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University of KwaZulu-Natal (1997)

Vegetation and ant dynamics in the southern Karoo

Adie, Hylton Ralph

Titre : Vegetation and ant dynamics in the southern Karoo.

Auteur : Adie, Hylton Ralph.

Université de soutenance : University of KwaZulu-Natal

Grade : Doctor of Philosophy (PhD) 1997

Résumé partiel
The aim of this thesis was to describe the structure and dynamics of ant and plant communities in the southern Karoo and to assess mechanisms of species coexistence in ant and plant communities. The role of species interactions in structuring natural communities was emphasised. Diversity indices were used to determine the importance of habitat in maintaining ant species diversity. Ant species diversity was not predicted by measurements of plant species diversity or vegetation structural diversity. Ant species richness was correlated with vegetation structural diversity but not with plant species diversity. Ant species appeared to respond to aspects of vegetation height. Although vegetation complexity influences ant species richness, competitive effects, particularly of dominant ant species, appear to suppress sub-ordinate ant species influencing measures of richness and diversity. Aggressive dominant ant species determine the distribution and abundance of sub-ordinate ant species. Interference competition for space was prevalent between dominant ant species and competitive success was a function of vegetation. It was not clear whether ants respond directly to physical conditions created by vegetation which then influences foraging activity or, alternatively, whether ants respond to productivity gradients which are affected by vegetation. Understanding vegetation dynamics is critical to interpreting patterns of ant species distribution and abundance. A patchy habitat disrupts the competitive dominance of aggressive dominant species, removing the potential towards habitat monopolisation, and therefore maintaining ant species diversity. In the Portulacaria afra rangeland, Pheidole sp. 2 was superior in well-shaded microhabitats but Messor capensis nested successfully under woody shrubs and several ant species (Tetramorium peringueyi, T. quadrispinosum, Monomorium alamarum, Ocymyrmex barbiger) persisted on bare nutrient-rich patches. Rangeland dominated by grass would favour Pheidole sp. 2 at the expense of other ant species which would be unable to establish successfully. There is no evidence supporting the notion that ant communities are at equilibrium. Rather, dominance hierarchies lead to the replacement of species over vegetation gradients with the tendency towards the aggressive acquisition and monopolisation of space. The coexistence of dominant ant species at study sites in the southern Karoo was a combination of territorial strategy and colonisation ability. In the P. afra rangeland most ant species escaped the severe competitive effect of Pheidole sp. 2 by persisting as fugitives on bare areas of local disturbance where Pheidole sp. 2 was less successful. At Tierberg, competitively inferior ant species with a decentralised territorial system coexisted with competitively superior ant species in an unstable equilibrium by pre-empting newly available space through the lateral expansion of territories.

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