Complex interactions between populations of native ants, non-native ants, and Japanese knotweed

Background/Question/Methods In the northeastern United States, Japanese knotweed (Fallopia japonica) is an introduced species commonly found along riparian zones. Like other plants with extrafloral nectaries, Japanese knotweed attracts ant guards that may defend the plant from herbivores. Located within the broader distribution of Japanese knotweed, the introduced European fire ant (Myrmica rubra) is found along coastal areas of Maine through Massachusetts, and inland within the Hoosic-Hudson watershed. Using data collected during the summers of 2005-2007 from the Hoosic-Hudson watershed, we evaluated the potential effect of mutualism between Japanese knotweed and ants for invasion patterns of both species.

Results/Conclusions Our results show a significant correlation between Japanese knotweed and M. rubra at both local (log-odds ratio = 2.8, P = 0.026) and watershed (χ²_df=1 = 11.35, P < 0.001) scales. Consistent with the results from a separate knotweed analysis, M. rubra was more commonly found in sinuous stretches of the Hoosic (z = 2.038, P = 0.0415). This result suggests that the correlation between these species is driven, at least in part, by corresponding habitat requirements. We found little evidence for a protective function of ants under natural conditions, consistent with the relatively low herbivory rates of Japanesese knotweed. Interestingly, preliminary results suggest that native ants, but not M. rubra are induced by simulated herbivory. A significant decline in native-ant diversity with increasing M. rubra density or presence (Poisson regression, z = -3.07, P = 0.002) therefore suggests the potential for complex dynamics between native ants, M. rubra, and Japanese knotweed.