Effects of remnant primary forests on feeding habits of ants in a secondary forest in Sarawak, Malaysia: An isotopic study

Primary forests are known to have positive effects on the biodiversity of surrounding secondary forests. However, it is unclear whether primary forest remnants affect the feeding habits of consumers that inhabit secondary forests. Here, we examined whether the feeding habits of ants (Formicidae: Hymenoptera) belonging to four taxonomic groups (Camponotus gigas, Odontomachus rixosus, Pachycondyla spp., and Polyrhachis spp.) differed in Macaranga secondary forests with different proportions of surrounding primary forest remnants in Sarawak, Malaysia. In the secondary forests, our previous study showed that species diversity of ants significantly increased as the proportion of surrounding primary forest remnants increased. We explored feeding habits by measuring carbon and nitrogen isotope ratios (δ13C and δ15N). We also estimated the diet age of the ants, which represents the lag time between primary production and utilization by ants, by performing radiocarbon (Δ14C) measurements. δ13C of ants and litter decreased with the increase in primary forest remnants, likely indicating a more closed canopy cover around the sites. Meanwhile, baseline-corrected δ15N of ants, which considered the variation in plant δ15N among study sites, or diet ages of the ants did not show significant response to the proportion of the surrounding primary forest remnants. δ13C, δ15N, and diet ages showed consistent differences among the ant taxa across different proportions of primary forest remnants. These isotopic patterns suggest that, in contrast to the effect on species diversity, surrounding primary forest remnants did not markedly affect the feeding habits of the ants that persisted in the secondary forests.