Long-Term Impact of Multiple Invasions on Fish Community Composition and Functional Richness

Invasive species are among the biggest threats to biodiversity and represent billions of dollars to the global economy. Invasive species pose a threat to the biodiversity of freshwater ecosystems, which are among the most vulnerable due to their isolation by terrestrial ecosystems and the impact of human activities. While invasive species in freshwater ecosystems are known to cause extinctions and therefore reduce species richness, little attention has been paid to their effects on functional richness. Here, we aimed to evaluate the species and functional richness of a wetland fish community that has been subjected to the introduction of exotic species in repeated events over four decades. Our study site, La Mintzita, was home to 15 native species, some of them endemic to the Central Mexican Plateau; the ongoing changes in the fish community have led to a current state where the richness of invasive species matches that of native species. Over the decades, functional richness has declined, dropping from 8.1 before 1985 to 2.8 in the 2020s. We found that in the 2020s, functional richness was significantly lower than expected by chance. The native species that might be locally extinct are those occupying similar functional niches as the invaders or those that are the most sensitive, thus indicating a process of fauna homogenisation. We highlight the importance of focusing conservation efforts on maintaining functional richness to preserve services derived from freshwater ecosystems.