Distribution of Birds in Relation to Major Biotic Communities

Pitelka, Frank

1. For the past 50 years, North American ornithologists have adhered to Merriam's life-zone concept in analyzing the facts of bird distribution. 2. The life-zone theory has been examined critically both from the experimental and faunistic aspects, and in no way does it clarify, but rather misrepresents the relations of biotic communities. 3. The concepts of modern ecology as an alternative view are considered briefly through an application of them to twenty-seven species of North American birds to show the relation of their distribution to major biotic communities. North American Tetraonidae as a taxonomic group, eleven species commonly associated with the deciduous forest, and four coniferous-forest species were tested for correlation with biomes by plotting local occurrences on individual maps. 4. The fundamental and largest unit of plant-animal communities is the biome. A diagram and map of the major communities of North America show the relationship and relative size of the climaxes together with broad ecotones and extensive subclimaxes. 5. Designation of breeding ranges of birds are usually inadequate due to a lack of evaluation of abundance and a lack of appreciation for the importance of community relations. In ten species of deciduous-forest birds, breeding ranges are differentiated into areas of common, uncommon, and rare occurrence within the community occupied by the particular species. 6. Three species of Tetraonidae are characteristic of the tundra biome, four of the coniferous forest biome, and three of the grassland biome; two species are characteristic of seral stages rather than of any climax and range over three or four biomes. 7. Seven of the eleven deciduous forest species are confined to that biome: two occupying climax deciduous growth within the biome occur in subclimax vegetation of the same life form in the coniferous forest biome, one species, Dryobates pubescens, occurs in deciduous growth over all of temperate North America. A race of the last species (Antrostomus vociferus) is confined to the deciduous forest region; other races while occurring in different climatic regions (chiefly Mexico), are found in communities in which the combination of vegetational life-forms appear comparable to the subclimax niche of the deciduous-forest race. 8. Correlation of birds with vegetation reveals no relation to specific dominants or groups of dominants of a single biome, consistent correlation, however, occurs between species and life forms of plants. 9. In species ranging over several biomes variation tends to show correlation with climatic factors; among species confined to one major community variation appears to be effected by geographic and biotic factors. 10. A true avian indicator of climax communities is confined to a niche which is available within that climax and barring biotic and geographic factors, does not vary subspecifically. 11. Distribution of birds confined to a biome may be controlled more by availability of the niche while species characteristic of seral stages in more than one biome may be limited more directly in distribution by physical factors. 12. A clearer understanding of the facts of bird distribution can be obtained through an appreciation of the laws of biotic succession and recognition of developmental as well as climax biotic communities over the continent.

Distribution of Birds in Relation to Major Biotic Communities

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