Invasive ants can have substantial and detrimental effects on co-occurring community members, especially other ants. However, the ecological factors that promote both their population growth and their negative influences remain elusive. Opportunistic associations between invasive ants and extrafloral nectary (EFN)-bearing plants are common and may fuel the population expansion and subsequent impacts of invasive ants on native communities. Recent studies have provided support for this hypothesis, showing that when invasive ants feed on carbohydrate-rich, mutualist-derived resources they can have increased colony and population growth, wider foraging ranges, and increased rates of activity and aggression towards co-occurring species. A puzzling question that arises from these dynamics is : ‘why don’t native ants take advantage of these available resources, given the demonstrated benefits to invaders?’ In this study, we examined the responses of a diverse assemblage of native ants and one invasive ant species in the dry tropical forests of Northern Australia to experimental increases of carbohydrate-rich nectar. We then examined the bacterial community housed in the guts of the entire ant assemblage for the presence of Nitrogen-fixing bacterial symbionts. These bacteria can modify dietary nitrogen inputs into a form that is digestible and may therefore free ants from strict nitrogen needs in their diets. Since feeding on carbohydrate-rich diets tends to shift the C:N balance, the presence of these microbes could allow their ant hosts to feed on a more carbohydrate-rich diet, such as one dominated by nectar. We found that only two species (out of ten total species sampled) responded significantly to our pulses of high nectar availability. These two species were also the only two species that harbored >2 N-fixing bacterial symbionts. Additionally, those species that harbored no N-fixing bacteria were not found on plants before or after pulses of nectar. Invasive Anoplolepis gracilipes (Yellow crazy ants) responded more strongly to nectar than any other species and harbored >2 N-fixing bacterial symbionts. These correlative data support the hypothesis that these bacterial symbionts mediate interactions between ants and plants. We are currently developing plans to examine these dynamics experimentally (What happens to these interactions when ants are either ‘cured’ of their bacterial symbionts or fed symbionts? What are the consequences of co-occurring communities? Are invasive species really different?)
