Opencast mines in South Africa are usually rehabilitated using soil that was stripped from the land prior to mining and stockpiled until it is needed for rehabilitation purposes. The soil is stockpiled for long periods during which essential nutrients required for productive plant growth are lost. When the soil is returned for rehabilitation purposes, heavy vehicles transporting and placing the soil increase the soil compaction to such a degree that water runs off the surface at high velocities, leading to erosion, and, eventually, land degradation. Plant growth is consequently limited due to acidic and impermeable soils with very low fertility, creating an inhospitable environment for the establishment of any biotic community, especially fauna that rely on the presence of vegetation.
Some animals can survive in these limiting environments as long as a food source and appropriate climatic conditions are present. Dung beetles mainly rely on the presence of dung to complete their lifecycles, flying as far as 850 meters (although it is suspected to be even further) to the dung source. As low-intensity grazing is a common post-closure land use for mines in developing countries such as South Africa, dung beetles will occur simultaneously with the presence of cattle.
Dung beetles are classified into mainly three groups based on how they process dung. Telecoprids (rollers) roll dung balls away from the dung pat and bury the dung ball elsewhere; endocoprids (dwellers) live and breed inside a dung pat; paracoprids (tunnellers) construct tunnels directly beneath dung pats and create broodball clusters at the bottom of the tunnels. Like termites and ants, dung beetles play important roles in the ecological processes of soil, particularly nutrient cycling. As tunneling dung beetles’ burrowing activities are associated with improved soil bioturbation and aeration, it has been observed that plant growth is enhanced where these insects have been active. It was therefore hypothesized that dung beetle activity would lead to similar enhancements on rehabilitated mine soils, providing that dung beetles could dig into the highly-compacted soil surface.
Our study, funded by Coaltech Research Association and published in the journal Applied Soil Ecology, investigated the effects of tunneling dung beetles on soil and plant properties within confinements that simulated conditions associated with rehabilitated opencast coal mines in South Africa. The confinements were constructed to simulate soil conditions that occur on rehabilitated mines with a soil profile consisting of a layer of waste coal, subsoil, and topsoil, and vegetated with typical mine rehabilitation grass species. Three treatments, namely dung + beetles, dung only, and no dung or beetles, were applied to separate confinements (with three replicates each), where after results were analyzed and compared in terms of the following: water infiltration rate, plant biomass yield and protein content, soil compaction, and soil nutrients. Dung beetles were collected from the wild and bred in a breeding facility before they were used for these experiments.
When dung beetles were placed inside the confinements, it was clear that compacted soil did not present a challenge to them. Dung beetles were found to significantly increase water infiltration rate, thereby reducing the likelihood of erosion on the rehabilitated landscape. As the selected species of dung beetles’ tunnels only go down to approximately 15 cm, water will likely not enter the spoils underneath the soil. Plant growth was similarly improved where dung beetles were active by yielding approximately double as much biomass compared to confinements with only dung and confinements with no dung or beetles.
Adding dung beetles to the conventional mine rehabilitation methods may accelerate the reclamation of post-mining soils and would provide job creation where individuals would need to be trained for dung beetle breeding programs. Although not part of our study, it was observed that dung beetles are capable of breeding in these harsh conditions, where we found brood balls containing eggs at the bottom of tunnels. It would be important to investigate the long-term effects of adding dung beetles and cattle dung to post-mining soils, including possible patterns of ecological succession that would indicate a positive trajectory in mine rehabilitation efforts.