Information relating to the early decline of native mammal species in the western third of Australia, before the establishment of rabbits and foxes, was sought from recollections of oldtimers, archival records, historical documents, and searches of museum collections. Based on the information discovered, the hypothesis is advanced that the early decline was caused by an exotic disease. The evidence available is suggestive of a first wave of mammal declines and extinctions in Western Australia (WA) commencing about 1875. From this single, contingent historical event, it is postulated that 33 species (about one third of the non-volant mammal fauna of WA) changed significantly in distribution and abundance.
The pattern of the decline both geographically and over time is consistent with epizootic disease as the primary factor, but probably interacting with drought and predation by feral cats as secondary factors. Much of the decline occurred before food shortages or habitat destruction caused by sheep grazing, habitat destruction caused by wheat farming, and changes in Aboriginal fire regimes.
Dated and localised records of disease affecting conspicuous (often pest) species, when integrated with other records of early declines, are suggestive of the epizootic spreading rapidly from the Shark Bay district in the 1880s. The lower south-west area was affected last, before 1920. Descriptions of clinical signs are imprecise but do refer to numerous dead or dying animals, mange, alopecia, eye disease, withered ears, and scabs on the nose. Plausible explanations of the source of the disease (south-east Asia), its transmission (mosquitoes), and the pattern and scale of its spread (exceptionally wet summers) are proposed. Other possible vectors are discussed but do not adequately explain the pattern and scale of the spread.
Many other early declines of mammal species in WA that are difficult to explain convincingly may have been influenced by this epizootic. Disease as a primary factor is considered to provide a more satisfactory explanation of early declines in WA than predation by feral cats. Impacts of these declines on ecosystem processes were likely to have been profound. The applicability of the conceptual model developed is discussed in relation to the early decline of some mammal species in eastern Australia, The need for ongoing, vigilant quarantine to minimise the risk of animal diseases being introduced is emphasised.