Elevational Patterns of Tropical Stream Insect Communities Are Associated With Annual Mean Temperature, Aluminium and Dissolved Oxygen Concentrations

ABSTRACT Although general elevational patterns have been well documented in terrestrial plants and animals, such patterns remain unclear for aquatic organisms—particularly, stream macroinvertebrates, which are highly sensitive to stream order and anthropogenically induced environmental changes. In this study, we surveyed headwater macroinvertebrates across an elevational gradient in the Pearl River basin, using a design that minimised the confounding effects of stream order, assessed their elevational patterns and the interactions between local physicochemical conditions and broad climatic variables. Taxonomic richness showed a clear monotonic decline with increasing elevation, correlated with decreases in annual mean temperature (AMT). Community density, however, showed no elevational pattern and was strongly affected by hydrological seasonality. Local environmental factors also played an important role in structuring communities. Aluminium (Al) was consistently associated with reduced overall richness and density and substantially reduced numbers of predators across all elevations. Meanwhile, dissolved oxygen (DO) concentrations showed a context‐dependent effect: being positively associated with predators in lowland streams but negatively associated with community density in highland streams. The elevational patterns of functional feeding groups were highly season‐dependent. Predator density was lower at higher elevations during the dry season. Conversely, shredder richness and density were higher at higher elevations exclusively during the wet season. By controlling for stream order, this study reveals the elevational patterns of tropical macroinvertebrates and suggests that Al and DO concentrations may play a role in structuring the community. Our findings underscore the need to monitor historically acidified catchments, as persistent Al leaching from polluted soils may pose a severe threat even when stream pH remains neutral. Ultimately, our findings suggest that broad climatic conditions alone are insufficient to predict the future of tropical stream biodiversity. Effective conservation must prioritise localised monitoring of environmental conditions such as metal toxicity and oxygen dynamics, especially in regions facing rapid land‐use change and chronic acidification.