Functional Stability Despite Taxonomic Changes in Mixed-Species Foraging Flock Participants Along an Elevational Gradient in Knuckles Montane Reserve, Sri Lanka 斯里兰卡纳克斯山地保护区 (Knuckles Montane Reserve) 海拔梯度上混合物种觅食群参与者的功能稳定性与分类变化

Abstract

Mixed-species foraging flocks (MSFs) are widely regarded as resilient communities in response to global habitat change. Sri Lanka is a tropical region well-known for its extensive body of research on MSFs and its highly diverse MSF assemblages, especially in lowland rainforests. Identifying MSFs in high-elevation ranges is increasingly important as climate change drives species to shift toward higher elevations. This study examined MSFs across three elevation gradients (high, mid, and low) along the same stretch of the northern flank of the Knuckles Mountain Range, specifically in Riverstan (high), upper Pitawala (mid), and the Bellan-Ela area (low). Using a line transect method, we observed 40 MSFs at each elevation and estimated foraging heights of each species and their abundance within observed MSFs. To compare MSF participants, we used interpolation and extrapolation (iNEXT) four-step analyses to assess taxonomic diversity. The functional diversity of MSF participants was measured using functional richness, functional divergence, functional evenness, functional dispersion, and Rao's quadratic entropy. Additionally, we compared the foraging heights of species across elevations. Our results revealed significant differences in taxonomic diversity among MSF participants along the elevational gradient. High elevations had lower taxonomic diversity, while low elevations had the highest diversity. Mid- and low-elevation assemblages were similar in Simpson diversity, though both exceeded high-elevation assemblages. However, functional diversity remained conserved across all elevational levels. Thirteen species demonstrated flexibility in changing their foraging heights at different elevations. This flexibility may be a strong indicator of the adaptability of MSF species when shifting habitats, suggesting that MSFs may serve as refuges that increase foraging efficiency during elevational shifts. Nonetheless, only three endemic species showed foraging-height flexibility, while 12 exhibited aversion to changing their adapted niche. These findings highlight the importance of studying highland MSFs to better understand their ecology and implement targeted conservation strategies.