A Trophic Comparison of Avifaunas

Lein, M. Ross (Museum of Comparative Zoology, Harvard University, Cambridge, Mass. 02138) 1972. A trophic comparison of Avifaunas. Syst. Zool. 21:135-150. This paper presents an attempt to compare the passerine and near-passerine bird faunas of the six fauLnal regions on a trophic basis. Indices of similarity are calculated using: a) the importance (percentage of total species) of families in the areas, and b) the importance of different trophic roles in the areas. The importance values for trophic roles were calculated by multiplying the importance values of the families by indices for the importance of applicable trophic roles for each family and summing the values for each troDhic role in each region. The faunal regions showed much higher similarity indices when compared on a trophic rather than a taxonomic basis. Also, the geographic patterns of taxonomic similarity are different from that for trophic similarity. High trophic similarity is related to the major ecological division "temperate" versus "tropical" rather than to the geographical proximity. An examination of the individual trophic roles reveals several facts. While some, such as gleaning invertebrates from foliage, have little variation in importance from region to region, others such as nectivory show high degrees of difference. Other roles with intermediate variability show individual discrepancies, such as the paucity of ground-feeding insectivores in the Australian region. Possible reasons for some of these phenomena are suggested. The families of birds that are specialists in or major contributors to the different trophic roles are surveyed to provide a number of examples of "ecological equivalence" between taxonomically remote groups. This ecological equivalence is often reflected in convergent morphology and behavior patterns. [Trophic levels; faunal regimes; Aves.] Zoogeography describes the geogiaphical distribution of animal groups and attempts to explain it. Traditionally, the groups considered are taxonomic. The present paper takes a different approach to the distribution of birds. It is an innovation and is based on major oversimplifications and generalizations. Nevertheless, I feel that the method here proposed is both valid and useful in the examination and comparison of bird faunas; moreover, it focuses attention on some important phenomena in their evolutionary history. Moreau (1966) has pointed out that much of value and significance is lost if a bird fauna is considered en bloc in a zoogeographic study. In an attempt to overcome this difficulty he divided the Ethiopian fauna into five major groups, broadly ecological as well as taxonomic. He pointed out (1966:5) that this method provided enlightening comparisons, with significant differences being obtained in various analyses. Here, I examine the distribution of trophic rather than taxonomic groups of birds. By a trophic group I mean all the species in an area that utilize a single food source in a similar manner; for example, feeding on invertebrates captured on the ground. A more refined examination, considering narrower divisions both of the food sources utilized, and of the behavioral patterns of the animals utilizing them, is both possible and desirable. Ecological Equivalence The concept of the ecological niche has undergone extensive modification since it was introduced (in somewhat different senses) by Joseph Grinnell (1917, 1924, 1928) and Charles Elton (1927). These changes and the differences between the Grinnellian-Hutchinsonian and the Eltonian niche concepts have been discussed by Elton and Miller (1954) and Miller (1967). It would appear, however, that the functional niche of Elton can be recognized, at least on broad levels. Recognition of the functional niche is inseparable from the idea of ecological equivalence that the same functional role in different communi-