Oscar Alberto Rojas Castillo:
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The expansion of oil palm plantations has led to extensive deforestation and land-use changes in tropical regions, severely affecting biodiversity. However, little attention has been given to its effects on freshwater ecosystems. This thesis investigates the impact of oil palm expansion on stream ecosystems, including habitat, water quality, leaf-litter breakdown, and the diversity of bacteria, fungi, macroinvertebrates, fish, and amphibians. Additionally, it addresses the mitigating effect of riparian buffers on these impacts. To do so, we conducted a sampling campaign in Guatemala investigating streams in primary forests, grazing lands, and oil palm plantations with and without riparian buffers. In addition, we conducted a meta-analysis review on the research contrasting stream taxon richness and community composition 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 breakdowns than the plantations without buffers, probably due to increased shredder abundance. The meta-analysis also detected the enhancing effect of riparian buffers on taxon richness across all groups. 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.