Celeste H. Wale

We integrate demography, morphology and genetics to assess the biology and population structure of the endemic Kimberley rock-rat (Zyzomys woodwardi), a habitat specialist of the monsoonal tropics. The species is confined to the extreme north-west of the continent and 16 adjacent islands that were part of the mainland during the last Pleistocene glaciation. Islands became isolated from ~6500 years before present (BP), with their current populations being considered relictual. This, together with habitat patchiness and sedentary behaviour, promotes marked genetic and morphological variation. Capture rates were high on islands. All populations show a strong preference for moist rocky gullies containing boulder substrates and vine forest floristics. Demographics show large seasonal variation in activity and movement, with breeding occurring throughout the year. There is no sexual dimorphism. Analyses of skull metrics showed clear evidence of dwarfism in island populations, contrary to gigantism commonly observed in murids. Genetic data showed that islands have smaller effective population sizes than does the mainland. There are strong genetic affinities among localities within geographically proximate groups, with evidence groups having remained isolated from one another since before the last glacial maximum. A reduction in the reliability of wet-season conditions, and more frequent prolonged dry periods, may directly affect the environmental niche occupied by the species. Introduced species, including feral cats (Felis catus), and increased fire, potentially compound these pressures and reduce population viability. Although the species is currently listed as Least Concern because of the extent of its distribution and local abundance, fragmentation leaves local populations vulnerable to extinction with the prospect of recolonisation problematic.

Endemic rodents constitute 19% of Australian terrestrial mammal species and this proportion is higher in arid zones and the north. We report substantial genetic diversity and population structure in the common rock‐rat Zyzomys argurus, a saxicoline murid whose range extends across northern Australia and into the continent's interior. Samples of 686 individuals from 68 locations, encompassing the western half of the species' range, provide one of the most geographically and numerically extensive genetic studies of an Australian rodent. The Great Sandy Desert, a prominent feature over the last 0.5 my bp acts as a significant barrier to gene flow. Continental islands, formed during the past 10 000 years following the end of the most recent Pleistocene glacial maximum, have populations with lower heterozygosity and marked differentiation from each other and the adjacent mainland. On the mainland, despite considerable differentiation between locations, there is only weak evidence for isolation by distance and where it occurs it is associated with drainage basins. In the Pilbara, analyses of genetic structure within localized sub‐locations, just a few kilometres apart, indicate considerable variation between them (F_ST) and inbreeding within (F_IS). We interpret this in the context of cyclonic and other extreme rainfall events that occur episodically, leading to boom‐bust population cycles. Populations decline into refugia of isolated rock patches during extended periods of poor resources with consequent low genetic diversity (H_e) and large differentiation from others. Populations that irrupt after major episodic rainfall increases resource states, have a more extensive distribution with greater heterozygosity because of gene flow between previously isolated refugia and less differentiation between the locations. These observations have conservation significance for threatened congeneric species and other Australian arid rodents by demonstrating, in this common exemplar, the reduced evolutionary potential imposed by both transient isolation due to climatic variation and longer term disruptions by geographic barriers.