Australian Rock-Mammals: A Phenomenon of the Seasonally Dry Tropics

Abstract

Mammal species specializing on rocky habitats constitute 11 percent of Australia's marsupial and rodent species. The majority of rock-dwelling species is limited to the seasonally dry tropics. These habitats experience a single wet season followed by a long dry season. Runoff from rocky escarpments, water absorption by large rock formations, and the presence of aquifers in rock result in greater water availability close to rock escarpments. Escarpment vegetation, therefore, is higher in species richness and plant productivity than the surrounding forested habitaat. These species-rich habitats provide a greater variety of potential foods for hebivores. The impact of large rock formations on surrounding vegetation is less marked in regions with a more evenly distributed or very low rainfall and/or a temperate climate. The impact of rock escarpments on vegetation in the seasonally dry tropics, rather than the existence of more opportunities for speciation via genetic isolation, is likely to have been instrumental in the evolution of the often regionally endemic rock faunas of Australia's dry tropics. THE EXISTENCE OF GEOGRAPHIC AND PAST CLIMATIC BARRIERS to gene flow is often emphasized in studies of speciation and zoogeographic differentiation (e.g., Keast et al. 1959; Kikkawa and Pearse 1969; Horton 1972, 1973; Keast 1981). Although speciation is unlikely to occur without geographically induced genetic isolation, the ecological framework within which natural selection occurs is likely to be equally crucial in determining whether genetic divergence occurs. The northwestern portion of Australia (the Timorian zoogeographic subregion; Kikkawa & Pearse 1969), has a highly endemic fauna. The region's fauna is usually assumed to have been the product of genetic isolation caused by climatic barriers to the eastern and southern regions of Australia. A large component of the Timorian subregion's endemic fauna is composed of species specializing on rock habitats. These habitats are composed of rock escarpments or isolated residual masses that can be more than 100 m above the surrounding country (e.g., Fig. 1). In western Arnhemland (Fig. 2A) alone there are two species of rock rat (Zyzomys woodwardi, Z. argurus), one species of rock ringtail possum (Pseudocheirus dahli), the rock pigeon (Petrophassa rufipennis), the black wallaroo (Macropus bernardus), a rock wallaby (Petrogale brachyotis), the nabarlek (Peradorcas concinna, a minature rock wallaby), together with species of gecko, skink, and frog, and a species each of python, honeyeater, fruit pigeon, grass wren, and shrike thrush. All these species are restricted to rock habitats. If genetic isolation alone had been sufficient to produce these rock specialists, then rock-dwelling species should be equally abundant in other parts of Australia. If, however, the rock-dwelling fauna is unusually larger than in other parts of Australia, then some ecological factor in addition to genetic isolation is required to explain the richness of rock faunas in Australia's dry tropics. The pattern of distribution of rock-dwelling mammal species across Australia is presented, and climatological correlates with species richness established. Vegetation of a rock escarpment in western Arnhemland is analyzed to determine the impact of climatic variables on rock habitats in the dry tropics. These results are compared with data from other climatic regions. The role of ecological factors in the evolution of endemic rock-faunas is assessed.