Fish invasions study shows human impact

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Human activity is most responsible for the propagation of alien fish species, according to a recent study.

The study was conducted by Belgian, Canadian and French scientists Fabien Leprieur, Olivier Beauchard, Simon Blanchet, Thierry Oberdorff, and Sbastien Brosse and was published in the journal PLoS Biology.

The authors said that three major theories have been proposed to explain the establishment of non-native species: the 'human activity' hypothesis, which says that humans disturb nature and increase pressure; the 'biotic resistance' hypothesis, which says that species-rich communities readily impede the establishment of alien species; and the 'biotic acceptance' theory, which predicts that suitable habitats for native species are also suitable for non-natives.

The authors used mathematical modelling to test which of these three hypotheses was best supported by the data and at the same time, sought to create global map of fish invasions (showing 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 that covered more than 80% of Earth's surface.

Human activity linkThe authors found that indicators of human activity in the river basins of the world were positively related to the number of established non-native fish species.

Furthermore, such indicators account for most of the global variation in non-native species richness, giving support to the human activity hypothesis.

They also demonstrate that the level of economic activity of a given river basin (expressed by the GDP) strongly determines the ease with which it can be invaded by non-native species.

They hypothesize three non-exclusive mechanisms that may account for this pattern: (1) economically rich areas are more prone to habitat disturbances (e.g., dams and reservoirs modifying river flows) that are known to facilitate the establishment of non-native species; (2) high rates of economic exchanges increase the propagule fluxes of non-native species via the ornamental fish trade, sport fishing, and aquaculture; (3) the increased demand for imported products associated with economic development increases the likelihood of unintentional introductions through importations.

Invasion hotspotsThey also identified six major invasion hotspots where non-native species represent more than a quarter of the total number of species present: the Pacific coast of North and Central America, southern South America, western and southern Europe, central Eurasia, South Africa and Madagascar, southern Australia, and New Zealand.

These six areas are also biodiversity hotspots with the highest proportion of extinction-prone fish species, according to the IUCN.

For more information, see the paper: Leprieur F, O Beauchard, S Blanchet, T Oberdorff, S Brosse (2008) Fish invasions in the world's river systems: when natural processes are blurred by human activities. PLoS Biology 6: e28 doi:10.1371/journal.pbio.0060028.