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Land‐use change and agricultural expansion have caused marked biodiversity loss in Southeast Asia, but impacts on freshwater communities have been very little studied. Semi‐aquatic bugs are abundant in streams, provide prey for many other animals, and are sensitive to environmental change, making them a relevant group for studying land‐use change.We investigated the effects of logging and conversion of forest to oil palm plantations on semi‐aquatic bugs in Sabah, Malaysia, and the potential value of retaining riparian buffer strips in plantations, by sampling across 12 rivers along an existing land‐use gradient. We recorded catchment, riparian, and stream‐scale environmental variables and surveyed semi‐aquatic bugs within streams in old‐growth forest, logged forest, and oil palm plantation with (OPB) and without buffer strips (OP). We recorded the abundance, richness, total biomass, and proportion of juveniles and winged adult individuals of all species, together with the sex ratio of a common morphospecies (Ptilomera sp.), as possible indicators of disturbance effects.Average abundance and average richness, but not total biomass, of all semi‐aquatic bugs were lower in areas with higher habitat disturbance. In particular, average abundance in old‐growth forest was more than two, four, and six times higher than that in logged forest, OPB, and OP, respectively. Average richness in old‐growth forest was higher than in logged forest by two species, but more than two and three times higher than in OPB and OP, respectively.The presence of riparian buffer strips in oil palm had little effect on the abundance and richness of semi‐aquatic bugs. We found no significant differences in the proportion of juveniles, winged adult individuals, or the sex ratio of Ptilomera sp. along the disturbance gradient.In conclusion, oil palm plantations were associated with lower average abundance and richness of semi‐aquatic bugs than forest sites, but community composition did not differ markedly between logged and old‐growth forests. We also found that the forested buffer strips maintained within our oil palm plantation study sites did not protect forest species of semi‐aquatic bugs. Maintaining forest may therefore provide the best option for the conservation of semi‐aquatic bugs, but further studies of the effects of land‐use change and management options are needed across Southeast Asia. [ABSTRACT FROM AUTHOR]

Length–biomass equations are relatively easy and cost‐effective for deriving insect biomass. However, the exact relationship can vary between taxa and geographical regions. Semi‐aquatic bugs are abundant and are indicators of freshwater quality, but there are no studies investigating the effect of habitat disturbance on their biomass, although it is useful in assessing ecological processes. We identified the best‐fit length–biomass models to predict the biomass of semi‐aquatic bugs (Hemiptera, Gerromorpha) collected from streams in Sabah, Malaysia. We used 259 juvenile and adult semi‐aquatic bugs to compare a range of plausible length–biomass functions, and to assess whether relationships differed across the following families and body forms: (1) Cylindrostethinae, Gerrinae, and Ptilomerinae, which are subfamilies within Gerridae consisting of small‐to‐large bugs that have long and slender bodies, (2) Halobatinae, a subfamily within Gerridae, consisting of small‐to‐medium bugs with wide heads and thoraxes as well as short abdomens, and (3) Veliidae, which are small bugs with stout bodies. Estimation used five fitting functions – linear regression, polynomial regression order 2, 3, and 4, and power regression – on the following groupings: three body forms combined; each body form with life stages (juvenile and adult) combined; and each body form with life stages separated. Power regressions were the best fit in predicting the biomass of semi‐aquatic bugs across life stages and body forms, and the predictive power of models was higher when the biomass of different body forms was calculated separately (specifically for Halobatinae and Veliidae). Splitting by life stages did not always result in additional improvement. The equations from this study expand the scope of possible future ecological research on semi‐aquatic bugs, particularly in Southeast Asia, by allowing more studies to consider biomass‐related questions. [ABSTRACT FROM AUTHOR]