The role of ecotones in the distribution of Andean birds.
For a set of 47 bird species whose limits (16 upper, 31 lower) coincided with the montane rain forest-cloud forest ecotone on the Cordillera Vilcabamba control transect, it was determined whether, as predicted, the species expanded or contracted their distributions in localities in which the homologous ecotones were displaced upward or downward relative to the control elevation. Where the ecotone was displaced away from species' centers of distribution, 36 of 41 species (88%) were found to have expanded their distributions. This result upholds the provisional assessment of the ecotone as a distributional barrier to these species in the control locality. Where the ecotone was shifted toward species' centers of distribution, 43 out of 44 species (98%) had failed to contract fully in distribution; they were found on the other side of the ecotone, in what had been predicted to be alien habitat. This result, not anticipated, is illustrative of a tendency of species to occupy a greater range of habitats near to vs. far from their centers of distribution. Widespread Andean bird species had expanded distributions, in both upward and downward directions, in the biogeographically isolated Cordillera de la Costa of Venezuela. Reduced avian species diversity in this range relative to the main Andean chain appears to have led to a general relaxation of distributional restraints. Competitive exclusion emerges as the factor of overriding importance in the exceedingly diverse Andean fauna. -from Author
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