Ants are known to provide valuable ecosystem services in agricultural landscapes, including oil palm plantations. Their communities are less diverse and more uneven in oil palm compared with forest, and this may increase their vulnerability to disturbance. This study quantifies ant communities in oil palm agroforestry and experimentally tests their robustness to a common-practice high-disturbance management intervention: removing understory vegetation.

Fieldwork was based at the Biodiversity and Ecosystem Function in Tropical Agriculture (BEFTA) Understory Vegetation Project in Sumatra, Indonesia, where three treatments varying in their degree of understory vegetation management were established in 2014: (1) widespread herbicide was applied removing all understory vegetation (Reduced); (2) herbicide was applied to the harvesting paths and circles, and other vegetation was allowed to grow (Normal – control); (3) no herbicide was applied (Enhanced). We measured ground-foraging ant communities before and after the treatments were implemented, using pitfall traps over 324 trap-nights (a trap-night is one trap set for one night). We investigated how ant abundance, species richness, species evenness, beta diversity, and community composition differed between the treatments.

We found 3507 ants across 68 species or morphospecies. Seven of these were highly abundant and accounted for 78% of individuals. Post-treatment ant abundance was lower in the reduced treatment (mean per plot: 84) than in the normal (159) and enhanced (131) treatments, which did not differ from each other. Species richness, species evenness, beta diversity and community composition were not affected by the vegetation treatments.

We recommend that oil palm growers maintain understory vegetation in oil palm plantations to support ground-foraging ants. Though not tested here, this may also improve ant-mediated ecosystem services, such as pest control, seed dispersal, nutrient redistribution, and the maintenance of soil health. This study demonstrates that enhancing habitat complexity through management practices can support biodiversity in monocrop landscapes.

Fieldwork was based in oil palm plantations at the Biodiversity and Ecosystem Function in Tropical Agriculture (BEFTA) Understory Vegetation Project in Sumatra, Indonesia, where three treatments varying in their degree of understory vegetation management were established in February 2014:

1. Reduced complexity (Reduced): Mature first-generation plantation with all understory vegetation removed by spraying herbicide.
2. Normal complexity (Normal): Mature first-generation plantation with understory vegetation removed from the harvesting paths and harvesting circle (a ~1.5m radius area around the base of each palm) using herbicide, and large woody vegetation removed manually. Other vegetation was allowed to grow. This is standard industry practice in these estates.
3. Enhanced complexity (Enhanced): Mature first-generation plantation with the same understory management as the normal complexity, except harvesting paths and circles were cleared by strimming rather than herbicide.

We measured ground-foraging ant communities before (March-April 2013) and after (September 2014) the treatments were implemented. We set three pitfall traps in each plot. Traps were 50 m apart, arranged in an equilateral triangle that was centred in each plot. Each trap was active for three days. This amounted to 108 traps over 324 trap-nights. Traps consisted of circular 20 cm diameter funnels, leading to a pot of 75% alcohol. Worker ants were identified to genus using keys, and to morphospecies or species where possible.

There are one dataset which shows the abundance of each ant morphospecies and their genus. One species (MS.5 Pheidole in dataset or Pheidole sp.1 in manuscript) may be a multispecies complex, as workers varied in size and colouration; there were no clear divisions along which to assign different species, so all individuals in this group were combined into a single morphospecies.