Background Normally, Crematogaster ant symbionts protect their Macaranga tree hosts from herbivorous insects, and in return are provided with nutrition in the form of food bodies, as well as nesting space inside the empty or easily hollowed-out stems. This system has been extensively studied as a model of ant-plant mutualistic interactions. Here, we report a novel finding of an exploiter species, the cavity-nesting wasp Dasyproctus agilis (F. Smith, 1858), which hollows out Macaranga stems, and uses the resulting space to provision its larvae with adult Diptera. We predicted that the Dasyproctus wasps, which are commonly found in highly disturbed habitats elsewhere, would be more common in Macaranga stems in an agricultural habitat (oil palm plantation), than in heavily logged forest. We also predict that because wasps nest in the same plant structures as the mutualistic ants, plants with greater numbers of wasps should have smaller ant colonies (although note that ants might also exclude wasps). Any such reduction in ant colony size could then reduce the ability of the mutualistic ants to protect plants from herbivorous insects. Methods We dissected 213 Macaranga pearsonii trees of all ants and wasps across the Malaysian state of Sabah to explore the extent of the phenomenon. To test our predictions, we then carried out further plot-based sampling of M. pearsonii plants in heavily logged forest and oil palm plantations in Sabah, Malaysian Borneo, and recorded the frequency of wasp colonisation. Next, we conducted plant-based sampling in oil palm plantations to explore the relationship between wasp abundance and Crematogaster ant colony size, and potential consequences for herbivory rates on M. pearsonii plants. Results We found that the wasps exploit the cavities of multiple Macaranga species in disturbed habitats across a wide geographical area. Wasp colonisation of M. pearsonii was more frequent in oil palm plantations than in heavily logged forests. We also found that plants colonised by larger numbers of wasps had smaller protective ant colonies, although causal direction remained unclear. There was no evidence for reduced ant protection of plants from herbivores in conjunction with wasp colonisation. Taken together, our results indicate that anthropogenic habitat change can result in colonisation by an exploitative species that potentially disrupts mutualistic interactions through pre-emption of the resources traded by mutualistic partners.
Leština et al. (Wed,) studied this question.