Adaptive Evolution and Shifts in Niche Occupation in Island Birds

Abstract

Adaptive evolution in island birds is investigated with special reference to the birds of Tasmania, a continental island 26,000 square miles in area that lies 140 miles off southern Australia. The avifauna is a typical insular one being numerically impoverished (only 43 species of passerine birds compared with 89 in equivalent habitats on the adjacent mainland), and lacking certain kinds of birds (e.g., true trunk feeders are absent). The study emphasizes shifts in vertical feeding zones and in morphological attributes associated with perching and feeding (e.g., bill, tarsus, hallux). The findings are as follows: 1. A series of island species have moved into the vacant trunk feeding and underexploited arboreal, foliage-gleaning, adaptive zones; 2. these are species that, on the adjacent Australian mainland, already feed to some slight extent in these zones; 3. the shifts invariably involve a broadening of, or increased diversity in, feeding; 4. there is a broad redivision of ecological roles and adaptive niches on the island; vacant niches are eliminated, and a new state of integration and balance is achieved within the avifauna. INSULAR AVIFAUNAS ARE numerically impoverished, in terms of numbers of species, relative to those of the larger landmasses. Whole taxonomic groups, and basic ecological types, of birds are missing in some cases. This absence applies even to the avifaunas of large, biotically fairly diverse, islands. Various intriguing ecological questions are posed. Do some adaptive zones, or ecological niches, remain empty in these cases? Are the vacant roles secondarily filled by other species? If so, what ecological and morphological adaptations do they make? What happens when a well-adapted new colonizer finds an unoccupied, or semivacant, niche? Finally, is there a comprehensive (and possibly periodic) reshuffling or redivision of ecological roles by the members of insular avifaunas so that all ecological opportunities are realized, and a new state of balance achieved? An investigation of these aspects forms the basis of the present paper. The avifauna2 considered is that of the island of Tasmania. This continental island lies about 140 miles to the south of Victoria, Australia, has an area of 26,000 square miles, and includes a wide range of avian habitats. It has only about half the number of species of breeding land and freshwater birds found in equivalent vegetation formations in southern Victoria. For small passerines the relative figures are 43 and 89 (table 1). ECOLOGICAL ATTRIBUTES OF