Tropical montane gradients elucidate the contributions of functional traits to competitive and environmental fitness

Functional traits can be classified as alpha or beta, based on their relative importance in determining a species competitive and environmental fitness, respectively. However, the link between a trait and its contribution to a particular aspect of fitness is not always straight-forward. We investigated phylogenetic and functional diversity for bird communities along a 200-2800 m elevational transect in the eastern Himalayas. We hypothesized that beta traits, associated with environmental tolerances, would exhibit a directional change in mean values, while alpha traits, linked to competitive strategies, would show a decrease in dispersion with elevation. Our findings showed that most functional traits exhibited a decrease in dispersion with elevation. However, surprisingly, the mean values of these traits also exhibited a significant relationship with elevation, suggesting their involvement in both competitive and environmental fitness. Furthermore, we observed that morphological traits, traditionally considered beta traits associated with environmental tolerance, were influenced more strongly by resource availability and habitat structure rather than aspects of temperature or air density. These results challenge the simplistic classification of traits as either alpha or beta. We suggest that future studies should carefully analyze the variation in mean values and dispersion of individual traits before assigning them solely to a particular category of fitness. The results contribute to a broader understanding of the complex interactions between functional traits, fitness, and environmental conditions in Himalayan bird communities.