A revision of the bioregionalisation of freshwater fish communities in the Australian Monsoonal Tropics

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.

The Kimberley, north‐western Australia, as a cradle of evolution and endemic biodiversity: An example using grunters (Terapontidae)

“…Australian freshwater biogeographical regions used in this study, based on Unmack (, ) and Shelley et al () (see Materials and Methods), with freshwater terapontid species total richness and endemic richness (in brackets) indicated by number. Some species are found across multiple provinces.…”

“…We coded all sampled taxa used in the tree based on their contemporary distributions within major freshwater biogeographical provinces for Australian species presented in Unmack (, ) [(1) Kimberley Province (K); (2) Northern Province (N); (3) Eastern Province (E); (4) Central Australian Province (C); (5) Murray‐Darling Province (D); (6) Pilbara Province (P)], or by their broader biogeographical region for non‐Australian species [(7) New Guinea (G); (8) Southeast Asia (S); or (9) Marine (M)]. The bioregionalization of freshwater fishes in the AMT, which encompasses the Kimberley and Northern provinces presented in Unmack (, ), was recently revised in Shelley et al () and the province boundaries were redefined. Specifically, the boundary of the Kimberley Province was extended east to include the Victoria River Basin (VRB; see Figure ), whereas the eastern Northern Province boundary was defined by the Great Dividing Range (as far north as the Wenlock River).…”

“…Molecular evidence suggests they might have evolved from a single freshwater invasion (Davis et al, ). Within Australia, we herein refer to the distribution of species by the freshwater biogeographical provinces defined in Unmack () and partially revised in (Shelley et al, ) as they are the most informative and relevant spatial units in this instance. Of the 38 obligate freshwater terapontid species found in Australia, 32 are shared between the Kimberley Province and Northern Province in the AMT.…”

“…To overcome the impact that this would have on the combined RAG1 + cytb tree, the variable sites from the mitochondrial sequences of individuals with heterospecific mtDNA haplotypes were removed (Shelley, Swearer, et al, 2018). Zheng and Weins (2015) investigated the impact of missing F I G U R E 3 Australian freshwater biogeographical regions used in this study, based on Unmack (2001Unmack ( , 2013 and Shelley et al (2019) (see Materials and Methods), with freshwater terapontid species total richness and endemic richness (in brackets) indicated by number. Some species are found across multiple provinces.…”

“…[(1) Kimberley Province (K); (2) Northern Province (N); (3) Eastern Province (E); (4) Central Australian Province (C); (5) Murray-Darling Province (D); (6) Pilbara Province (P)], or by their broader biogeographical region for non-Australian species [(7) New Guinea (G); (8) Southeast Asia (S); or (9) Marine (M)]. The bioregionalization of freshwater fishes in the AMT, which encompasses the Kimberley and Northern provinces presented inUnmack (2001Unmack ( , 2013, was recently revised inShelley et al (2019) and the province boundaries were redefined.…”