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

Thesis (PhD)--Stellenbosch University, 2013.

ENGLISH ABSTRACT: 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.

AFRIKAANSE OPSOMMING: Die Argentynse indringermier, Linepithema humile, is wydverspreid en is onder andere na die Kaapse Blommeryk (KBR) van Suid-Afrika gebring. Die miere het hulself suksesvol gevestig en het versprei na beide stads- en natuurlike omgewings. Selfs met die potensiële negatiewe effekte op die KBR, ‘n biodiversiteitskern, het baie min studies sovêr op hierdie mier in Suid-Afrika gefokus. Selfs minder is bekend oor die inheemse miere in die KBR, wat dui op die gebrek aan kennis van die oorspronklike miergemeenskapstuktuur en die bedreiging wat L. humile inhou vir ons inheemse miere en uiteindelik ook vir die KBR. In die Fynbosbioom, beset L. humile verspreidings wat wedersyds uitsluitlik teenoor die verspreiding is van baie van die dominante inheemse mierspesies, asook Pheidole megacephala wat die oostelike eskarp van die land beset. Ons het hulpbronontginning ondersoek: i) onder beheerde laboraturimtoetstande, ii) blomnektargebruik in die veld en iii) dieetveranderinge as ‘n reaksie op die vlakke van L. humile inval, en ook die interspesie wisselwerking tussen die inheemse miere en L. humile. Ons het laboraturiumbiotoetse gebruik om vas te stel of inheemse miere enige biotiese teenstand bied teen die verspreiding van L. humile. Fynbosmiere het nie met L. humile gekompeteer nie ten spyte van gelykgemaakte koloniegroottes, wat geen biotiese weerstand deur die gemeenskap aandui nie. Linepithema humile was in staat om veel meer werkers te werf as drie van die inheemse Fynbosmierspesies wat bestudeer is, en het ingemeng met hulle werwing deur aggresiewe gedrag. As hierdie oneffektiewe kompetisie van die inheemse Fynbosmiere onder laboratoriumtoestande ge-ektrapoleer word na veldtoestande, sal dit moonlik ’n faktor wees wat bydra tot die suksesvolle inval van die Fynbos deur L. humile. Op ‘n meer positiewe noot, P. megacephala het superioriteit teenoor L. humile getoon in kompetisie, en die mortaliteitssyfers van L. humile was enorm , wat impliseer dat hierdie inheemse mierspesie tog teen L. humile biotiese weerstand bied. Die Fynbosblomnektar vermeerder in die winter, daarom het ons die kos-soek aktiwiteit van die inheemse dominante mier Anoplolepis custodiens en L. humile op nektarprodiserende Proteaceae spesies, asook nesdigtheid om die blomplante ondersoek. Ons het ook die grond kos-soek aktiwiteit van miere in die studieplotte en blomsamestelling van proteaplante geassesseer.

Doctoral