Text by Aurele Toussaint
The biodiversity of freshwater fish fauna disturbed by invasive species
Non-native species introductions are recognized as one of the major drivers responsible of the ongoing sixth biodiversity crisis, along with climate change, pollution and habitat loss. However, this factor is most often only addressed in terms of increasing or decreasing in number of species, leaving unclear its impact on the functional diversity of communities (i.e. the diversity of ecological roles provide by species in ecosystems). An international team of researchers from the Institute of Ecology and Earth Sciences (Department of Botany/ University of Tartu/ Estonia), the Evolution and Biodiversity Laboratory (EDB – CNRS / University of Toulouse III Paul Sabatier / IRD) and the Center for Marine Biodiversity, Exploitation and Conservation (MARBEC – CNRS / University Montpellier / IRD / Ifremer) has just shown that while the number of fish species per river increased on average by 15% in the world’s rivers, the diversity of their functional attributes increased on average by 150% under the pressure of non-native fish introductions. These results, published in the November issue of the journal Ecology Letters, highlight the need to consider different facets of biodiversity in order to evaluate the impact of biological invasions on ecosystems.
Human activities has promoted biotic exchanges between regions for centuries, for food, commercial, ornamental and even involuntary purposes by carrying organisms unknowingly. As a result, several hundreds of species of freshwater fish are now established outside their native area. The survey of these introductions in the rivers of the world is rather well known and has already shown that the non-natives species introductions have resulted in an increase of about 15%, on average, of the number of species per river. However, it remained to be tested whether these non-native species share or not similar ecological characteristics to the species already present (native species) and therefore have or not led to an increase in the functional diversity of the communities. Toward this goal researchers have measured the morphological traits associated with locomotion and nutrition for more than 9,000 species of freshwater fish out of the 13,000 currently known. We demonstrated in this study that the 15% average increase in the number of species per river simultaneously caused an unexpected 150% increase of the functional diversity of the community!
While the very large rivers, such as the Amazon or Mississippi, have undergone few functional changes under the introduction of non-native species, the small rivers have undergone an explosion of their functional diversity because the introduced species have very different ecological traits from those of the species already present. This trend is particularly striking in arid and Mediterranean rivers, which naturally host few highly specialized species. Thus, in the Alpes-Maritimes for example, the Siagne river hosts historically 9 fish native species, but received under the effect of the human activities 6 non-native species. Among them the Carp, originating from Asia and Black Bass native to North America which have multiplied the original functional diversity of the community by more than 40.
More generally, at the world scale, we observed an increase in the average body size of fishes and an overrepresentation of laterally flattened species (such as carp or black bass) in communities under the introduction pressure. These functional changes are probably related to the effects of dams that favor the establishment of non-native species able to move and survive in a stagnant environment.
As conclusion of this study, we already knew that non-native species have a strong impact on the taxonomic diversity worldwide but we here demonstrate that considering the functional role of organisms is clearly a key point in order to better understand the impact of non-native species on biological communities.
Citation: Toussaint, A., Charpin, N., Beauchard, O., Grenouillet, G., Oberdorff, T., Tedesco, P. A., … & Villéger, S. (2018). Non‐native species led to marked shifts in functional diversity of the world freshwater fish faunas. Ecology Letters, 21: 1649–1659 (link to full text)
Global spread of non‐native species profoundly changed the world biodiversity patterns, but how it translates into functional changes remains unanswered at the world scale. We here show that while in two centuries the number of fish species per river increased on average by 15% in 1569 basins worldwide, the diversity of their functional attributes (i.e. functional richness) increased on average by 150%. The inflation of functional richness was paired with changes in the functional structure of assemblages, with shifts of species position toward the border of the functional space of assemblages (i.e. increased functional divergence). Non‐native species moreover caused shifts in functional identity toward higher body sized and less elongated species for most of assemblages throughout the world. Although varying between rivers and biogeographic realms, such changes in the different facets of functional diversity might still increase in the future through increasing species invasion and may further modify ecosystem functioning.