Abstract
The Australian freshwater fish fauna is very unique, but poorly understood. In the Australian Monsoonal Tropics (AMT) biome of northern Australia, the number of described and candidate species has nearly doubled since the last attempt to analyse freshwater fish species composition patterns and determine a bioregionalisation scheme. Here, we utilise the most complete database of catchment‐scale freshwater fish distributions from the AMT to date to: (a) reanalyze spatial patterns of species richness, endemism and turnover of freshwater fishes; (b) propose a biogeographic regionalisation based on species turnover; (c) assess the relationship between species turnover and patterns of environmental change and historic drainage connectivity; and (d) identify sampling gaps. Biogeographic provinces were identified using an agglomerative cluster analysis of a Simpson's beta (β sim) dissimilarity matrix. A generalised dissimilarity model incorporating eighteen environmental variables was used to investigate the environmental correlates of species turnover. Observed and estimated species richness and endemism were calculated and inventory completeness was estimated based on the ratio of observed to estimated species richness. Three major freshwater fish biogeographic provinces and 14 subprovinces are proposed. These differ substantially from the current bioregionalisation scheme. Species turnover was most strongly influenced by environmental variables that are interpreted to reflect changes in terrain (catchment relief and confinement), geology and climate (runoff perenniality, stream density), and biotic responses to climate (net primary productivity). Past connectivity between rivers during low sea‐level events is also influential highlighting the importance of historical processes in explaining contemporary patterns of biodiversity in the AMT. The inclusion of 49 newly discovered species and candidate species only reinforced known focal points of species richness and endemism in the AMT. However, a number of key sampling gaps remain that need to be filled to fully characterise the proposed bioregionalisation.
Highlights
In comparison to other continents, Australian freshwater fish communities are considered highly unique but species poor, largely due to Australia's arid climate and long isolation from other land masses (Unmack, 2001, 2013)
Our goals were to: (a) map spatial patterns of species richness, endemism and turnover between river catchments; (b) propose an updated biogeographic regionalisation of freshwater fishes based on the spatial turnover of species; (c) assess the relationship between species turnover and patterns of environmental change and historic drainage connectivity; and (d) identify survey gaps that require further sampling
We considered endemism in relation to the Australian Monsoonal Tropics (AMT); that is, species distributed outside the study region may still be considered endemic to a given catchment, subprovince, or province within the AMT
Summary
In comparison to other continents, Australian freshwater fish communities are considered highly unique but species poor, largely due to Australia's arid climate and long isolation from other land masses (Unmack, 2001, 2013). The AMT encompasses 33% of Australia's landmass, but contains 65% of all freshwater fishes, and the encompassed provinces contain a moderate number of species compared to others around the globe (Abell et al, 2008). The number of described and candidate species in the AMT has increased by 50% since the turn of the millennium, compared to a background rate of 35% across all of Australia (see Allen, Midgley, & Allen, 2002; Le Feuvre, Dempster, Shelley, & Swearer, 2016 for comparison). There is a substantial need for increased biogeographic research in this region
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