Riparian buffers mitigate impacts of oil palm plantations on aquatic macroinvertebrate community structure in tropical streams of Borneo

Southeast Asia is undergoing extensive conversion to oil palm (Elaeis guineensis) monocultures with fast growth expected in West Africa and South America. Despite rapid conversion rates, few tropical studies evaluate land-use change impacts on stream communities, with even fewer studies examining oil palm plantations (OP). In this pioneer study we compare macroinvertebrate community assemblages and their relationships with physico-chemical properties in OP streams bordered by different riparian buffer types to streams in a lowland dipterocarp forest in Sabah, Malaysian Borneo. Our study sites were in increasing levels of riparian disturbance (reduced vegetation density, diversity and structural complexity): (i) Native reference forested sites (NF); (ii) OP streams buffered with patches of natural forest (OPF); (iii) OP streams buffered with untreated oil palm (no chemical application at buffer sites; OPOP); and (iv) OP streams without buffer (OPNB). Macroinvertebrates were collected from leaf litter bags deployed with leaves of varying quality - native Macaranga tanarius and exotic oil palm, and retrieved after 14 days. We found that although invertebrates are typically influenced by resource quality, no differences were detected in their assemblages between the leaf species. All litter bags were dominated by collector-gatherers, with few shredding invertebrates, indicating that they were using the leaves as a habitat rather than a food source. We found no significant differences in macroinvertebrate abundance, richness and sensitive taxa richness (EPT) regardless of the rate of riparian disturbance. However, PERMANOVA showed significant differences between the macroinvertebrate community composition of heavily disturbed OPNB and pristine NF streams. Retaining high quality riparian buffers in OPF streams and lower quality understory in OPOP streams posed intermediate effects on macroinvertebrate community composition. The differences between sites were explained by the increase in nutrients, particularly potassium and phosphorus, stream water temperatures and erosion levels that negatively correlated with macroinvertebrate assemblages in oil palm streams. These findings support the use of riparian buffers in oil palm plantations to mitigate land conversion impacts on macroinvertebrate communities in tropical streams.