The expansion of oil palm plantations has caused significant deforestation and land-use change, negatively impacting tropical ecosystems and biodiversity. However, limited research focuses on the effects on freshwater systems. This thesis assesses the impact of the oil palm expansion on stream ecosystems and investigates the effectiveness of riparian buffers in mitigating these impacts. To do so, we conducted a sampling campaign in Guatemala addressing the benthic-macroinvertebrate (surber-net samples) and biofilm microbial communities (metabarcoding), the leaf-litter breakdown (leaf-litter bags experiment), and habitat and water-quality of streams surrounded by primary forest, grazing lands, and oil palm plantations with and without riparian buffers. In addition, we conducted a meta-analysis review on all available research contrasting taxon richness and communities of macroinvertebrates, amphibians, and fish in streams in forests and oil palm plantations.

Streams in oil palm plantations without riparian buffers were warmer, more turbid, carried less silica and macroinvertebrate and fungal richness, showed reduced microbial and total leaf-litter breakdown, and hosted fewer shredders, wood-decomposing fungi, and more generalist and invasive species compared to primary forests. Streams in grazing lands showed higher water conductivity and temperature, lower dissolved oxygen concentration, and lower macroinvertebrate and bacterial richness but similar leaf-litter breakdown as primary forests.

Streams in oil palm plantations with riparian buffers resembled primary forests, with similar temperatures, dissolved oxygen concentrations, substratum, canopy, and macroinvertebrate and microbial richness. These showed higher leaf-litter breakdown than the plantations without buffers, probably due to the increase in shredders. The enhancing effect of riparian buffers on taxon richness was also detected across all groups by the meta-analysis.

Plantations with buffers hosted 24% fewer taxa than primary forests and a similar richness with disturbed forests, as opposed to a difference of 44% and 19%, respectively, when buffers were absent. However, buffers alone cannot fully replace the role of protected areas since these were unable to conserve a forest community. Our findings showed that the oil palm expansion needs to be at the expense of unforested land uses; furthermore, these plantations need to preserve native riparian buffers to avoid species loss and maintain the integrity of freshwater ecosystems.