Attraction of ants by an invasive Acacia

1. Invasive plants are often released from the herbivores of their native range, but may also be deprived of their co-evolved mutualists. In southern Portugal Acacia dealbata has become naturalised in secondary woodland habitats and is apparently not damaged by local herbivores. It possesses inactive extra-floral nectaries (EFNs). 2. Artificial damage to leaves, mimicking herbivore attack, induced extra-floral nectar production on both adult trees and seedlings. This response was restricted to individual leaves rather than systemic. 3. Following EFN activation, trees were tended by the invasive Argentine ant Linepithema humile. Seedlings received a tenfold greater visitation rate from either L. humile or the native ant Plagiolepis pygmaea, which appears to displace the former. Eight days after the damage treatment the ants and nectar had largely gone. 4. There was no indication that either species of ant would defend the plant against potential herbivores. 5. This is the first recorded ant-plant interaction among two invasive species. Given the lack of natural herbivore damage and the absence of suitable ants in this novel community, nectar induction by A. dealbata is likely to be a dysfunctional response to damage in its invasive range, with little possibility of developing into a mutualism.