Impacts of Modified Forest Landscapes on Tropical Fish Assemblages

ABSTRACT Tropical freshwater ecosystems are highly biodiverse and increasingly threatened by anthropogenic pressures, yet the ecological processes structuring their communities remain poorly understood. Here, we examine the effects of hilltop forest conversion to commercially important agro‐crop landscapes (tea and cardamom) on fish assemblages in submontane streams of the Sinharaja Forest Reserve, with particular focus on the Critically Endangered, narrow‐range endemic loach Schistura madhavai . While changes at the community level were not robustly detectable, species‐level responses were more pronounced, with S. madhavai showing a preference for forested streams. Forest modification altered environmental gradients (hydrology, water chemistry, substrate, microclimate, and detritus accumulation), concurrently shifting habitat conditions preferred by the focal species, thereby influencing its occupancy. Associations with environmental variables also differed between ecological levels (e.g., detritus, used here as a food‐mediated proxy for benthic insectivores, was more important at the species‐level than at the community‐level), suggesting that species‐specific niche requirements influence the magnitude of responses to disturbance. However, such responses may go unnoticed at the community‐level because they are buffered by compensatory dynamics among species and obscured by the coarse resolution of community‐level metrics. Thus, environmental filtering may appear weak at broader scales despite strong responses among ecologically specialized taxa. Our findings demonstrate that indicator species can provide a more sensitive measure of biodiversity change than community‐level metrics alone. We also highlight the importance of incorporating species‐specific ecological requirements into conservation planning, particularly for range‐restricted endemics inhabiting threatened tropical freshwater ecosystems.