Ecological Similarity of Shrub-Desert Avifaunas of Australia and North America

The hypothesis that species and communities occupying vegetationally similar environments on different continents should be similar in ecology has become widely accepted, although quantitative support for this hypothesis is meagre. As an indirect assessment of the likelihood of ecological and evolutionary convergence, I quantified features of community structure and of breeding biology, behavior, and life history of bird species occupying several sites in semi-arid shrub deserts in the Great Basin of North America and the interior of Australia. Study sites on both continents were statistically indistinguishable in most quantitative measures of vegetation structure. Breeding-bird assemblages contained the same number of species per unit area, but densities of individuals in Australia averaged half those in North America. A cluster analysis based on a multivariate similarity matrix derived from 14 ecological and life-history characteristics of the species revealed little close matching of Australian species with a North American counterpart, although two such pairings suggest possible ecological convergence. The frequencies of most ecological/life-history traits among species breeding on local plots differed substantially between continents. The Australian species were characterized by longer breeding periods with more breeding attempts, greater social aggregation during breeding and feeding activities, greater use of domed nests, and sedentariness or nomadism rather than seasonal migration. The avifaunas did not differ in mean clutch sizes, the incidence of polygyny, nesting height, territory size, or general dietary patterns. Despite the close matching of vegetation structure on the two continents, differences between the species and communities are more striking than the similarities. These differences may be related to climatic differences between the areas. Although long-term mean precipitation in the Australian and North American shrub-desert regions does not differ, the monthly and yearly variance in Australia is substantially greater. Droughts may be prolonged and widespread, and this factor, together with soil-nutrient limitations, may result in generally lower and more erratic primary and secondary production in the Australian deserts. Close convergence may be lacking because, despite the similarities in habitat structure, the climates and resource levels of Australian and North American shrub deserts differ in fundamental ways, and the avifaunas are of differing phylogenetic derivation. These comparisons emphasize that (1) bird communities are frequently open, so understanding the composition and dynamics of local assemblages requires a broad-scale perspective, and (2) generalizations or explanations developed for one region of the world may not apply in other regions, even if the habitats are similar.