; 17: 375–382See all References. Physiological studies indicate that these invaders are energetically less efficient in fresh water than more ancient freshwater species11xTaylor, P.M. and Harris, R.R. J. Comp. Physiol. 1986; 156: 323–329Crossref | Scopus (26)See all References, 12xMcMahon, R.F. Am. Zool. 1996; 36: 339–363See all References. Yet, the creation of reservoirs (as havens for acclimation and as stepping stones) and transport vectors has extended their ranges7xMordukhai-Boltovskoi, P.D. Int. Rev. Gesamten Hydrobiol. 1979; 64: 1–38CrossrefSee all References, 8xJazdzewski, K. Crustaceana (Suppl.). 1980; 6: 84–107See all References and made invasions from saline habitats more likely and serious. The implication is that within the modern era we can expect many more independent invasions to occur from saline to freshwater habitats, especially from the Ponto-Caspian region. Human transport and habitat alteration have accelerated the pace of such invasions, and they have the potential for major impacts on aquatic ecosystems.