Strong seasonal and elevational dynamics in community assembly mechanisms of tropical montane birds in the Eastern Himalayas

Abstract

Ecologists have made significant progress in studying spatial variation in community assembly along elevational gradients. However, there has been comparatively less focus on temporal variation, particularly at intra-annual scales, despite the notable influence of seasonality on ecological communities. Our study revealed strong seasonal differences in elevational community structures, highlighting the importance of considering both time and space when studying assembly processes. Summer communities across all elevations, displayed a wider dispersion in functional and evolutionary features compared to winter communities, linked to increased competition for resources, perhaps mediated via morphological traits. Conversely, winter communities, and those at higher elevations regardless of season, exhibited a smaller, nested subset of the available functional and evolutionary variation indicating stronger environmental filtering. Notably, the use of metrics sensitive to deeper evolutionary relationships (i.e., Mean Pairwie Distance; MPD) provided insights into potential seasonal emigration of entire taxonomic clades. This aligns with phylogenetic conservatism of altitudinal migration, implying a coordinated movement of closely related species to lower elevations during winter. By incorporating seasonality into our analysis, we were able to provide deeper undertanding of how bird communities are structured along elevational gradients. We argue that future research that considers seasonality, particularly for mobile animals, is crucial for a more comprehensive understanding of how communities respond to environmental variation and resource bottlenecks, especially at short, intra-annual scales.