Assemblages of surface-active arthropods in pristine and disturbed savannah.

Abstract

Savannahs are structurally complex ecosystems rich in biodiversity, and the conversion of savannahs into human-modified landscapes poses a significant risk to organisms with limited dispersal abilities. Surface-active arthropods have limited dispersal abilities and are sensitive to changes in their environment; as a result, they are restricted to specific habitats with the resources that they need. Surface-active arthropods are essential in ecosystem processes, such as playing a direct or indirect role in water infiltration into the soil and its storage, decomposition of plant and animal matter, nutrient cycling, humification and biological pest control. Many landscapes of natural vegetation in South Africa have been transformed into croplands, residential and commercial areas. In Mpumalanga province, croplands (including fruit plantations) dominate the landscape where savannah vegetation occurred previously. This study investigated the influences of disturbance (transformed savannah vs pristine savannah) and season on the diversity of four groups of surface-active arthropods (ants, beetles, millipedes and spiders) in the savannah biome in Mpumalanga province. The objectives of the study were to determine 1) whether flightless arthropod assemblages differ between disturbed and pristine savannah, and 2) whether functional guilds of flightless arthropod assemblages differ between summer and winter. Field sampling using pitfall traps was done to collect the surface-active arthropods in pristine and disturbed savannah during summer and winter. All samples were sorted into morphospecies and identified into family or genus where possible and then assigned into functional guilds. Functional guilds were allocated based on the feeding habits of the surface-active arthropods, resulting in three distinct functional guilds (detritivores, herbivores and carnivores) being identified. Surface-active arthropods with diverse feeding habits were lumped to form a fourth functional guild, the “diverse functional guild”. I found significant differences in the species composition of ants, beetles and spiders between disturbed and pristine savannahs. The species richness and abundance of ants was significantly greater in the disturbed than pristine savannah, while beetles and spiders had similar species richness and abundance between the two habitat types. The number of unique species of the surface-active arthropods in the disturbed savannah were two times more than those in the pristine savannah. The species richness and composition of all functional guilds of the surface-active arthropods differed between summer and winter, with greater species richness in summer than in winter. Herbivorous and carnivorous arthropods had significantly higher abundance in summer than in winter, while the abundance of detritivores and the “diverse functional guild” was similar between the two seasons. This study shows that different vegetation types support different compositions of surface-active arthropods and that there may be a positive relationship between vegetation structure and the assemblages of surface-active arthropods in the savannah. Furthermore, the study highlights a potentially positive relationship between the assemblages of herbivorous and carnivorous arthropods. I recommend that it may be essential to consider both disturbed and pristine habitat types in the conservation of surface-active arthropods, especially in South Africa where most of the land is outside of formal conservation areas. This assertion is supported by the greater number of unique species of surface-active arthropods found in the disturbed than in the pristine savannah. My study provides relevant baseline information because little is known about the assemblages of surface-active arthropods in savannah landscapes dominated by croplands. Furthermore, my study gives some insight on how the assemblages of surface-active arthropods may respond to disturbance in the savannah.