Interactions between habitat disturbances and complexity: the effect on ant communities

Abstract

Hluhluwe-iMfolozi Park is home to a number of indigenous and endemic species. As its mandate is to protect and preserve biodiversity, any factors that may result in the loss of species should be monitored carefully. This study aims to look at two such factors; the alien invasive plant Chromolaena odorata and intensive grazing by large mammalian ungulates (resulting in grazing lawns). Disturbances such as these typically result in the modification in the structural complexity of the habitat which in turn affects its associated fauna. This study compared the structural change of these two disturbances and their adjacent uninvaded habitats and bunch grass habitats respectively. By using a set of manipulated and mensurative (natural) treatments the change in habitat structural complexity was then related to ant diversity, assemblage formation, competitive interactions, resource acquisition, resource monopolisation and ant body size. Both disturbances revealed that structural complexity within the natural experiments did not significantly alter ant assemblage composition, diversity or competitive interactions. However, at microhabitat scale, as was exhibited by manipulated treatments, structural complexity did appear to play a role in the organisation of ant communities, their competitive interactions and body size. The baited traps which were less complex in structure tended to support the larger species of ants which acquired resources at a faster rate and in higher numbers. Assemblage composition of ants and the diversity of ants were not significantly altered by C. odorata. This could suggest that ants respond to structural complexity rather than plant species richness. Therefore it would be wise to employ the use of other biological indicators such as spiders to test the effect of alien invasive plants on ecosystems. On the other hand, as a result of heavy grazing, a unique assemblage composition of ants was found on grazing lawns when compared with their adjacent bunch grass sites. This would suggest that grazing is a vital component of the savanna system as it adds heterogeneity to the system, resulting in a biodiversity increase.