Fish Invasions in the World's River Systems: When Natural Processes Are Blurred by Human Activities

Abstract

Because species invasions are a principal driver of the human-induced biodiversity crisis, the identification of the major determinants of global invasions is a prerequisite for adopting sound conservation policies. Three major hypotheses, which are not necessarily mutually exclusive, have been proposed to explain the establishment of non-native species: the “human activity” hypothesis, which argues that human activities facilitate the establishment of non-native species by disturbing natural landscapes and by increasing propagule pressure; the “biotic resistance” hypothesis, predicting that species-rich communities will readily impede the establishment of non-native species; and the “biotic acceptance” hypothesis, predicting that environmentally suitable habitats for native species are also suitable for non-native species. We tested these hypotheses and report here a global map of fish invasions (i.e., the number of non-native fish species established per river basin) using an original worldwide dataset of freshwater fish occurrences, environmental variables, and human activity indicators for 1,055 river basins covering more than 80% of Earth's surface. First, we identified six major invasion hotspots where non-native species represent more than a quarter of the total number of species. According to the World Conservation Union, these areas are also characterised by the highest proportion of threatened fish species. Second, we show that the human activity indicators account for most of the global variation in non-native species richness, which is highly consistent with the “human activity” hypothesis. In contrast, our results do not provide support for either the “biotic acceptance” or the “biotic resistance” hypothesis. We show that the biogeography of fish invasions matches the geography of human impact at the global scale, which means that natural processes are blurred by human activities in driving fish invasions in the world's river systems. In view of our findings, we fear massive invasions in developing countries with a growing economy as already experienced in developed countries. Anticipating such potential biodiversity threats should therefore be a priority.

Homogenization patterns of the world’s freshwater fish faunas

“…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.…”

“…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).…”

“…The resulting taxonomic changes affect both the number of species in each locality (α-diversity) and the similarity in species composition among localities (β-diversity). Although the way extirpations and introductions affect α-diversity patterns has been fairly well documented at the world scale (5,6), their effect on β-diversity remains little known. However, local and regional studies report that introduced species can increase the taxonomic similarity of assemblages in terms of species composition (i.e., decreased β-diversity), a process called taxonomic homogenization (7)(8)(9)(10).…”

“…We used a global database on historical and current freshwater fish occurrences over 1,054 river basins (6). These changes in species composition are largely attributable to human activities (6,18,19) and have mainly occurred during the past 2 centuries (20)(21)(22).…”

“…These changes in species composition are largely attributable to human activities (6,18,19) and have mainly occurred during the past 2 centuries (20)(21)(22). We computed Jaccard's similarity between each pair of river basins for historical and current fauna composition (9,23).…”