Invasive species and habitat degradation in Iberian streams: an explicit analysis of their role and interactive effects on freshwater fish biodiversity loss

Abstract

The diversity of life on Earth is under the so called biodiversity crisis, which is specially pressing in freshwater ecosystems. Habitat loss and degradation and invasive species are commonly cited as the main causes. Distinguishing the role of each extinction driver and their potential interactions through a mechanistic understanding of impact is crucial for achieving conservation goals. 2. We analyze whether freshwater fish invasive species are mere passengers co-occurring in the biodiversity loss process driven by habitat degradation or as main drivers of the decline of native fish communities in an Iberian basin. Moreover, since few invaded ecosystems are free from habitat loss and degradation, we also tested whether native species simply responded to the abundance of invasive species or if habitat degradation modified the functional relationships between natives and invasive species. 3. We found invasive species to be leading the decline of freshwater fish native communities, while habitat degradation neither played an active role nor influenced invasive species per capita effect on natives. Lower reaches and areas close to reservoirs held the most seriously injured fish communities independently of their habitat degradation status. Then Mediterranean freshwater fish show some resilience to habitat perturbations while invasive species should be raised to the center of attention of conservation actions. Moreover, the essential ecological role that hydrologically stable reaches might play for native communities’ persistence in highly fluctuating environments, such as the Mediterranean, is endangered by the proliferation of invasive species in those environments. 4. Synthesis and applications: Conservation efforts to reduce biodiversity loss among Mediterranean areas freshwater fish communities should focus on mitigating the effect of invasive species especially in better conserved areas. However, the high cost and low efficiency of management actions against invasive species may difficult the effective fight against invasive threats, while new tools such as harder legislation could help reduce the current introduction rates. The roles of different drivers leading the decay of native communities should not be directly extrapolated across taxonomic groups and/or environments, but be analyzed in different particular situations in order to tackle objective management plans facing the current biodiversity problem.