Author: Fabien Leprieur
Position: Postdoctoral researcher
Institution: Laboratoire Evolution et Diversité Biologique UMR 5174, Université Paul Sabatier, Bâtiment 4R3, 31062 Toulouse cedex 4 France
E-mail: leprieur@cict.fr
Additional Authors: Olivier Beauchard; Simon Blanchet; Thierry Oberdorff; Sébastien Brosse
Submitted Date: March 04, 2008
Published Date: March 5, 2008
This comment was originally posted as a “Reader Response” on the publication date indicated above. All Reader Responses are now available as comments.

R.E. Gozlan (RG) made three comments on our study analyzing global patterns of non-native freshwater fish species richness.

First, we agree with RG that the term “invasion” is often confusing as an invasive species can be defined as either an established species spreading over a large area or an established species having harmful ecological impacts [1]. In our article, we defined an “invasion” as the end product of a succession of three stages (i.e. introduction, establishment and spread) [1-2]. We therefore recognize “biological invasions” as a process instead of a dichotomous classification (invasive or not) [1]. As mentioned in the article, our study focuses on the establishment stage of the invasion process.

RG stated that we incorrectly used the term “invasion” and would have preferred us to use the term “introduction”. As mentioned by RG, the term “introduction” refers to the release of a species outside its native range, and therefore account only for the initial dispersal stage of the invasion process [1-2]. We agree with this definition, which was given in the introduction of our article. But, a problem arises when RG proposed: “In building the database on self reproducing populations of novel species within a given river basin, the authors have analysed fish introductions rather than fish invasions”. In this case, RG misemploys the term “introduction” since the release of a species should not be confounded with the establishment of an introduced species (the fact that an introduced species had self reproducing populations, see [1,3]). We therefore confirm that we analyzed the number of non-native fish species established per river basin, as mentioned in the article.

Second, RG advanced: “In calculating human activity, the authors used a combination of human population density and GDP. This combination has generated a bias toward wealthy populated regions”. RG apparently missed the point here by mixing results of our modelling approach with the picture of the percentage of non-native species by river basin (Figure 1A), which was used to describe “invasion hotspots”. Our modelling approach did not focus on the percentage of non-native fishes. Rather, it aimed at explaining the number of non-native species established.

RG criticizes the fact that river basins of Australia and New Zealand emerged as “invasion hotspots” rather than those of South-East Asia by advancing that Asia is characterized by the greatest number of fish introductions associated to aquaculture. RG’s remark is quite unfair especially since Fig.1B well describes the pattern he mentioned. Once again RG misinterprets our results by confusing introduction and establishment.

Last, we agree with RG’s opinion that not all non-native species have negative ecological impacts. However, because of a sampling effect, the more non-native species establish in a region, the higher is the probability that some of them will become harmful for natives [4]. Then, if there is one case where the precaution principle should be applied in conservation ecology, this is actually the one. We reiterate that our point was not to identify ecological impacts but to analyze the global pattern of fish invasions at the establishment stage.

To conclude, we suppose that RG missed the main point of our paper, as we did not repeated well known generalities (i.e. the trivial role of humans in fish introductions), but tested for the first time at the global scale the relative role of three major hypotheses proposed to explain geographical differences in non-native (established) species richness. We are therefore confident that our study provides an additional knowledge to the field of biological invasions.

1. Lockwood JL et al. (2007) Invasion Ecology. Blackwell Publ.

2. Williamson M (1996) Biological invasions. Chapman & Hall.

3. Copp GH et al. (2005) J Appl Ichthyol 21: 242-262.

4. Pyšek P, Richardson DM (2006) J Biogeogr 33: 2040–2050.