Controls of number of bird species on montane islands in the Great Basin

Johnson, Ned

This work makes specific reference to species found in Great Basin National Park. Stepwise multiple regression and path coefficients are used to clarify patterns of inter-correlation and causality in an analysis of controls of Boreal bird species numbers on 20 mountaintop islands and 11 sample areas within the adjacent 'continents' of the Sierra Nevada-Cascade Mountains and Rocky Mountains in the western United States. Total area and area of forest-woodland, variables of significance in other, similar studies, here predict only from 28 to 45% of the variation in total species number. In contrast, an index of habitat diversity explains 91% of the variation in total bird species. Habitat diversity, in turn, is under complex control, with area of forest-woodland, width of barrier, and elevation and latitude of highest peak being significant. A minor distance effect is shown only for permanent resident birds; diversity of this group is reduced by increasing relative isolation from source stocks on the continents. The permanent resident birds may represent a non-equilibrium situation as has been proposed for mammals of the Great Basin islands. But because forest habitats and associated resources required by birds are also impoverished in the Great Basin, partly through difficulty of access of propagules and presumably because of general severity of climate, many bird species are lacking from certain islands because their environmental needs are not met. On the most impoverished mountain islands the Boreal avifaunas are reduced to 14 species, each in a separate genus and each with a distinctive ecologic role. The problem of inter-correlation of effects of area, distance, and habitat deserves increased attention in analytical insular biogeography.

Controls of number of bird species on montane islands in the Great Basin

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