Removing understory vegetation in oil palm agroforestry reduces ground-foraging ant abundance but not species richness

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.