The role of dung beetles in reducing greenhouse gas emissions from cattle farming

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.

with the dung collected as the cattle entered the barn for within-stall milking. When indoors, the cattle was fed additional silage, a standard concentrate (Maituri 20 and Aminomaituri 30, Raisio Oyj, Raisio, Finland), and magnesium-selenium minerals (Pihatto-Melli; Raisio Oyj, Raisio, Finland). No animal in the herd had been given antibiotics or antiparasitic treatments.

Methods used in flux measurements
Gas samples were taken with a syringe after 5, 10, 20, and 30 minutes of the chamber being sealed, and injected into glass vials (3-ml Labco Exetainers® with double septa, Labco Ltd., Buckinghamshire, UK). CH4 and N2O were then quantified in parts per million (ppm by volume) by gas chromatographs (HP 5890 Series II, Hewlett Packard, Palo Alto, CA, U.S.A.) equipped with thermal conductivity, flame ionization and electron capture detectors. Ambient air temperature in the shade, next to each chamber at 20 cm height, was recorded during the sampling of all gases, for later scaling of gas fluxes to temperatures. For further details, see Penttilä et al. 1 .

Calculation of CH4 emissions from grazing animals
We used data about the proportion of each manure management system used in Finland and the methane producing capacity of the manure management systems (Eq. 1).

Supplementary Material 2: Additional results.
As our paper focuses on estimating the relative contribution of dung beetle mediated effects on GHG emissions at the level of dung pats, pastures and the life cycle of beef and milk production, we focus the presentation in the main paper on these proportions. Here, we give the detailed analyses and specific numbers underlying these proportions. The step-by-step calculations for these estimates are provided in an Excel spreadsheet, Supplementary Material 4: Flux Calculations.

Supplementary Material 3: Sensitivity analysis
To examine which pasture flux components (dung pats, contribution of the beetles, dung-free areas of the pasture) had the largest influence in the results, we carried out a sensitivity analysis, where we varied the length of the grazing season, the flux from the dung-free areas of the pasture, the flux from dung (dung flux rate varied, but beetle effect kept constant), and the contribution of the dung beetles. Such variation may be due to various abiotic (e.g. temperature, moisture, climate, soil type) and biotic (e.g. dung beetle abundance and community composition, vegetation type) factors. As response variables, we used the total annual flux of the pasture (which could have dung pats with or without dung beetles in it, or be devoid of dung) and the emissions over the life cycle of milk and beef production. For the LCA sensitivity analysis, we chose the scenario without LULUCF emissions (see Table 1 and Leip et al (2010) 1 ).
For all but grazing season length, the range of values used for the explanatory variables were derived using the value observed / calculated in this study as a starting point, ± three times the value as the upper and lower ranges. The length of the grazing season was allowed to vary from 0 to 365 days per year.
The annual CH4 flux at the pasture scale was most sensitive to the variation in grazing season length and flux rate from dung pats, while changes in fluxes from dung-free pasture areas and dung beetle contribution had relatively little influence on the results (Fig. S2). Although changes in the effect of dung beetles had a proportionally large influence on the life cycle emissions from milk and beef production, the total effect of the dung beetles in the LCA remained small: changing the dung beetle effect on fluxes at the dung pat scale from a 14.5% reduction (estimate from this field study) to 43% reduction only resulted in 0.21% and 0.36% reduction in the LCA emissions of milk and beef, respectively.
The annual N2O fluxes at the pasture scale were the most sensitive to variation in the flux from the dung-free pasture area, compared to which all other sources of variation were minor (Fig. S.3). This is due to the magnitude of the N2O flux from dung pats and from dung-free areas. As the dung-free areas are much larger (96% vs. 4%), they will dominate the overall fluxes. For CH4, the pattern is the opposite, as dung pats are hotspots of CH4 emission. Thus, emissions from pats play a larger role in the pasture CH4 budget compared with the N2O budget. Similar to the life cycle of CH4 emissions, the N2O emission also showed a large proportional response to variation in the effect of dung beetles.
Nonetheless, the total effect remained small, as a reduction of 6% (three times the observed value of 2%) only resulted in reductions of 0.34% and 0.62% in LCA emissions from milk and beef, respectively.