Numerical abundance of invasive ants and monopolisation of exudate-producing resources - a chicken and egg situation

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. 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. 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. 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.