Dung beetle assemblages on reclaimed coal mines in eMalahleni (South Africa) their environmental associations and tunnelling ability on compacted soils

Abstract

Opencast coal mining operations are a major contributor to habitat destruction through the removal of soil, vegetation and fauna from an area. Habitat loss and fragmentation is known to adversely impact invertebrates because of their small size and limited dispersal range that also applies to local dung beetle assemblages. Dung beetles have the potential to aid in reclamation efforts through their beneficial activities in soil although there is little known about their assemblages on reclaimed mine land. Additionally, highly compacted soils are a feature of reclaimed mine sites that may pose a significant challenge to tunnelling dung beetles and may limit their remediation benefits. This study aimed to describe the local dung beetle assemblage in terms of species richness and abundance on reclaimed mine sites in comparison to reference areas (including cattle farms and a protected area). Furthermore, it aimed to determine if dung beetles can tunnel into compacted soils, and how these soils may influence their tunnelling depth as shown through penetration resistance measurements. Dung beetles were collected using standard baited pitfall traps from five reclaimed mined sites and three reference sites (two cattle farms and a Telperion Nature Reserve) from January 2015 to April 2017. Various abiotic factors that could influence the distribution of beetles were measured including soil bulk density, vegetation cover, humidity and soil texture. Dung beetle abundance was found to be significantly higher at the Telperion Nature Reserve than any other site (F (7, 56) = 8.613, p<0.05). Species richness was found to be higher at reference sites than reclaimed sites with a single exception (F (7,56) = 17.61, p<0.05). These differences were attributed primarily to the absence of dung on the reclaimed sites, and environmental differences in the soil and vegetation profile. Dung beetle assemblages on reclaimed mined sites were found to differ significantly from the reference sites (R=0.55, p<0.05). While increasing vegetation cover, sand percentage (F (1, 30) = 5.46, p<0.05; R2= 0.15) and bulk density on sandier soils (F (1, 30) = 8.61, p<0.05; R2= 0.22) were found to be affiliated with higher species richness. Increasing clay percentage on the other hand showed to be affiliated with a decreased species richness (F (1, 30) = 5.58, p<0.05; R2= 0.16). To determine the influence of a change in penetration resistance on dung beetle tunnelling ability, three beetle species were used, namely: Onitis alexis Klug, 1835, Digitonthophagus gazella (Fabricius, 1787) and Euoniticellus intermedius (Reiche, 1849). Five individuals of each species (at an approximately even sex ratio) were placed on 30 separate 1 kg cattle dung pats where they were left to tunnel for 14 days. Dung pats were place on the soil surface with a range of penetration resistance levels between 100 kPa and 5 000 kPa. Digitonthophagus gazella showed a negative correlation with increasing penetration resistance (p<0.05; R2=0.65). While Onitis alexis tunnel depth showed no correlation to penetration resistance, Euoniticellus intermedius increased tunnel depth with increasing penetration resistance (p<0.05; R2=0.35). Although tunnelling depth was notably shallower than previously observed for the individual species, all three species could tunnel past the site average of 3 193 kPa as well as the equipment threshold value of 5 000 kPa. Live eggs and larvae were found in multiple brood balls that gives credence to the fact that dung beetles could complete their life cycle in reclaimed mined soils. Reclaimed mine sites supported a relatively high diversity of beetles in lower abundance and these results indicate the potential to improve assemblage structure with the incorporation of large herbivores on site. Differences in species richness between sites were attributed to vegetation cover, soil texture and bulk density differences between sites. Dung beetle species that were well adapted to mined sites were identified and recommended, should breeding be necessary to enhance their beneficial activities. Beetles active on mined sites occurred at great enough numbers to suggest that the population present in the area will be sufficient for rehabilitation efforts, provided dung becomes available.