Dung Beetles Help Reducing Greenhouse Gas Emissions From Cattle Farming
Categories: On The Farm
Dung beetles do just what their name suggests: they use the manure, or dung, of other animals in some unique ways! These interesting insects fly around in search of manure deposits, or pats, from herbivores like cows and elephants. Dung beetles come in a variety of colors, from dull and glossy black to metallic green and red. Ancient Egyptians thought very highly of the dung beetle, also known as the scarab (from their taxonomic family name, Scarabaeidae). They believed the dung beetle kept the Earth revolving like a giant ball of dung, linking the insect to Khepri, the Egyptian god of the rising sun.
Agriculture is one of the largest anthropogenic sources of greenhouse gases (GHGs), with dairy and beef production accounting for nearly two-thirds of emissions. Several recent papers suggest that dung beetles may affect fluxes of GHGs from cattle farming. Here, we put these previous findings into context. Using Finland as an example, we assessed GHG emissions at three scales: the dung pat, pasture ecosystem, and whole lifecycle of milk or beef production. At the first two levels, dung beetles reduced GHG emissions by up to 7% and 12% respectively, mainly through large reductions in methane (CH4) emissions. However, at the lifecycle level, dung beetles accounted for only a 0.05–0.13% reduction of overall GHG emissions. This mismatch derives from the fact that in intensive production systems, only a limited fraction of all cow pats end up on pastures, offering limited scope for dung beetle mitigation of GHG fluxes. In contrast, we suggest that the effects of dung beetles may be accentuated in tropical countries, where more manure is left on pastures, and dung beetles remove and aerate dung faster, and that this is thus a key area for future research. These considerations give a new perspective on previous results perspective, and suggest that studies of biotic effects on GHG emissions from dung pats on a global scale are a priority for current research.
Greenhouse gases are the biggest contributors to global warming and climate change1. GHG emissions from agriculture and associated land use change (LUC) were estimated to contribute 30% (8.0 Gt CO2e yr-1) of the global anthropogenic emissions in 20102. Of GHG emissions from agriculture, livestock production accounts for around two-thirds (Fig. 1a), with direct emissions (4.6 Gt CO2e yr−1) emanating mainly from digestion by livestock2 (Fig. 1b). Dairy and beef production alone have been estimated to account for 60% of the total emissions of livestock production (Fig. 1c), with both enteric fermentation and fluxes from manure and its management being major contributors of GHGs2 (Fig. 1b).
Production of both meat and milk is projected to increase with a growing world population3. Grazing of beef and dairy livestock on grasslands, rather than intensive indoor production, can be used to mitigate livestock-related GHG emissions via the carbon sink of grass-fed production systems4. However, while grassland systems often act as sinks of CH45,6, manure deposition may turn them from sinks to sources of GHGs6,7. Thus, the mitigation potential of manure management is an important issue for the livestock industry3,8.
Dung beetles (Scarabaeidae: Scarabaeinae, Aphodiinae, Geotrupidae) are some of the most important invertebrate contributors to dung decomposition in both temperate and tropical agricultural grasslands9,10,11,12,13. As such, they may help mitigate GHG emissions and aid carbon sequestration through removing dung deposited on the pastures, increasing grass growth and fertilization14. It has recently been shown that dung beetles have the potential to reduce GHG emissions from dung pats deposited on pastures7,15. Yet, how these effects compare to other GHG sources in livestock farming remains unknown, as the impact of dung beetles has not been included in more comprehensive lifecycle assessments of meat and dairy products.
In this study we use Finland as an example as this is one of the few regions of the world where all aspects of cattle farming and dung beetle impacts are well-documented. We adopt previously measured effects at the level of the dung pat to derive new estimates at the level the pasture ecosystem, and over the whole lifecycle of milk or beef production. By identifying the factors limiting the scope for dung beetle mitigation of GHG emissions in this highly-industrialized production system, we then point to other, still poorly-explored production systems in both temperate and tropical regions, where dung beetles are particularly likely to have an effect, and suggest that further research is urgently needed in these areas.