Community dynamics and structure of ant populations on ash disposal sites under rehabilitation at hendrina power Station, South Africa

Abstract

Ants can be considered as useful indicators of ecosystem recovery following the rehabilitation of ash dams, associated with power stations. However, few studies were conducted on the indicative value of ant species in disturbed industrialized areas in South Africa. This study at Hendrina Power Station (Mpumalanga, South Africa) reports on a preliminary investigation of the indicative value of ant species, changes and trends in the composition and diversity of the ant community during succession and also the habitat characteristics involved in the successional trends in the structure of the ant community on ash dams. The study was conducted over a period of three years. Pitfall trapping, sweepnetting and hand collections were used as sampling techniques. Ant fauna at Hendrina Power Station were highly diverse, with 34 species recorded on the ash dams and 38 species recorded in natural grassland. Two successional stages were discernible on the ash dams - a pioneer stage represented by 0-3 year old ash dam grids and a second older successional stage represented by 4-9 year old rehabilitated ash dam grids. The third and most advanced successional stage was represented by the natural grassland. The pioneer stage of succession was characterized by the presence of pioneer plant species, such as Tagetes minuta and Cyperus esculentus, a low vegetation cover, a low organic matter content and a high gravel content within the soil. These areas had a low ant species diversity and was dominated by only a few opportunistic, early colonizing ant species, such as Cardiocondyla shuckardi, Monomorium albopilosum and Monomorium sp.2. The second stage of succession was characterized by the presence of stable plant communities such as Hyparrhenia hirta and Eragrostis curvula, a high vegetation cover, a high organic matter content and high sand content. These areas had a higher ant species diversity than the pioneer stage of succession, which were dominated by the pugnacious ant, Anoplolepis custodiens and other generalized ant species. Although ant species diversity on the 4-9 year old rehabilitated ash dam grids corresponds with that of the natural grassland and were also dominated by A. custodiens, more -vii- specialized ant species, such as Camponotus spp., Cerapachys sp., Crematogaster sp., Leptogenys intermedia etc. were caught within the natural grassland, where the diversity of the habitat is more diverse, than on the ash dams. Ant species richness as a measure of ant recolonization or resilience was significantly positively correlated with plant species diversity, percentage vegetation cover and sand content of the substrate and seemed to be enhanced by these environmental variables. Various ant species were indicative of various soil and vegetation factors, associated with different stages of succession, and could therefore be considered as useful parameters of rehabilitation success. Ant species diversity varied significantly on a seasonal basis, being significantly lower in winter, compared with spring, summer and autumn. Ant species showed particular seasonal activities, where the majority of ant species were active during warmer months. However some ant species were active in winter, creating spatial and temporal ant diversity on the ash dams and confirmed the importance of functional diversity in ecosystems. These seasonal activities are typical for ants and reflected the effects of weather and resource availability on the activity of ant species. Lastly it can be concluded that a diverse ant community had established on the ash dams over the short term, but the absence of specialized ant species on the ash dams emphasized the need for additional management strategies, in order to meet not only the demands of these ant species on ash dams, but also the demands of sustainability in the long term. These management strategies and other recommendations are briefly