Experimental addition of Onthophagus binodis and Geotrupes spiniger to pasture ecosystems in New Zealand and an investigation of affected biotic and abiotic soil factors

New Zealand has a serious environmental pollution problem caused by intensive farming, and a lack of co-evolved dung beetle species to clean up the waste produced by these introduced herbivores. Our endemic dung beetles evolved in isolation from large herbivores such as cattle, sheep and horses and consequently don't use the dung of these species. New exotic species of dung beetle have been introduced into New Zealand with the purpose of establishing populations that will combat this agricultural pollution. All of the species approved for introduction share the trait of being paracoprid or tunnelling. They dig tunnels below the dung pat and construct brood chambers from dung brought down from the surface. Significant benefits from this behaviour have been documented in other countries where these beetles are already established, however there is a paucity of studies documenting these effects in New Zealand soils. The value of this research lies in the investigation of exotic dung beetles in novel soil types. Here we aimed to demonstrate in Otago soils the benefits to soil physical structure, nutrient profiles, and invertebrate species (particularly earthworms) which have been found in other environments. A field study was conducted from February to June 2015 at two Otago sites – one a lowland intensive farming system (Mosgiel) and the other an upland less modified system (near Middlemarch) with some native tussock grass. Onthophagus binodis and Geotrupes spiniger beetles were added to an enclosed field plot and effects on bulk density, availability of essential plant nutrients, organic matter and soil invertebrate abundance were measured after 4 months. No decrease in earthworm abundance was found and no negative effects on any other invertebrate species at either site occurred, despite sites exhibiting very diverse fauna. O. binodis beetles were also observed to be active much later into the season than expected – activity in the middle of May was seen at both the lowland and upland sites. While this study found no negative effects of the introduction of dung beetles to these pasture sites, it also found no significant positive effects. Lack of effect was in large part assumed to be due to the low numbers of beetles available and used in the field study – less than is typical of a natural dung pat colony.