Arthropod and plant diversity in maize agro-ecosystems of South Africa

Abstract

Agricultural intensification in the twentieth century has led to rapid biodiversity decreases on farmland. In sub-Saharan Africa, where rapid population increases and high direct dependence on natural resources coincide, biodiversity loss due to land-use change is of particular concern. Stock-grazing and dryland crop agriculture are two prominent and growing land-uses in the Grassland and Savanna Biomes of South Africa. Maize (Zea mays L.) represents the most important grain crop, with an approximate annual production of 128 million tons of maize grain on approximately 31 million hectares of land. Understanding what effect farmland management regimes have on the complexity and interactions of biota in remnant semi-natural ecosystems is a necessary step towards a sustainable future for biodiversity in agro-ecosystems. However, there has not been a considerable effort to understand the effects of these agricultural disturbances on species, structural, or functional diversity in South Africa’s grassy biomes. The research project described in this thesis aimed to address the knowledge gap regarding biodiversity of maize agro-ecosystems in the Grassland and Savanna Biomes of South Africa by providing insight into the observational patterns of taxonomic and functional diversity, compositional structure and diversity relationships of two major groups of biota (vascular plants and plant-associated arthropods) in relation to an agricultural disturbance gradient at regional and local scales. Surveys were conducted in six provinces of South Africa, namely North-West, Mpumalanga, KwaZulu-Natal, Limpopo, Free State and the Eastern Cape. The transformation of semi-natural grassland and savanna into maize fields resulted in severely decreased species diversity, functional diversity and abundance as well as marked changes in species composition of plants and arthropods. However, there was no evidence for reduced levels of species diversity, functional diversity or trait abundance of plants and arthropods at medium disturbance intensity marginal vegetation (30-100 m from the maize field edges) compared to low-disturbance intensity rangelands. The pattern was consistent across the Grassland and Savanna Biomes. This suggests that the possible disturbance effects of maize fields do not have considerable negative effects on either the diversity or species assemblages of plant and arthropod communities at ≥30 m from the area of active cultivation. Uncultivated semi-natural vegetation of the Grassland and Savanna Biomes had distinct arthropod assemblages although these distinctions were better explained by geographical position than by plant features such as tree and grass cover. There was also evidence for positive relationships between low-growing (>2m) plant species and arthropod richness, diversity and abundance in maize fields and in uncultivated vegetation. The patterns recorded in this study suggest that crop field margins ≥ 30 m from the site of active cultivation are valuable conservation sites for the continued persistence of beneficial species and functional diversity of non-crop plants and arthropods within the agricultural environment