One of the reasons I first became interested in specialization and generalization was due to the results of some of my work on communities of cicadas (e.g., Mac Nally and Doolan 1986, Mac Nally 1988b) and birds (e.g., Mac Nally 1989,1990a, 1995), each involving distributions in relation to a variety of habitat types. The most remarkable feature was the range of ‘habitat versatility’ – ubiquity – displayed by species. For example, the cicada Abricta curvicosta occupied a wide range of forest, woodland and shrubland habitats, while several other species were ‘tall forest specialists’ (Psaltoda moerens, Henicopsaltria eydouxi, Thopha saccata) or ‘shrubland specialists’ (Cystosoma saundersii) (Mac Nally and Doolan 1986). Other species exhibited intermediate levels of habitat versatility. Similarly, birds such as the grey shrike-thrush (Colluricincla harmonica) and laughing kookaburra (Dacelo novaeguineae) occupied all five major forest and woodland types that were studied, whereas other species were restricted to only a single kind of forest or woodland (e.g., red-capped robin, Petroica goodenovii; yellow rosella, Platycercus flaveolus) (Mac Nally 1989). Species occupying practically any combination of the five habitat types were recorded. Such disparities in ubiquity among species, which typify many studies (e.g., Collins et al. 1982, Williams 1988, Woinarski et al. 1988), demand an explanation and certainly represent an important component of the degree of ecological specialization or generalization of species in its broadest sense.

Although recent work on biogeographic patterns is beginning to clarify the picture for the reasons for differences in the extent of geographic ranges among species (e.g., Brown and Gibson 1983, Hengeveld 1990), the determinants of variation in ubiquity remain less clear.