Events occurring in the world around us have led to a tacit acknowledgment that we are in the midst of a cycle of dramatic changes in our climate. In 1546, a writer named John Heywood published a collection of Renaissance proverbs. It is the first known mention of the now commonly used idiom, “You can’t see the forest for the trees,” which carries the meaning of not understanding the whole of a situation because of concentration on the details. Oddly enough, this is becoming ever more apparent the deeper researchers study the influence of ecology on carbon sequestration in relation to climate change—the element carbon and its derivative carbon dioxide having become the measure by which such change is scaled.
The public at large have been exhorted to plant more and more trees, and this seems to have become the widespread focus of activity aimed at trapping more carbon in the landscape in order to reduce the amount of carbon dioxide in the atmosphere (this is the greenhouse gas along with methane that is seen as the chief culprit in planet-wide warming). Heywood’s proverb is particularly apt, because a team of 15 scientists from several research institutions published a paper in April, 20221, that studies how the presence or absence of animals in a landscape can influence the whole question of carbon sequestration in vegetation for good or ill. The question is much more complex than just planting trees. Research in Guyana has shown that the presence of a diversity of medium- to large-bodied wild animal species within an ecosystem can boost the vegetative sequestration of carbon in a forested area by 4-5 times. This is now called “animating the carbon cycle.” However, it requires protecting or restoring animal species to a critical density to promote necessary movement and interaction on a natural scale. This process is termed “trophic2 rewilding,” and applies as much to seascapes as landscapes. Trophic rewilding introduces a whole other level of complexity into the carbon capture scenario.
The team led by Oswald Schmitz, a professor at Yale University, uses, as an example to illustrate the effect of animal presence, the fabled wildebeest migration in central Africa. In the first half of the 20th century, the wildebeest herds were decimated to around 300,000 animals by a viral disease called “rindepest,” and by poaching and constraints on habitat. The loss of active trampling and grazing during the migration period of the time encouraged the growth of standing grass in the Serengeti, which in turn fueled frequent and intense wildfires, releasing carbon into the atmosphere from plants and soil. The Serengeti changed from being a net carbon sink to a net carbon source. The wildebeest population is now around 1.2 million, resulting in increased grazing and trampling of the grasses, which reduces the incidence of wildfire, while the animal’s carbon containing dung is incorporated into the soil by insect action. Calculations show that the Serengeti is back to being a carbon sink, storing an additional 4.4 million tonnes of carbon dioxide over the level when the herds were reduced.
The controls animals exert come from foraging and movement encouraging the dispersion of seeds across landscapes, from burrowing, wallowing and ecosystem engineering, but not only land animals are involved. There is a process known as the “whale pump,” which involves complex interactions providing whales’ contribution to the seaborne carbon cycle. Fish, too, play an important part, not because of their size, but because of their abundance worldwide. It should be remembered, however, that it is the larger bodied animals that can contribute the most to an ecosystem’s enhanced uptake. It is those animals, like elephants and whales, whose populations we have in the past decimated and continue to do so by over-hunting, over-fishing, and the influences of human expansion. It is, for instance, the latter that is once again threatening the wildebeest’s migration.
Schmitz and his colleagues studied nine animal groups to derive their conclusions. These are wildebeest, sea otters, gray wolves, sharks, muskox, and fish, all needing protection, and elephants, bison and whales that need their numbers restoring. He says, “Many animal species exert very strong control over the carbon cycle,” and he believes an opportunity to employ their influence on carbon uptake is being missed. The research group concluded that protecting or restoring the populations of just those nine animal groups studied could “collectively facilitate the additional capture of 6.41 billion tonnes of carbon dioxide annually,” going a long way toward the 10 billion tonnes that need to be removed from the atmosphere each year to meet net zero emissions by 2050.
Schmitz’s work on gray wolves in the forests of Canada indicates that by controlling the foraging behavior and numbers of large herbivores, such as moose, through predation, encourages the growth of young trees that would otherwise be destroyed. His group estimates that gray wolves in those forests have the potential to remove extra carbon from the atmosphere equivalent to the emissions of 33-71 million cars per year. Schmitz points out, however, that conversely, wolves predating on elk in the North American grasslands can reduce the amount of carbon those grasslands can hold. Elk feces fertilize the soil and encourage grass growth. In areas not subject to wildfires, that is beneficial to carbon storage. This is evidence of the kind of complexity that faces ecologists in determining whether animals or vegetation are likely to be more beneficial in particular scenarios. However, Schmitz says, “[It is clear that] the science shows the dynamics of carbon uptake and storage fundamentally changes with the presence or absence of animals.”
Cristina Banks-Leite, a conservation ecologist at Imperial College in London, adds, “In Brazilian forests… roughly 80 percent of all trees are reliant on animals for seed dispersal or pollination. We can see how those trees would just not survive very long without those animals. Christopher Sandom, a rewilding expert and lecturer at Sussex University in the UK, sounds a warning note that animating the carbon cycle isn’t a panacea, not that Schmitz’s group claims that it is: “Research has shown that nature is a complex set of interlocking processes that may not give you the expected outcome.” There have been many instances in the past of unexpected disasters arising out of the lack of consideration of the wider consequences. For instance, the well-documented introduction of mongooses to Hawaii to control rat populations that has decimated the avian population, resulting in several bird species becoming endangered.
Just planting more trees, while undoubtedly beneficial, is an over-simplistic approach that ignores the underlying complexities of the whole carbon sequestration equation. What Schmitz’s research underlines is that the presence of suitable numbers of indigenous animals in their natural habitat offers a significant way of amplifying the carbon uptake of natural landscapes and seascapes that is largely being ignored. It is, perhaps, a semblance of the return to the natural balance that existed before humankind interfered. In turn, it urges us to correct the depredation that humanity has and is exercising on the numbers of those animals and the reduction of their habitats.
1. Trophic rewilding can expand natural climate solutions—Oswald J. Schmitz et al., March 2023.
2. Trophic—of, or relating to, the process of getting or eating food.
This article is based on information from the above paper and a BBC publication by Fernando Duarte, November 20, 2023.
© David Cuin 2023