Principal Investigator
At a Glance
The last five years have brought growing recognition of the role of animals in the global carbon cycle, along with optimistic projections about the possibility of synergies between biodiversity conservation and carbon storage. African savannas are home to the world’s greatest diversity of large mammals and are also considered attractive habitats for carbon offsets, because savannas can vary more than tenfold in vegetation biomass and can in theory be induced to switch between low-carbon and high-carbon states. Yet, savanna carbon storage, and hence the viability and sustainability of offset projects, depend on the interplay between herbivory, fire, and rainfall—and these interdependencies are not yet understood. Robert Pringle’s research group is using remote sensing and field experiments to understand how elephants and other large herbivores influence above- and below-ground carbon stocks in conjunction with broader environmental and land-use changes.
Research Highlight
Large herbivores eat vast quantities of plant biomass and alter ecosystems in many ways. But how do they affect ecosystem carbon budgets? This question has shot to the forefront of ecology over the last five years with a stream of high-profile publications by leading scientists, some of which offer a bullish assessment of the potential that herbivores may increase net carbon storage. However, the available data do not yet paint a clear picture, much less a rosy view. Some models and data indicate that “megaherbivores” such as elephants may boost carbon storage by dispersing seeds of large trees with dense wood, which in turn are harder for elephants to topple. Similarly, dense herds of grazers, such as wildebeest, may enhance tree cover by trimming grass that would otherwise fuel fire. Other studies, however, show that elephants can cause substantial reductions in aboveground tree biomass, especially in conjunction with fire. Meanwhile, scientists are only beginning to investigate the impact of these aboveground processes on soils, which hold the majority of total carbon stored in savannas.
Understanding the extent to which large herbivores positively, negatively, or neutrally influence carbon sequestration and storage is essential for evaluating the viability of carbon offset projects and the proposition that these could be extended to include conservation and restoration of threatened wildlife. Until then, the jury is out, and a 2023 Atlantic article quotes Pringle as cautioning that wishful thinking should not be allowed to drive carbon-policy decisions (https://www.theatlantic.com/science/archive/2023/12/elephant-conservation-carbon-storage-climate-change/676204/).
The Pringle group is searching for answers in dry savannas in Kenya and a wetter savanna-forest matrix in Mozambique. One forthcoming study from Kenya used high-resolution LiDAR to map all trees and understory plants in 36 experimental plots that exclude different herbivore species. The study shows that herbivores reduce herbaceous plant biomass by roughly an order of magnitude, and that elephants alone reduce tree cover by roughly fourfold despite having no effect on the total number of trees. Other ongoing work is analyzing the carbon content of soils at different depths in the same herbivore-removal plots.
Wildlife populations in Mozambique’s Gorongosa National Park are steadily growing after being driven to the brink of extirpation during a civil war in the 1980s. This dynamic scenario presents opportunities to study how the ”rewilding” of large herbivores influences carbon stocks. Preliminary analyses using remote sensing suggest a dramatic and unexpected trend: carbon stored in aboveground woody vegetation has increased by as much as 6 Mg per ha (megagrams per hectare) from 2010-2020 even as populations of elephant and other wildlife have increased, while the surrounding human-dominated buffer zone shows the opposite trend (Figure 5.1). These trends may be driven by fire, which has decreased inside the park—perhaps owing to reduction of fuel loads by grazing herbivores—while remaining relatively constant in the buffer zone (Figure 5.2). Remotely sensed estimates of soil carbon stocks suggest a parallel pattern, although this result requires further validation. As in Kenya, the team is pursuing a deeper understanding by using experimental herbivore removals together with soils analysis and high-resolution drone-based LiDAR imaging.
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Aboveground woody biomass increased in Gorongosa National Park while declining in the human-dominated buffer zone. The largest increases in aboveground woody biomass, predicted using spaceborne radar and LiDAR at 100m resolution, were along drainage lines, particularly in the gorges of the elevated Rift Valley rims in the western and eastern sections of the park.
Burnt area and fire intensity have declined in Gorongosa National Park while remaining relatively constant in the buffer zone. Declines in proportional burnt area in Gorongosa may be due to the recovering densities of grazing herbivores, which reduce fuel for fires. The map illustrates absolute burn frequency change, which ranges from -1 (11/11 years burnt 2001–2011 and 0/11 years burnt 2012–2022) to +1 (0/11 years burnt 2001–2011 and 11/11 years burnt 2012–2022). Seasonal timing of fire (Burnt date) was comparable between Gorongosa and the buffer zone, but fire intensity in the park has declined to a level comparable with the buffer zone, again consistent with reductions in fuel loads by grazing herbivores.
Together, the preliminary results from this work are in keeping with the idea that large herbivores can indeed have profound effects on terrestrial carbon storage. The direction of these effects, however, is not straightforwardly predictable and likely hinges on the interplay between the living and non-living environments.