Use of ground-dwelling arthropods as bioindicators of ecological condition in grassland and forest vegetation at eThekwini Municipality in KwaZulu-Natal.

Abstract

Epigaeic arthropods are among the most diverse and abundant group of animals. They are important in the functioning of terrestrial ecosystems. Assemblages of arthropods may be affected by vegetation type, seasonality and disturbances such as alien plant invasion. The aim of this study was to develop a bioindicator tool for monitoring ecological conditions of the KwaZulu-Natal Sandstone Sourveld (KZNSS), which is one of the most threatened grasslands in KwaZulu-Natal. The objectives of this study were to 1) determine species abundance and species richness of ground-dwelling arthropods; 2) determine the effect of season on ground-dwelling arthropods; 3) determine functional diversity of ground-dwelling arthropods; and 4) determine if ground-dwelling arthropods distribution differs in three vegetation types in Tanglewood and Giba Gorge nature reserves in the KwaZulu-Natal Sandstone Sourveld. Ground-dwelling arthropods were sampled during the wet and dry seasons at two sites in each of the nature reserves. At each site, pitfall trap sampling was carried out in three vegetation types, namely intact grassland, riverine or scarp forest and a disturbed grassland vegetation. The disturbed grassland vegetation was an ecotone between grassland and forest dominated by alien invasive plants. Ants, beetles, spiders, terrestrial crustaceans, sand crickets, roaches, termites, millipedes, lacewigs, hemipterans and woodlice were the sampled taxa in both reserves. A total of 6 150 specimens belonging to 60 morpho-species were sampled in Giba Gorge and Tanglewood Nature Reserve. Species abundance and richness varied among reserves as greater epigaeic arthropod abundance was observed in Tanglewood than in Giba Gorge Nature Reserve. However, Giba Gorge was the richer reserve than Tanglewood. Distribution of epigaeic arthropods varied among vegetation types, higher arthropod abundance was recorded in the forest than the disturbed and intact grassland. While higher species richness of epigaeic arthropods was recorded in an intact grassland. Ants were the most abundant and richest taxon at both sites and greater abundance of ants was recorded in the disturbed grassland which shows that ants are opportunistic organisms. Seasonality played an important role in epigaeic arthropod species abundance and richness. Higher abundance and richness was observed in the wet season. This shows that the wet season provides optimum food resources, temperature, soil moisture and diverse vegetation structure which is favourable to epigaeic arthropods. Functional diversity varied across vegetation types. Forest supported a greater abundance of decomposers and predators while the disturbed grassland supported greater abundance of generalists and herbivores. This shows that functional guilds have different food resources and niche requirements and vegetation type plays an important role in functional diversity. Generalist arthropods were more abundant in the dry season unlike the predators, herbivores and decomposers which were more abundant in the wet season. Generalists have broad diets and are able to survive under unfavourable conditions. A terrestrial crustacean, Talitriator africana occurred in all vegetation types but was more abundant in forest. An ant, Pheidole sp.02 (megacephala gp.) was the most widespread species. However, it was more abundant in disturbed grassland. These results suggest that T. africana and Pheidole sp.02 (megacephala gp.) could be useful potential indicators for monitoring ecological conditions in the KwaZulu-Natal Sandstone Sourveld. Forests and grasslands should be conserved because they supported higher numbers of epigaeic arthropods and functional guilds. Arthropods play an important role in the functioning of terrestrial ecosystems. Disturbed grassland should be managed and restored back to a grassland because grasslands play an important role in the functioning of the ecosystem by providing direct and indirect ecosystem services