INVASION NOTE Impact of a mutualism between an invasive ant and honeydew-producing insects on a functionally important tree on a tropical island R. Gaigher • M. J. Samways • J. Henwood • K. Jolliffe Received: 22 February 2010 / Accepted: 27 December 2010 / Published online: 19 January 2011 Ó Springer Science+Business Media B.V. 2011 Abstract Mutualisms between invasive ants and honeydew-producing Hemiptera have the potential to result in unusually high population levels of both partners, with subsequent major changes to ecosys- tem composition and dynamics. We assessed the relationship between the invasive ant, Pheidole megacephala, and its hemipteran mutualists, Dysmi- coccus sp. and Pulvinaria urbicola, on Cousine Island, Seychelles. We also assessed the impacts of the mutualism on the condition of the hemipteran host plant, Pisonia grandis, a native and functionally important tree species. There was a strong positive relationship between Ph. megacephala activity and hemipteran abundance, and the exclusion of ants from Pi. grandis resulted in a significant decline in Pu. urbicola abundance. High abundance of the mutualists was strongly associated with damage to the Pi. grandis forest. This indicates that the mutu- alism is contributing to the massive increase in the population levels of the mutualist species, and is intensifying their impacts on the island. The widespread trophobiosis and its associated high densities of mutualists pose serious threats to the ecosystem, highlighting the need to control the ant and associated hemipteran populations. Keywords Pheidole megacephala Á Big-headed ant Á Ant-hemipteran interaction Á Trophobiosis Á Pulvinaria urbicola Á Cousine Island Á Seychelles Introduction Positive interactions between introduced species can be instrumental in their establishment and population growth (Abbott and Green 2007; Simberloff and Von Holle 1999) and can be major determinants of their invasion success (Helms and Vinson 2003). Trophobiosis between ants and honeydew-producing Hemiptera is a prime example of a positive interac- tion that can facilitate the success of the species involved (Helms and Vinson 2003; Holway et al. 2002). These mutualisms involve the protection and sanitation of Hemiptera in return for honeydew, a carbohydrate-rich byproduct of hemipteran metabo- lism, as a food source for the ants (Ho ¨lldobler and Wilson 1990). Studies on invasive ant-hemipteran mutualisms on islands have demonstrated that these positive inter- actions can lead to unusually high densities of the mutualists (Abbott and Green 2007). This can cause serious damage to the native communities (Handler R. Gaigher (&) Á M. J. Samways Department Conservation Ecology and Entomology, Faculty of AgriSciences, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa e-mail: reneg@sun.ac.za J. Henwood Á K. Jolliffe Cousine Island, Mahe ´, Victoria, P.O. Box 977, Seychelles 123 Biol Invasions (2011) 13:1717–1721 DOI 10.1007/s10530-010-9934-1