PS 76-168
Assessing the importance of “unwanted” mutualisms between invasive ants and invasive honeydew-producing insects in native forests of the Northern Mariana Islands
Ants represent one of the most ecologically damaging groups of invasive species largely because of their strong negative impacts on native arthropod diversity, disruption of established mutualisms, and alteration of trophic structure. Oceanic islands may be particularly susceptible to the deleterious effects of ant invasions, in part because their native biotas often evolved in the absence of ants. One factor increasingly implicated in the success of ant invasions is the formation of facultative food-for-protection mutualisms with honeydew-producing insects (HPIs). These highly generalized interactions can strongly impact native plant communities when ants and HPIs reach high densities. Preliminary observations in the native limestone forests on the Pacific island of Guam suggest that the invasive white-footed ant (Technomyrmex spp.) has reached densities comparable to those reported in other island ant invasions, perhaps as a result of its association with invasive HPIs. To further explore these observations, 208 trees representing 13 common limestone forest species on the islands of Guam, Saipan, and Rota had branches randomly clipped from their canopies. The presence, abundance, and identity of ants and HPIs were noted on these branches, and ants nesting or foraging on the trunks of trees were also recorded.
Results/Conclusions
Trees in Guam were much more likely to have ants present in the canopy or on the trunk, with 94.3% (100/106) of sampled trees on Guam containing invasive ants compared to 79.1% (53/67) on Saipan and 57.1% (20/35) on Rota. The same pattern was observed for HPIs: 47.2% (50/106) of sampled branches on Guam had honeydew-producing aphids, scale insects, or mealybugs, compared to 14.9% (10/67) on Saipan and 25.7% (9/35) on Rota. Guam, Saipan, and Rota all had similar invasive ant species richness, though Technomyrmex spp. was present and at high densities only on Guam. Follow-up research is needed to further elucidate whether the elevated densities of invasive ants and HPIs on Guam are directly related, though it is clear that Technomyrmex spp.—the numerically dominant ant on Guam—actively tends HPIs. Certain tree species were more vulnerable to infestation by HPIs, most notably the fast-growing Macaranga thompsonii. These results are valuable because they contribute to our understanding of the factors influencing ant invasion success and the impacts of ant invasions on plant communities via interactions with HPIs. They also add to a growing body of evidence related to the ecological consequences of Technomyrmex spp. introductions.