On the edge: habitat patchiness and island isolation fracture the distribution of the endemic Kimberley rock-rat

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.