Exploring the influence of ancient and historic megaherbivore extirpations on the global methane budget

Abstract

Globally, large-bodied wild mammals are in peril. Because "megamammals" have a disproportionate influence on vegetation, trophic interactions, and ecosystem function, declining populations are of considerable conservation concern. However, this is not new; trophic downgrading occurred in the past, including the African rinderpest epizootic of the 1890s, the massive Great Plains bison kill-off in the 1860s, and the terminal Pleistocene extinction of megafauna. Examining the consequences of these earlier events yields insights into contemporary ecosystem function. Here, we focus on changes in methane emissions, produced as a byproduct of enteric fermentation by herbivores. Although methane is ∼200 times less abundant than carbon dioxide in the atmosphere, the greater efficiency of methane in trapping radiation leads to a significant role in radiative forcing of climate. Using global datasets of late Quaternary mammals, domestic livestock, and human population from the United Nations as well as literature sources, we develop a series of allometric regressions relating mammal body mass to population density and CH 4 production, which allows estimation of methane production by wild and domestic herbivores for each historic or ancient time period. We find the extirpation of megaherbivores reduced global enteric emissions between 2.2-69.6 Tg CH 4 y −1 during the various time periods, representing a decrease of 0.8-34.8% of the overall inputs to tropospheric input. Our analyses suggest that large-bodied mammals have a greater influence on methane emissions than previously appreciated and, further, that changes in the source pool from herbivores can influence global biogeochemical cycles and, potentially, climate.

Glacial/interglacial wetland, biomass burning, and geologic methane emissions constrained by dual stable isotopic CH4ice core records

“…1) directly support the hypothesis (120, 121) of higher fire activity during the Holocene and the LGM compared with previous interglacials and glacials, respectively. At the same time, the largely unchanged CH 4 levels suggest that direct CH 4 emissions from large animals are confined to the lower end of values found in the literature (128).…”

Megafauna and ecosystem function from the Pleistocene to the Anthropocene

“…Although domestic livestock are recognized as a major source of methane emissions, wild animals also produce substantial methane, and these emissions scale allometrically with body mass (22,39,91). Smith et al (22) predict detectable decreases in the global methane budget related to the extirpation of megaherbivores and estimate the greenhouse gas effect of Pleistocene megafaunal loss to be a global cooling of 0.08-0.20°C.…”

The Rise of the Anthroposphere since 50,000 Years: An Ecological Replacement of Megaherbivores by Humans in Terrestrial Ecosystems?

“…These effects mimic those of megaherbivores respiring and fermenting large amounts of vegetation that also contributed to the the greenhouse gas composition of the atmosphere (e.g., Smith et al, 2010bSmith et al, , 2016a.…”

“…Moreover, emissions of methane by megaherbivores may have had an effect on greenhouse gas concentration in the atmosphere and therefore on climate (Smith et al, 2010b(Smith et al, , 2016aBrook and Severinghaus, 2011).…”

“…Elephant and rhinoceros damage trees (Schüle, 1992;Omeja et al, 2014;Terborgh et al, 2016) Large scale deforestation to gain forage areas for domestic herbivores and crop fields (Godwin, 1944;Williams, 2008) Nutrient cycling and fertilization Recycling and spreading of nutrients away from hotspots and counteracting gravity Doughty et al, 2013Doughty et al, , 2016b Active fertilization of agricultural areas: manuring, plowing, cultivation of legumes in rotation (Galloway et al, 2013); recycling human and animal waste (Melillo, 2012); increasing P export (Boyle et al, 2015) Megafruit and seed dispersal Promoting dispersal of plants with megaseeds (Guix, 2009;Corlett, 2013) Domestication of megafruit plant species (e.g., avocado, chocolate, squashes Kistler et al, 2015) Emissions of greenhouse gas Methane emissions through digestive fermentation (Smith et al, 2016a) CO 2 emissions through forest clearing (Ruddiman and Ellis, 2009). Methane emissions by domestic ruminants, rice paddies (Fuller et al, 2011) In some cases, humans have limited the negative impact of the loss of megaherbivores for the plant species producing very large fruits and they behaved as a partial replacement of large fruits dispersers (Guimarães et al, 2008).…”