Distribution on Environmental Gradients: Theory and a Preliminary Interpretation of Distributional Patterns in the Avifauna of the Cordillera Vilcabamba, Peru

A new theoretical approach to the study of distribution is presented in this paper. The central concerns the types of interactions between organisms and their surroundings which may function to impose limits on the occurrence of species on a smooth unifactorial environmental gradient. The theory is constructed of a set of three complementary and mutually exclusive models which arbitrarily are given the power of accounting for all possible distributions. Each of the models predicts a different patter of distribution within a group of organisms and each contains two or more unique features which serve to distinguish it from the others. In their simplest form the models state that the occurrence of species is limited respectively by: (i) physical or biological conditions that vary in parallel with the measured gradient, (ii) competitive exclusion and (iii) environmental discontinuities (ecotones). Predictions of each model include (a) the shape of population density curves, (b) the shape of congruity (faunal attenuation) curves, (c) distributional patterns at the termini of gradients, and (d) the form oft he frequency distribution of ecological amplitudes. Application of the theory is demonstrated with data obtained in a study of the distribution of bird species on a uniform elevational gradient in the Eastern Andes of Peru. A series of four expeditions to the Northern Massif of the Cordillera Vilcabamba, a vast undisturbed mountain wilderness, provided information on the upper and lower limits of occurrence of over 410 species of forest birds. Faunal composition and the relative abundances of many species wee estimated at each of 15 stations through large netted samples of birds (170—604 individuals). Following a protocol described in the text, the upper and lower limits of 261 species were assigned to one or another of the three models. Certain limitations of method result in a small number of unavoidable errors in these assignments; hence the outcome of the partitioning procedure is only a first approximation. As evaluated by this preliminary analysis, the three mechanisms of distributional limitation differ appreciably in their importance in the Vilcabamba avifauna. Ecotones account for less than 20% of the distributional limits, competitive exclusion for about one—third of the limits and gradually changing conditions along the gradient for about one—half of the limits.