Wild Animals contribute to Climate Change too

Just in case you were considering taking seriously the dire warnings about the effect of cow burps on the climate, Pennsylvania State University has done some research on the contributions of wild ruminants (e.g. deer, buffalo) on greenhouse gas emissions, both pre-settlement and today.  All those pro-wild-animal environmentalists might have to re-think blaming greenhouse gas emissions solely on man or domestic livestock.  Gee, those bad buffalo were contributing almost as much greenhouse gas (GHG) prior to European settlers as cows do today.  Even wild animals are out to ruin the planet.

In North America, there are many native ruminants, i.e. herbivore animals with a complex digestive system, a major compartment of which is the rumen. Some present-day examples are the bison (Bison bison), the elk (or wapiti, Cervus Canadensis), or the deer (white-tailed, Odocoileus virginianus or mule, Odocoileus hemionus)…

Methane is a potent GHG and domestic ruminants have been blamed by many for a large portion of the global GHG emissions, thus having a significant impact on climate change. The importance, or rather unimportance, of livestock to global GHG emissions has been discussed by us and by others (see, for example, Hristov, 2008); here, we’ll focus on wild ruminants…

The bison was by far the most important wild ruminant methane emission source in the pre-settlement period.  The size of the animal, its high DMI, and its sheer numbers were determining its role as the greatest wild ruminant methane emitter. As the bison pre-settlement population size estimates vastly differ between sources (see Reynolds et al., 2003), we calculated emissions for 3 different scenarios: high (75 million), low (30 million), and medium (50 million bison) population sizes. The high population size is based on estimates by the famous 19th century naturalist and writer, Ernest Thompson Seton, and the low population size is based on the number of animals the available range at the time could support (estimated by McHugh, 1972). In all cases, the bison methane emissions represented between 84 and 93% of all emissions from wild ruminants in the pre-settlement period.

Kelliher and Clark (2010) came to similar conclusions; these authors estimated that methane emissions from bison in the 10 states encompassing the historical range of this animal were close to the present-day methane emissions by cattle in these states (2.2 vs. 2.5 Tg methane/yr, respectively). It has to be pointed out that our bison population (plains bison) estimates include bison in the Canadian Prairie Provinces (Alberta, Saskatchewan, and Manitoba), which would likely overestimate methane emission for the contiguous United States. According to Reynolds’ distribution map (Reynolds et al., 2003), however, the Canadian portion of the plains bison range was relatively small compared to the distribution range in the contiguous United States.

Overall, methane emissions from bison, elk, and deer in the pre-settlement period in the contiguous United States were about 70% (medium bison population size) of the current emissions from farmed ruminants in the U.S.; data for current (2008) methane and GHG emissions in the U.S. are from the EPA “Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 – 2008” report. If the high bison population estimate is taken for this comparison, wild ruminants in the pre-settlement period emitted as much methane as the current domestic ruminants in the United States. Present-day livestock methane emissions are primarily from cattle; the contribution of sheep and goats to the total emissions is miniscule (about 1%). Estimates for methane emissions from horses (about 0.17 Tg/yr) and swine (about 0.10 Tg/yr) were published by EPA but are not included in this analysis. It is worth mentioning that in this most recent EPA report, GHG emissions from agriculture made up about 6% of the total GHG emission in the U.S. for 2008 (427.5 vs. 6,956.8 Tg CO2 Eq./yr, respectively). Methane from enteric fermentation was 140.8 Tg CO2 Eq./yr, representing 25% of total methane emissions in the U.S. Emissions of methane and nitrous oxide from enteric fermentation and manure management (i.e., the total livestock contribution to GHG emissions) was 202.9 Tg CO2 Eq./yr, or 47% of the agricultural emission, but only 2.9% of all GHG emissions in the U.S.

Present-day methane emissions from wild ruminants (excluding moose, mountain sheep, goat, antelope, and caribou) were also estimated. Population data were from the Feldhamer book (bison and deer), or the RMEF (elk). Body weight, DMI, and methane emission per unit of DMI were the same as for the pre-settlement calculations. With these assumptions, it can be estimated that present-day methane emission from the major wild ruminant species in the U.S. are about 0.23 Tg/yr, which is only 3.6% of the emissions from domestic ruminants. Due to its population size (estimated at 25 million; Miller et al., 2003) the white-tailed deer is the largest present-day wild ruminant contributor to GHG emissions in the contiguous United States.

– Dr. Alexander Hristov, associate professor of nutrition, Penn State Department of Dairy and Animal Science


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