Effects of biotic resistance and resource availability on the invasion success of the Argentine ant, Linepithema humile (Mayr), in the Cape Floristic Region, South Africa.
Mothapo, Natasha Palesa
Wossler, Theresa Clair
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The invasive Argentine ant, Linepithema humile, is widespread and has been introduced into the Cape Floristic Region (CFR) of South Africa. It has successfully established and spread into both urban and natural environments. Even with its potential negative effects on the CFR, a biodiversity hotspot, very few studies have focused on this ant in South Africa. Even less is known about the indigenous ants to the CFR highlighting the paucity in our knowledge of resident ant community structure and the threat of L. humile on our native ants and ultimately the CFR. In the Fynbos biome, L. humile occupies distributions mutually exclusive to those of many of the dominant native ants, as well as to Pheidole megacephala which occupies the eastern escarpment of the country. We investigated resource exploitation: i) under controlled laboratory conditions, ii) floral nectar utilisation in the field and iii) diet switching in response to levels of L. humile invasion, as well as interspecific interactions between resident ants and L. humile. We used laboratory bioassays to ascertain whether resident ants posed any biotic resistance to the spread of L. humile. Fynbos ants were not competitive towards L. humile despite equalised colony sizes, suggesting no biotic resistance from this community. Linepithema humile was able to recruit far more workers than three of the resident Fynbos native ants studied and interfered with their recruitment through aggressive behaviour. If this ineffectual competition from native Fynbos ants under these laboratory conditions is extrapolated to field conditions, it may be one factor currently contributing to the successful invasion of the Fynbos by L. humile. On a more positive note, P. megacephala showed competitive superiority and L. humile suffered huge mortality rates, implying that this resident ant species may actually be offering biotic resistance to L. humile. The abundance of floral nectar in the Fynbos increases during winter and so we measured the foraging activity of the native dominant ant Anoplolepis custodiens and L. humile on nectar producing proteacea species as well as nest density around the flowering plants. In addition, the ground foraging activity of ants in the study plots and floral composition of these protea plants were assessed. Elemental stable-isotope analysis of δ13C and δ15N and C:N ratios, which are the contribution of carbohydrates and protein to the diet, was used to study the foraging ecology of L. humile and some of Fynbos native ant species along an invasion continuum. Linepithema humile effectively exploited Fynbos floral resources, showed diet flexibility by feeding on carbohydrate resources in winter but also supplemented their diet with protein, likely from predation or scavenging on native arthropods. Linepithema humile altered the diets of some native ant species and also changed species assemblages both on the ground and in the Proteacea inflorescences. Linepithema humile responded more efficiently to fluctuating resources provided by floral nectar than native Fynbos ants and outcompeted resident ants through aggression when competing for a shared resource. This aggression of L. humile, together with their ability to monopolise fluctuating carbohydrate resources promotes ecological dominance and invasion success of this ant species, especially in areas with nectar producing Proteacea species.