Riparian zones offset community decline in true flies (Diptera) after lowland rainforest conversion to smallholder cash crops in Sumatra

Abstract Tropical land-use change is a major driver of global biodiversity loss, yet its effects on flies (Diptera) remain underexplored, particularly in Southeast Asia. The present study addresses this knowledge gap by comparing Diptera abundance, diversity and community composition between lowland rainforest and smallholder monocultures of rubber and oil palm in lowland Sumatra, both on riparian soils as well as upland, well-drained soils. 6589 Diptera specimens were collected by canopy fogging from 36 research plots (eight per land use) in two landscapes—‘Bukit Duabelas’ and ‘Harapan’—in Jambi Province, Sumatra, Indonesia. Based on 5907 specimens from the ten most abundant families (Cecidomyiidae, Ceratopogonidae, Chironomidae, Chloropidae, Dolichopodidae, Drosophilidae, Limoniidae, Milichiidae, Phoridae, Sciaridae, sorted to 706 morphospecies from 29 subfamilies and 96 genera), we found that average Diptera abundance did not differ across land uses, but diversity was higher in lowland rainforest compared to monocultures, and community composition different. Patterns varied among families, with some affected by land use and others by landscape. Irrespective of land use, riparian zones in Harapan consistently showed higher Diptera abundance and diversity than upland zones, alongside differing community composition, primarily driven by increased abundances and diversity of Chironomidae and Dolichopodidae. These insights highlight that tropical rainforest conversion negatively affects Diptera populations, depending on landscape context and observed family, and that riparian zones can support greater Diptera abundance and diversity than upland zones. Overall, protecting and restoring riparian zones to buffers would thus likely benefit conservation and sustainability efforts in Southeast Asian cash crop agriculture.