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.