Results and Discussion

The Palearctic and Nearctic realms are the two most invaded realms (Fig. 1, Fig. 2A, and Table 1), with a mean of six and four nonnative species per basin, respectively, contrasting with the Afrotropical and Neotropical realms, where, on average, only two nonnative species have been introduced per basin (Fig. 2A). Extirpations were less frequent than introductions, with, on average, less than one species extirpated per basin (Fig. 2B). The number of species shared by two basins has increased slightly at the world scale and for most realms (i.e., less than 0.5 additional shared species; Fig. 2C), although the increase is higher for the Nearctic and Palearctic realms (i.e., up to 2 additional shared species; Fig. 2C).

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Fig. 1.

Patterns of nonnative species introductions in the six realms. (A) Distribution of the number of nonnative species in basins. (B) Percentage of occurrence of nonnative species in each realm. The vertical dashed line separates nonnative species that have been introduced in only one basin (Right) from nonnative species that have been introduced in at least two basins (Left).

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Fig. 2.

Changes in fish assemblage structure induced by human activities across biogeographical regions and at the world scale. The number of nonnative species introduced per river basin (A), number of native species extirpated per river basin (B), and increase in the number of species shared by river basin pairs between the historical and the current situations (C) are shown. The historical situation indicates assemblages before introductions and extirpations attributable to human activities and corresponds to the preindustrial period. The current situation indicates today’s assemblages (native species − extirpated species + nonnative species). Values are mean (±SEM) over basins from each biogeographical realm or over all the basins considered at the world scale. SEM bars, although present in the figure, are sometimes not visible because of their low value.

Following species introductions and extirpations, uniqueness has decreased by 13% at the global scale (i.e., from a historical value of 81% to a current value of 68%) and by less than 7% in all realms except the Nearctic and Palearctic, which showed a 15% drop and a 9% drop, respectively (Fig. 3A). Historical pairwise taxonomic similarity across basins was low in the six realms (mean < 10%; Fig. 3B). Mean taxonomic similarity has increased over the past 2 centuries by less than 0.3% in the Afrotropical, Australian, Neotropical, and Oriental realms (Fig. 3B). In contrast, it increased by 1.1% in the Nearctic and 0.6% in the Palearctic (Fig. 3B). At the global scale, mean taxonomic similarity has increased by only 0.5%.

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Fig. 3.

Changes in taxonomic uniqueness and taxonomic similarity among fish faunas between historical and current situations for the six realms and at the world scale. The taxonomic uniqueness (percentage of river basin pairs sharing no species) (A), mean taxonomic similarity between pairs of river basins (measured using Jaccard’s similarity index) (B), and proportion of river basin pairs showing taxonomic homogenization (“h,” red), no changes (“0”, white) or taxonomic differentiation (“d,” blue) between historical and current situations (C) are shown. The historical situation indicates assemblages before introductions and extirpations attributable to human activities and corresponds to the preindustrial period. The current situation indicates today’s assemblages (native species − extirpated species + nonnative species).

These patterns were confirmed by the analysis of complementary distribution parameters (Table 2). For instance, the median value of pairwise taxonomic similarity remains low (<0.2%) at the world scale and for the Afrotropical, Neotropical, and Oriental realms. In contrast, for the Nearctic and Palearctic realms, the median values increased by 2.56% and 0.99%, respectively (Table 2). For all six realms, the distribution became less right-skewed and less peaked (Table 2), confirming the tendency toward a loss of uniqueness and an increase in taxonomic similarity. Nevertheless, it is noteworthy that the proportion of basin pairs becoming less similar (i.e., experiencing taxonomic differentiation) exceeds 17% in the six realms (Fig. 3C). In the Afrotropical and Neotropical realms, more than 70% of basin pairs have not experienced change in their taxonomic similarity and more than half of the remaining basin pairs experienced taxonomic differentiation (Fig. 3C). Furthermore, the two realms with the highest frequency of differentiation (25.4% in the Nearctic and 27.3% in the Palearctic) are also those where homogenization is the most frequent (36% and 28%, respectively; Fig. 3C).

Overall, these results indicate a weak homogenization at the global and realm scales, although it is stronger in the Nearctic and Palearctic. Previous studies at the regional scale reported a greater increase of taxonomic similarity (i.e., 7–20%) over a wide range of plant and animal phyla, which led to the forecast of a similarly high increase of taxonomic similarity at the global scale (7, 9, 11, 17, 18, 24, 25). Our results are in contrast to this last idea, because in our study, the taxonomic similarity of fish faunas remains largely unchanged at the world scale (i.e., 0.5%).

Analyses at the basin scale confirmed the trends observed at the world and realm scales. Indeed, taxonomic homogenization occurred in most of the basins, even though a few displayed differentiation (Fig. 4 A and B). The null model revealed that most of the homogenizations observed are significantly higher than expected, except in the Afrotropical and Neotropical realms (Fig. 4 C and D). Likewise, most of the differentiations are of lower magnitude than expected. This is the case for the basins of Western Europe (Fig. 4 C and D); given the high number of nonnative species they received, if nonnative species introduction had been random, the intensity of differentiation would have been higher than observed.

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Fig. 4.

World maps showing the mean change in taxonomic similarity between each basin and the other basins belonging to the same realm (). The direction and intensity of change from the historical situation to the current situation (A) and the proportion of basins in each class of per realm (B) are shown. (C) Significance of the observed taxonomic homogenization or taxonomic differentiation against a null model testing random native species extirpation and nonnative species introduction. The “+” symbol indicates that the changes observed in taxonomic similarity are significantly greater than expected under the null hypothesis (P > 0.975), whereas the “−” symbol indicates that the changes observed in taxonomic similarity are significantly lower than expected under the null hypothesis (P < 0.025). ns indicates that the changes observed in taxonomic similarity are not significantly different from the values expected under the null hypothesis. (D) Per realm proportion of basins in each level of significance of .

Finally, using generalized least squares models, we showed that the factor primarily correlating with change in mean taxonomic similarity per river basin is the number of nonnative species introduced (Table 3): The higher the nonnative species richness, the greater is the increase in mean taxonomic similarity. This positive relationship also varies according to the historical taxonomic similarity as well as the historical number of native species (significant interactions terms in Table 3). Specifically, these interactions indicate that the increase in taxonomic similarity with species richness is more marked for basins with historically low native richness and low taxonomic similarity.

Overall, human preference for a small number of species and the resulting uneven frequency of introduction of nonnative species (Fig. 1B) either caused taxonomic homogenization or at least mitigated the intensity of taxonomic differentiation. These changes in β-diversity are roughly consistent with the global distribution of nonnative fish richness (α-diversity) (6). Of the six introduction hotspots defined by Leprieur et al. (6), five have experienced marked taxonomic homogenization, namely, the Pacific Coast of North and Central America, southern South America, central Eurasia, South Africa and Madagascar, southern Australia, and New Zealand (Fig. 4A). This confirms that nonnative species are responsible for changes in both α- and β-diversity, a trend also confirmed by the strong contribution of the nonnative species to the change in taxonomic similarity (Table 3). The last introduction hotspot, Western and Southern Europe, exhibits a particular pattern because of differentiation in some basins (Fig. 4 A and B), even though the overall Western and Southern European trend is toward homogenization. This trend was consistent with previous regional studies showing that nonnative species identity (exotic or translocated) can have an opposite effect on change in taxonomic similarity (11). It should, however, be noted that although we detected a general trend toward homogenization at global, regional, and basin scales (Figs. 3 and 4), the homogenization strength remained lower than that reported in the literature (7, 9, 11, 17, 18, 24, 25), testifying that the Homogocene or New Pangea era (16) is still not the case for most of the earth’s freshwater fish faunas. However, the relatively high levels of homogenization and differentiation noted for the Palearctic and Nearctic realms indicate a need for a better understanding of the ecological consequences of human alterations in β-diversity.

Even though policies have been implemented to prevent new introductions, we should be aware that the increase of global exchanges will certainly promote new introductions through ballast water, fish farming, and the aquarium trade (6, 22, 26, 27). In this context, predicting consecutive future changes in taxonomic similarity is a challenging issue because the identity of the future nonnative species may either induce taxonomic homogenization or differentiation (11, 28). In addition, the impact of nonnative species introductions on ecosystem functioning has been mostly addressed through species-centered approaches on a limited number of invasive species (29), whereas the ecological consequences of taxonomic homogenization remain largely unknown to date (cf. 13, 14, 30). Forthcoming studies should therefore focus on assessing how changes in β-diversity affect ecological and evolutionary processes from local to global scales (31). Based on our results, we appeal for studies to focus on the areas that experience high homogenization trends. Because most of these areas are also recognized as gathering a high number of threatened species and facing a large range of human-induced physical disturbances (3, 6), local and detailed studies are urgently needed to decide whether it is appropriate or not to set up efficient control, remediation, and eradication procedures (32). In addition to ecological concerns, further studies are needed to answer societal questions, such as how taxonomic homogenization will modify our perception of biodiversity through the loss of cultural landmarks.