Reply to the comment by Reid and Hudson on “Rate of species introductions in the Great Lakes via ships’ ballast water and sediments”

Abstract

Drake and David M. Lodge 555 With Reid and Hudson (2008), we lament the weak representation of taxonomic expertise in modern biology. In fact, we have advocated increased funding for research in systematics and programs for training taxonomists (Lodge 2002). We are cognizant of our own lack of expertise in taxonomy and how the lack of taxonomic information and expertise hinders progress in research and management of biological invasions. Nevertheless, taxonomic expertise does not explain the differences between the conclusions of our papers (Drake and Lodge 2007a, 2007b) and the conclusions Reid and Hudson think we ought to have drawn. Rather, their disagreement is with our conceptual framework for thinking about invasive species and the uncertainties concerning species that might establish in regions outside their native ranges. Specifically, they assess invasion potential differently than we did. That is, to categorize species that have not invaded the Great Lakes, and for which a paucity of distributional and ecological information exists, we adopted different conceptual criteria than Reid and Hudson to designate these species as indigenous or nonindigenous. In addition, for species that have not invaded the Great Lakes, it is impossible to say if they might become established if they were to be introduced, leaving room for informed judgments and improvement of those judgments as new information becomes available. At issue are four species of freshwater copepods that we collected from the ballast water of ships entering the Great Lakes: Microcyclops rubellus, Microcyclops varicans, Paracyclops chiltoni, and Maraenobiotus insignipes. Reid and Hudson believe the first three should be considered indigenous to the Great Lakes because they occur elsewhere in North America and are reported from the Great Lakes in Hudson and Lesko (2003). Unfortunately, Hudson and Lesko (2003), an Internet source, was not available when the taxonomic component of this work was being performed, does not appear in a peer-reviewed journal, and does not refer to previously published reports of these species. Furthermore, Hudson and Lesko (2003) indicate that these species were not collected in the Great Lakes prior to 1984. Thus, we were and are justified in considering these species to be nonindigenous to the Great Lakes. Proper analysis and publication of previously collected specimens may yet reveal that these species were established prior to our collection of them from ballast water, in which case we would consider them as established, nonindigenous species. Concerning Maraenobiotus insignipes, Reid and Hudson concede that it has not been reported from the Great Lakes, but think we should have excluded it on the basis of its habitat, which Reid and Hudson report is primarily “streams, seeps, and damp moss”. Ideally, for all species we collected, we would have attempted to indicate their ability to establish in various Great Lakes habitats. Unfortunately, the Great Lakes ecosystem is so heterogeneous and the habitat tolerances of zooplankton are so poorly documented that we concluded that the only designation we could reliably apply to all the species we collected was whether or not they were known to tolerate fresh waters. Of course, the species we collected are just a sample of a much larger species pool, for which our primary goal was to estimate its size. An unknown fraction of this pool will not be able to inhabit the Great Lakes for various reasons. That this fraction is unknown is nonetheless a poor reason for rejecting our estimate of the total pool. Reid and Hudson also object to our categorization of Oithona similis as “freshwater” (Drake and Lodge 2007b). As can be understood from the context and the more complete listing in Annex 1 of Drake and Lodge (2007b), our designation indicates we were unable to rule out freshwater tolerance. This is different than asserting it to prefer fresh-