Ecosystem structure and function of afrotropical streams with contrasting land use

Land use changes can strongly influence stream ecosystems, yet these effects remain poorly documented in many hotspots of deforestation such as tropical Africa. The few studies conducted in this region have mostly focused on structural aspects of stream integrity; sparse data are available for key ecosystem processes such as ecosystem metabolism and litter decomposition. To address this knowledge gap, we measured multiple ecosystem functions in rainforest and nearby agricultural (farm) streams located in an afrotropical deforestation hotspot. In two farm and forest stream pairs, we quantified the standing stock biomass of basal resources and invertebrate functional feeding groups, measured rates of litter decomposition (microbial and shredder-mediated) and modelled invertebrate secondary production. We also calculated whole-stream metabolic parameters over multiple weeks. To test the regional generality of our results, we then reanalysed a published dataset and assessed the functional composition of invertebrate communities from many local streams. We found that farm streams had greater algal standing stocks, gross primary production (GPP) and production-to-respiration ratios than forest streams, although they remained strongly heterotrophic with relatively low GPP values. Agricultural land use also led to the almost complete extirpation of invertebrate shredders, resulting in much slower litter decomposition rates at farm than forest sites despite comparable rates of microbial decomposition. Finally, farm streams had lower invertebrate biomass despite abundant basal resources, leading to a 41%-66% reduction in estimated invertebrate production. We speculate that the functional response of our streams to agricultural land use (i.e., a modest increase in GPP but large declines in invertebrate production and shredder-mediated litter decomposition rates) could be common across the afrotropics due to low fertiliser usage in this region and/or features of the local fauna. Although our study focuses a small number of streams, the large effect sizes that we report suggest that afrotropical streams could be strongly affected by rapid land use changes in the region, highlighting the need for additional research, monitoring, and conservation efforts in this poorly studied area of the world that is under increasing human pressures.