Samuel Chak Lam Ho , Simon Hart, Michael Hammer, Peter Baxter, Peter Unmack, Salit Kark

Freshwater ecosystems cover less than 3% of the earth’s surface, yet support nearly 10% of all known animal species, majorly represented by freshwater fishes (69%) and amphibians (24%), both of which are highly threatened groups. Geographically isolated freshwater species, such as those inhabiting islands, are at high risk. Australia, with ~9300 islands, is home to diverse island freshwater fauna. However, the lack of published literature on their island occurrence, threats, and management impedes effective conservation across islands. We aim to describe the distributional patterns of amphibians and freshwater fishes on islands and analyze the island characteristics that influence these patterns. We compiled the first database of occurrences of amphibians and freshwater fishes of Australia’s islands. Utilizing the database, we used regression analysis to examine the main drivers of distributional pattern, species richness, and species composition on Australia’s islands. We found 102 amphibians and 95 freshwater fishes, 55 fishes were obligate freshwater species, 21 were euryhaline and 19 were diadromous. Both amphibians and freshwater fish richness were higher on islands with more precipitation. While freshwater fishes were less diverse on islands than amphibians, potentially due to lower survey efforts, fishes had a higher proportion of threatened and exotic species. Islands closer to the mainland hosted higher amphibian richness, which likely retained mainland amphibian assemblages, or were more easily colonized. In contrast, larger islands hosted more freshwater fishes, where diverse habitats were likely to sustain more species. Using the new database we compiled; we found that amphibian and freshwater fishes occurrences on islands likely resulted from different drivers. This study provides a baseline for follow-up studies on phylogeny and biogeography. This research contributes to the conservation of amphibians and freshwater fishes on islands by revealing potential hotspots of extinction risk and lays the groundwork for future spatial prioritization work.