Avian Habitat Classification for Alaska

Abstract

A primary requirement of a habitat classification system is that it reflect the actual use by the inhabiting taxon, in this instance, birds. In Alaska, as elsewhere, there is a predictable relationship between the occurrence of a bird species and of its characteristic niche-gestalt (see James 1971), the configuration of which is based largely on the basic life forms of the vegetation (Pitelka 1941 and subsequent authors), topography, and the physical nature of the substrate. During many years of field work across most of Alaska, I have consciously attempted to identify habitat niche characteristics common to a bird species throughout its Alaska range and, hence, generally to define its niche-gestalt. Simultaneously, I have observed species associations that form more or less consistently when several species have similarities in the configuration of their niche-gestalten. Using this double-ended approach -individual species niche-gestalten and species associations in certain environmental situations-I have constructed an avian habitat classification system for Alaska, a system based upon its use by birds (table 1). The classification system has been improved over the years through innumerable discussions with other ornithologists and ecologists and with botanists, geologists, and oceanographers. It is not a vegetation or landform classification system, but one describing habitats used by birds. The classification may not be entirely applicable to other taxa, since organisms of different taxa have widely different perceptual worlds. However, with minor modification, such as possible expansion of ground cover details for small mammals, it can be adapted for other primarily terrestrial taxa. To serve as a basic overall system, this classification must be broad. Researchers doing detailed studies within any of these large habitat units may wish to subdivide them further. Suggestions for further subdivisions or for types of characteristics to be used are listed under some of the habitats below, but the important point is to use meaningful characteristics, i.e., ones that obviously are affecting the ecology of the community under study. Additional descriptive physiographic adjectives may prove useful, e.g., riparian, estuarian, lowland, coastal, alpine, etc. A number of environmental features contribute to the niche-gestalt of a bird species, but based on my overview, the two morphologic features with the greatest impact on Alaskan avian community ecology relate to the occurrence and characteristics of water and of woody plant growth. Hence, these features are pervasive in this classification system. Some of the habitats recognized below, particularly the vegetative ones, often form a continuum based on a moisture gradient. Such continua cause classification problems, particularly in relation to shrub heights and dominances. Generally, a woody habitat should take its name from the tallest canopy of woody plants present in sufficient amounts to attract breeding birds of that community, even though this canopy coverage may be sparse. Alternatively, however, a compound name may be used to describe sites intermediate between two habitats, e.g., 'I.ow-medium shrub thicket, with the dominant habitat type listed first. In a few instances, instead of a continuum, habitats form more of a hybrid through