Insect Conservation and Diversity (2008) 1, 208–214 doi: 10.1111/j.1752-4598.2008.00026.x © 2008 The Authors 208 Journal compilation © 2008 The Royal Entomological Society Blackwell Publishing Ltd Numerical abundance of invasive ants and monopolisation of exudate-producing resources – a chicken and egg situation THOMAS H. OLIVER, 1 TIM PETTITT, 2 SIMON R. LEATHER 1 and JAMES M. COOK 3 1 Division of Biology, Imperial College London, Silwood Park Campus, Ascot, Berkshire, UK, 2 The Eden Project, Bodelva, Cornwall, UK, 3 School of Biological Sciences, Royal Holloway University of London, Egham, Surrey, UK Abstract. 1. Invasive ants commonly reach abnormally high abundances and have severe impacts on the ecosystems they invade. Current invasion theory recognises that not only negative interactions, such as natural enemy release, but positive interactions, such as facilitation, are important in causing this increased abundance. 2. For invasive ants, facilitation can occur through mutualism with exudate-producing plants and insects. To obtain such partnerships, however, invaders must first displace native ants, whose communities are highly structured around such resources. 3. By manipulating the abundance of an invasive ant relative to a native, we show that a minimum threshold abundance exists for invasive ants to monopolise exudate-producing resources. In addition, we show that behavioural dominance is context dependent and varies with spatial location and numerical abundance. 4. Thus, we suggest a ‘facilitation-threshold’ hypothesis of ant invasion, whereby a minimum abundance of invasive ants is required before facilitation and behavioural dominance can drive abundance rapidly upwards through positive feedback. Key words. Behavioural interference, competition, displacement, ecological dominance, facilitation, Homoptera, numerical abundance, Technomyrmex albipes. Introduction Invasive ants are a common problem worldwide and have serious negative impacts on ecosystems. Often reaching abnormally high abundances and functioning as predators, herbivores, competitors and ecosystem engineers, they disrupt native communities and may cause the loss of endemic biodiversity (for a comprehensive review, see Holway et al., 2002). There are several theories to explain how alien species reach such high densities compared with those in their native ranges. The most widely cited of these are based on negative interactions, such as release from the natural enemies and competitors that limit populations in the native range (e.g. Keane & Crawley, 2002). Positive species interactions, however, such as direct mutualisms and indirect cascade effects, are now also becoming increasingly implicated as important facilitators of biological invasion (Stachowicz, 2001; Bruno et al., 2003). For ants, the importance of positive interactions with sugary exudate-producing plants (e.g. extrafloral nectary bearing Passi- flora) and insects (Lycaenidae and Hemiptera: Sternorrhyncha, previously Homoptera; Carver et al., 1991) for determining community structure cannot be overestimated (Blüthgen et al., 2000, 2004; Wimp & Whitham, 2001). Ecological dominance in ant communities is strongly associated with the ability to exclude competitors and monopolise exudate-producing resources (Fiedler, 2001; Blüthgen et al., 2004). Unsurprisingly, invasive ants are most likely to be able to infiltrate native communities by forging positive interactions with native or co-invading alien Sternorrhyncha (honeydew-producing insects) (Ness & Bronstein, 2004; Abbott & Green, 2007; Lach, 2007). Indeed, invasiveness in ant genera is associated strongly with trophobiosis with Sternor- rhyncha (T. H. Oliver, unpublished results). These sternorrhynchan mutualists probably provide the resources that allow ants to forage widely and achieve high abundances (Stadler & Dixon, 2005). Population growth is further facilitated by reduced intra-specific aggression between invaders and the formation of large unicolonial ‘supercolonies’ (Holway et al., 2002). When abundant, ants can achieve numerical dominance improving both Correspondence: Thomas Oliver, CEH Wallingford, Maclean Building, Benson Lane, Crowmarsh Gifford, Oxfordshire, OX10 8BB, UK. E-mail: toliver@ceh.ac.uk