Range restriction leads to narrower ecological niches and greater extinction risk in Australian freshwater fish

Human-induced environmental changes are accelerating biodiversity loss. Identifying which life-history traits increase extinction risk is important to inform proactive conservation. While geographically or numerically rare species are typically more vulnerable, ecological specialization may also increase extinction risk particularly when associated with rarity. We investigate whether regionally endemic freshwater fishes have more specialized diets and habitat requirements than more widely distributed, closely related species. We then use this information to assess extinction risk. Using closely-related widespread and endemic congeneric pairings from the Kimberley region of north-western Australia, we investigate whether there are ontogenetic diet shifts in 13 species and if some of these ontogenetic trophic units (OTUs) have narrow dietary niches. Using qualitative measures of habitat and presence/absence data, we also assess habitat specialization in 32 species. Overall, range-restricted species had narrower ecological niches. Ontogenetic diet shifts existed in 12 of 13 species and range-restricted species were more specialized for some or all of their OTUs compared to their widespread congenerics. Endemic species had a higher degree of variance in habitat use compared to their widespread congenerics, showing they had more specialized habitat requirements. As specialization is linked to extinction risk, the narrow niche breadth of small-ranged endemic fishes makes them more vulnerable to extinction than more cosmopolitan species. As many endemics from the Kimberley region have small ranges and/or low abundances, they may have an increased risk of extinction. By identifying which endemic species have narrow ecological niches, our study provides essential information for targeting proactive conservation efforts.