Taxonomic rarity and functional originality of freshwater fishes and their responses to anthropogenic habitat alterations

Abstract

The human alteration of habitats is a major driver in the decline of biodiversity worldwide. However, the relationships between different forms of rarity, including both taxonomic and functional aspects, are poorly known. Two relationships were examined in this study: (i) between the different forms of taxonomic rarity and functional originality of lotic fishes; and (ii) between rarity indices and the degree of human alterations. Stream fish communities at 429 stream and river sites were sampled between 2013 and 2018. The taxonomic rarity of fishes was characterized based on their local rarity, habitat specificity and geographical restrictedness. The functional originality of each species was also quantified by using ecomorphological traits, and the relationships between the different indices and their responses to human habitat degradation were explored. Only weak or medium level correlations were found among the indices of taxonomic rarity. A combined taxonomic rarity index showed that overall taxonomic rarity was determined most strongly by local rarity, followed by habitat specificity, and finally, geographical restrictedness. Functional originality was not related either to individual rarity indices or the combined taxonomic rarity index. Mean taxonomic rarity of species was correlated with the degree of habitat degradation of the sites. Contrary to taxonomic rarity, mean functional originality of species increased with the degree of human alteration. This study shows that a more exact characterization of species rarity requires multiple metrics, including functional ones, because these indices respond differently to human alterations. Combined indices of rarity can help to prioritize species level conservation actions, although single rarity metrics are still needed to adapt these actions to species attributes (e.g. endemism or unique functions). Species level conservation would benefit from including the evaluation of functional rarity into conservation policy.

Temporal change in functional rarity in marine fish assemblages

“…For example, we observed higher levels of taxonomic rarity relative to the null expectation (figure 3a,b), as well as reduced evenness, for given levels of richness, in the west coast system (figure 3c) as opposed to the east coast system (figure 3d). Since increased taxonomic rarity can be associated with habitat complexity [45][46][47], this result could reflect the increased structural heterogeneity of the west coast, as well as contrasts in warming trends, and/or recovery from historical fishing pressures [33,48]. Historically, the east coast system (North Sea region) has been more heavily exploited than the west coast system (Celtic Sea area), but, since the beginning of this time series in 1985, fishing pressure has been largely similar in both areas (see electronic supplementary material, figure S7).…”