The broader ecological effects of aphid tending by the Argentine ant

Background/Question/Methods

Studies of mutualistic interactions generally focus on each member within the partnership but often do not consider the broader ecological consequences of mutualisms. These ecological effects can be important; mutualisms can influence the structure of food webs. Previous studies of ant-hemipteran mutualisms have shown that ants typically increase the reproductive success of hemipteran partners and affect the diversity of predators that consume hemipterans. Some studies also noted that fewer leaf-chewing herbivores occur on plants with mutualistic ants. This reduction in herbivory is an effect of ant presence and can indirectly benefit plant reproduction. When introduced ant species invade new areas, they often eliminate other ant species and take over mutualistic roles vacated by native ants. Currently, it is unclear if introduced ants can offer the same indirect plant benefits that native ants once provided. To test whether the invasive Argentine ant (Linepithema humile) provides similar indirect plant benefits, we planted 80 cotton seedlings (Gossypium hirsutum) and either inoculated plants with cotton aphids (Aphis gossypii) or removed aphids on a weekly basis. We conducted surveys of the arthropod community on plants once per week for 21 weeks. At the end of the season we harvested and quantified plant biomass and reproduction.

Results/Conclusions

In several respects, the Argentine ant altered arthropod food webs in ways consistent with previous studies of ant-aphid interactions. First, the Argentine ant assiduously tended cotton aphids on cotton plants and ant abundance on plants was significantly correlated with aphid abundance. Although generalist predator densities overall were not affected by Argentine ants, aphid specialist consumers significantly increased. However, Lepidoptera were not deterred by the presence of the Argentine ant and were found equally between treatments. Despite the absence of herbivore deterrence, reproductive fitness (measured by seed weight) was greater for plants with the mutualism present. In this system, the reproductive benefit plants received from hosting aphids accrues because of ant behavior toward pollinators. In the absence of aphids, ants dissuade pollinators from visiting flowers; when aphids are present, ants are distracted from flowers and collect honeydew. This study provides evidence of indirect plant benefits of hosting aphids by a mechanism that may mimic the role of extrafloral nectar in other plants.