Trophic rewilding is a gigaton carbon capture opportunity and a lesson in systems thinking
🐘🦬 Trophic rewilding has gigaton potential for enhanced carbon capture and storage. 🐳🐟 Those animals provide valuable ecosystem engineering and lessons in systems thinking. 🐺🐯
A fascinating new perspective in Nature1 on trophic2 rewilding:
Conserving and rewilding of animals can have a huge impact on ecosystems in terms of their ability to capture and store carbon.
The chart below shows a number of species along with their respective emission-related drivers.
Concrete examples of rewilding and its impact on carbon
🐃🐃🐃 The Wildebeest population in Africa makes sure that the grasslands doesn't grow too high and dense, preventing large fires. The population was at a low point in the early 20th century, turning the Serengeti into a carbon source. With repopulation efforts, the Serengeti is now a carbon sink again.
Apparently, the evidence base is solid enough to make statements like these: "the amount of carbon stored in the Serengeti increased linearly by 15% with every increase of 100,000 animals."
🐘🐘🐘 African forest elephants "control carbon storage by dispersing seeds of carbon-dense overstory woody species, and by foraging and trampling understory vegetation, which enables overstory trees, released from competition, to grow larger and produce more carbon-dense biomass."
🌳🌳🌳 BTW, different continent, but: "In Amazonian forests, [particularly carbon-rich tree species] represent 1% of tree diversity, but store 50% of the forest carbon".
🐳🐳🐳 and the Whale Pump: "They control the carbon cycle via the so-called ‘whale pump’, which involves feeding at ocean depth and released nutrients in the excrement as they breathe and rest in surface waters, which thereby stimulates phytoplankton production."
☃️☃️☃️ Permafrost: "Trophic rewilding could be impactful in the Arctic where ~500Gt of organic carbon is stored in the Yedoma permafrost, and where high densities of large animals could prevent massive CH4 release from permafrost melting. Herds of large animals compact snow, which keeps soil in a frozen state."
Impact
Of course, not all species will have a positive impact on the carbon balance. But the paper concludes that even accounting for negative effects (such as methane emissions by the animals), the overall effect is expected to be vastly positive.
Here’s the overview:
The third largest contributors are wolves (in forests!3) with about 260mt of CO2e per year. The key driver is that they reduce the population of large herbivores, thereby enabling higher density of deciduous trees.4 By the way, the Predation Risk Hypothesis indicates that the wolves not only reduce the herbivore population directly but also influence its behavior5 and therefore the distribution of herbivores in an area, which can presumably have a much greater impact on the tree population.
The second largest contributors are bison with about 600mt of CO2e per year. Bison impact the carbon balance in many ways: Their grazing induces increased plant growth; their grazing also prevents large-area wildfires; their manure adds nutrients to the soil, promoting plant growth; trampling and wallowing leads to soil compaction; trampling can also lead to soil aeration and seed dispersal.
The by far largest contributor is fish with a gigantic 5.5 gigatons of CO2 per year potential6. The key driver here is the ocean carbon biological pump, which essentially leads to carbon being “exported” downward from the upper7 to the lower ocean zones.8
Broader patterns of decarbonization: systems thinking
My goal here is to identify the patterns underlying the observations. In the case of trophic rewilding, mostly systems thinking comes to mind. At least three important aspects of systems thinking are present: Interconnectedness & interdependence, feedback loops and resilience.
1. Interconnectedness & interdependence
Elements in a system influence each other. The presence or absence of a particular species can have a cascading effect on the entire system, including its carbon balance. Managing ecosystems from a systems perspective requires understanding the complex interactions between different elements and their impact on the system's overall functioning.
2. Feedback loops
Feedback loops describe the way in which a system can respond to changes, with positive feedback loops reinforcing changes and negative feedback loops counterbalancing them.
Trophic rewilding can create positive feedback loops, where the reintroduction of a species leads to increased carbon capture and storage, which in turn enhances the habitat and resources available for other species, leading to further positive effects on the carbon balance. Understanding and leveraging such feedback loops is crucial to creating self-sustaining ecosystems.
In addition to the feedback loops mentioned above, there is the famous9 Yellowstone example, where the feedback loop was broken when the wolves 🐺🐺🐺 were extirpated in the early 1900s.
Wolves prey on elk, which leads to a decrease in elk numbers and a decrease in overgrazing. This allows for a healthy population of aspen, willow, and cottonwood trees, which in turn provide habitat for beavers. Beavers build dams, which create ponds that provide a habitat for fish and other aquatic organisms. These ponds also slow the flow of water, allowing sediment to settle out and creating wetland areas. These wetlands are highly effective at storing carbon, and as they expand, they create a feedback loop that further enhances the ecosystem's ability to store carbon.10
With the last wolf shot in 1926, an important feedback loop broke down and the ecosystem lost its balance. With the re-introduction of wolves around 2000, the Yellowstone ecosystem was able to restore at least some of its former properties.11
3. Resilience
Resilience refers to the ability of a system to maintain its structure and function even in the face of disturbance or change.
Ecosystems with high levels of biodiversity and functional redundancy are more resilient to disturbances and better able to maintain their carbon balance. By reintroducing keystone species and restoring ecological processes, trophic rewilding can enhance the resilience of ecosystems and their ability to withstand and recover from shocks and stressors.
One example is forests with a more diverse structure that are better to withstand pests, disease, extreme weather events, or fires. Another example is functional wetlands that help mitigate the impacts of storm surges and flooding and can reduce erosion.
In conclusion
Animal ecosystems are complex and fascinating. They represent a huge potential for climate and carbon impact. And they are an invitation to contemplate how interconnected our world is.
👉 What are your favorite ecosystem examples?
Link to Nature paper, accessible version of the same paper, good summary on One Earth — full citation of Nature paper: Schmitz, O.J., Sylvén, M., Atwood, T.B. et al. Trophic rewilding can expand natural climate solutions. Nat. Clim. Chang. 13, 324–333 (2023). https://doi.org/10.1038/s41558-023-01631-6
"Trophic" refers to the feeding relationships between different species in a food chain or food web.
Interestingly, (re-)introducing wolves in grasslands has the opposite effect, leading to reduced carbon uptake: More wolves result in fewer herbivores, which in turn leads to reduced stimulation of root growth (=smaller roots), resulting in diminished ecosystem productivity. See Effects of gray wolf-induced trophic cascades on ecosystem carbon cycling.
AKA its “foraging strategy”.
With a large margin of error of ± 4.40.
Called “euphotic”.
There are many more effects — this is just a subset I mention in this paragraph.
See, for example, Wolves and the Ecology of Fear: Can Predation Risk Structure Ecosystems? and many other sources.
There are, however, also voices that are more skeptical and suspect that some ecosystems can never be brought back to their former “performance”: Have returning wolves really saved Yellowstone? — High Country News – Know the West (hcn.org)