The humble yet industrious dung beetles of the Karoo are important little creatures worthy of big scientific research.
So says Lavhelesani (Lavhe) Simba, who will be receiving her PhD degree at Stellenbosch University’s December graduation ceremony this week.
Simba, a former student in the Faculty of AgriScience’s Department of Conservation Ecology and Entomology, tracked the Karoo’s dung beetles during her study. Her dissertation is titled The relative importance of species richness and functional diversity for ecosystem functions performed by dung beetles (Coleoptera: Scarabaeidae) in South Africa.
Simba has been fascinated by insects from a very young age. While growing up in a rural village in Limpopo, Simba and her siblings would often go searching for locusts and termites to eat. Later, as a student at the University of Venda, her interest in entomology deepened.
The opportunity to study the Karoo’s scarabs came along when the BioGaps project was initiated by the South African Biodiversity Institute to fill gaps in the scientific understanding of the Karoo’s biodiversity.
As a result, Simba’s PhD study focused on dung beetles living on six livestock farms and in five protected areas, all located in the Nama and Succulent Karoo biomes. These protected areas included the Karoo, Camdeboo and Tankwa Karoo National Parks; Anysberg Nature Reserve; and Tierberg, managed by the South African Environmental Observation Network. Together, her 11 study sites stretched over an area of approximately 500 km2 across three provinces.
The study’s aim was to determine the effect of changes in land use and rainfall on dung beetle diversity in the region. More specifically, Simba investigated the factors that influence dung beetle abundance, species richness and functional diversity within protected areas and rangelands. The specific influential factors she chose to investigate were rainfall, mammal stocking rates, soil texture and vegetation cover.
The value of the humble dung beetle
Dung beetles play a crucial role in land-based ecosystems, Simba points out. “While using and burying dung as a food and nesting source, they perform a variety of ecosystem services,” Simba says. Ultimately, both nature and humans benefit from these services.
“Dung beetles fertilise the soil, clean pastures, contribute to nutrient cycling and soil quality, and provide pollination services,” Simba explains. “Dung removal also controls flies and livestock-internal parasites, thereby providing pest control.
“Moreover, nutrient cycling and the fact that dung beetles rework the soil have a positive influence on soil fertility, thereby increasing plant productivity.”
These insects are also the ideal subject for biological study in an evolving world. “They are indicators of ecological change in the region,” Simba says. “Dung beetles are sensitive to changes in rainfall patterns and land use practices.”
How was it done?
This study is one of the first to assess dung beetle communities along a rainfall gradient, and is also novel in assessing the role played by land use.
Simba conducted her research across a rainfall gradient in the winter and summer rainfall regions of the semi-arid Nama and Succulent Karoo, where the extent of biodiversity is still largely unknown.
The Karoo region is characterised by summer rainfall in the east and winter rainfall in the west. Mean annual precipitation ranges from 500 mm in the east to well below 100 mm in the northwest.
Simba identified 35 different dung beetle species in the study region, these species ranging in colour and size. Some are the size of a R5 coin, and others the size of tiny beads (ranging from 2 mm to 22 mm in length).
She used both soil pitfall traps with cow dung balls in the centre of a buried plastic bucket, and water pitfall traps with a wire harp holding dung bait suspended above a buried bucket. The use of two methods was motivated by the desire to study dung beetles with different nesting behaviours. Simba collected almost 9 090 dung beetles during her study.
The insects were classified into one of three groups according to their behavioural traits: rollers (species that make a dung ball, roll it and bury it at a distance from the dung pile), tunnellers (who bury the dung directly under the dung pile) or dwellers (who feed and nest on the dung pile). These specific behavioural traits influence how the different species participate in ecosystem functions like dung removal.
Simba also identified 15 of the species as ‘indicator species,’ which were associated with different environmental variables. An indicator species serves as a measure of the environmental conditions in a given region.
What the land revealed
Protected areas (with more dung-producing mammalian species) and farms (with fewer such species) did not differ in terms of dung beetle presence or absence, species richness or the body length of the dung beetle species found. Nevertheless, dung beetle abundance was found to be greater in protected areas than on farms.
This finding is surprising as there was a positive correlation between dung beetle abundance and increased stocking rates on the studied farms. Simba explains, “This may be because there were dung types that are more appropriate for breeding available in the protected areas (as opposed to the farms), including larger dung pats than just the pelleted dung of sheep.
“This result suggests that the species in these protected areas were more similar to each other [in other words, the species richness was lower], dividing up niches more finely, with greater competition between species,” Simba says.
Surprisingly, the greater abundance of dung beetles in the protected areas also did not result in greater functional diversity — in other words, greater diversity in those components that influence how an ecosystem operates or functions. In fact, functional diversity was significantly lower in the protected areas than on the farms.
Dung beetle species richness and abundance both increased significantly with an increase in, respectively, vegetation cover, mammalian stocking rates and rainfall.
Dung beetle species richness was driven by rainfall and the availability of dung, and less so by dung diversity, Simba points out. “Land use did not affect dung beetle species richness, probably because other factors, like rainfall, are far more important in this region,” she says.
In this study, short-term rainfall (over the five days before the sampling date) was a better predictor of species richness than medium-term rainfall (over 5 years) or long-term rainfall (over 30 years). Species abundance was, however, best explained by long-term rainfall.
What the future holds for dung beetles
“Future climate projections indicate extreme changes in rainfall in this area; my results here suggest that this would have dramatic impacts on dung beetle communities,” Simba says.
Many dung beetles stay dormant in the soil during the dry season when conditions are unfavourable. This study suggests that some species cannot tolerate very dry conditions and might have retreated further away from the dry areas during periods of drought.
Simba also investigated whether certain functional groups of dung beetle are more efficient than others at performing a given ecosystem function. It turns out that rollers are potentially more efficient at removing dung and tunnellers, in turn, are more efficient at increasing plant productivity. This still needs further investigation, she says.
“This demonstrates that the different functional groups make different contributions to ecosystem functions in the region,” Simba explains. “The Karoo needs both roller and tunneller beetles to deliver ecosystem benefits.”
Overall, the findings emphasise the importance of assessing all community attributes when determining how dung beetle communities are affected by specific changes, Simba says.
“The study shows just how important rainfall is in an arid area, not only for plants and vertebrates, but also for the little guys in the ground that you rarely see.
“Efforts to protect dung beetles will support a functioning ecosystem in this region.”