Animals storing carbon dioxide would seem to be a strange idea, at least on the surface. However, animals may be able to contribute positively to this growing human-generated problem.
More than a million wildebeest roam across East Africa’s vast Serengeti grassland. Their annual migration is one of the largest movements of animals on the planet. Their hooves churn up the dirt, and hungry mouths devour huge quantities of plant life as they travel.
There weren’t always this many wildebeest, and the story of this large antelope reveals the impact wildlife can have on the amount of carbon present in our planet’s atmosphere. While it is tempting to only look at technical solutions, such as renewable energy, to solve the problem of climate change, we may have allies in the natural world. Increasing certain populations of animals, such as wildebeest, can help tackle climate change, according to scientists.
In the first half of the 20th Century, a combination of a viral disease called rinderpest, spread by cattle, along with poaching and loss of habitat, decimated wildebeest herds, causing their numbers to fall to about 240,000. With fewer animals grazing, the volume of grass and other plants on the Serengeti increased. This might seem like a good thing for sequestering carbon, but in fact it provided fuel for more frequent and intense wildfires. This meant that much of the carbon stored within the plants and soil of the savannah was released into the atmosphere, turning the region from a net sink of carbon to a net source.
The wildebeest numbers recovered by the 1970’s, with the introduction of a vaccine for cattle, and the average annual numbers of wildebeest currently stands at around 1.2 million. This means that, once more, the Serengeti has been transformed into a giant reservoir that absorbs more carbon than it releases, helping to lower the concentration of CO2 in the atmosphere. And for every 100,000 additional wildebeest in the Serengeti, the amount of carbon stored rises by 15%, according to Oswald Schmitz, a professor of population and community ecology at Yale University.
Schmitz is one of a group of researchers using wildebeest as an example to argue that animal rewilding could be an important solution in tackling climate change. They concluded that protecting, or restoring, the populations of just nine specific groups of animals could “collectively facilitate the additional capture of 6.41 billion tonnes of carbon dioxide annually”. To put this figure in context, climate scientists estimate that up to 10 billion tonnes of CO2 will need to be removed from our atmosphere globally every year to achieve the goal of reaching net-zero emissions by 2050.
The nine major animal groups that Schmitz and his colleagues say could make the most difference are marine fish, whales, sharks, grey wolves, wildebeest, sea otters, musk oxen, African forest elephants, and American bison. Of these, marine fish could the biggest impact, says Schmitz. The researchers estimate these animals could help absorb up to five billion tonnes of CO2 a year. Fish are capable of capturing carbon in different ways, which include eating carbon-rich plankton near the surface, and releasing fecal pellets that can sink rapidly to the bottom, where they are sequestered. Also, when fish die, they sink to the bottom of the ocean where the carbon in their bodies is also sequestered.
Scientists have found that sharks have a positive impact on the marine carbon cycle by eating herbivorous fish or restricting the areas where those fish live. That limits their preys’ ability to eat marine vegetation, which is crucial for carbon absorption. Tiger sharks, for example, help to keep populations of sea cows in check, allowing seagrass meadows to flourish, which in turn help to store huge amounts of carbon around the world. The sharks themselves also directly store carbon in their own poo as it sinks to the sea floor.
Like sharks, whales do a lot through their poo. Their excrement is rich in nutrients and acts as a fertiliser, stimulating the growth of phytoplankton – microscopic plants that consume carbon dioxide. Even those that feed deep in the ocean still need to swim up to the surface to breathe and poo, creating what has become known as the “whale pump” for pulling carbon out of the atmosphere. The sheer size of many whales means they also store large quantities of carbon in their bodies. When they die and sink to the bottom of the ocean they can remain there for centuries as they slowly decay and are consumed by the creatures that live there.
Grey wolves living in Canada’s boreal forest, one of the world’s most important carbon sinks, have an indirect influence on the woodland’s carbon storage capacity. By controlling the foraging behaviour and abundance of large herbivorous animals such as moose through predation, the wolves help to promote the growth of young trees. Without the wolves, the herbivores browse upon saplings and thin out the forest growth. Schmitz and his colleagues estimate that grey wolves in the boreal forests have the potential to remove carbon from the atmosphere equivalent to the emissions of between 33-71 million cars per year.
It’s a delicate balance though, and Schmitz points out that different ecosystems can actually benefit from having more large herbivores. Grey wolves that prey on elk in the North American grasslands, for example, can reduce the amount of carbon those landscapes can hold. Elk feces can fertilise the soil and stimulate the growth of grass. In ecosystems where wildfires are not common, more vegetation can be a store of carbon. And this is often where the complexity lies: One animal can have a beneficial effect on the climate in one ecosystem but may not in another. Yet they can also play a crucial role in both.
In the Arctic, Musk oxen are a favoured prey of wolves, yet these large herbivores have a “huge influence” in tackling climate change by protecting the frozen soil, says Schmitz. “By grazing and trampling they protect against permafrost thawing,” he says. “If permafrost thaws, it can release many millions to billions of tonnes of methane.” Methane is a potent greenhouse gas.
These complex relationships are one reason why Christopher Sandom, a rewilding expert and biology lecturer at the University of Sussex, UK, warns that replenishing animal populations will not be a silver bullet for climate change. “Research has shown that nature is a complex set of interlocking processes that may not give you the expected outcome,” he says. “Rewilding cannot be seen as a panacea. We must not simply think that nature can suck all the carbon up and not adopt measures across the board to reduce man-made emissions.” He adds, however, that it’s clear that animals and their conservation need to be included in the discussions around climate change. “Planting trees is indeed important, but nature shows they need animals to help them grow,” he says.
Fascinating, and an education for all who think they know the answers to the problems of climate change. It’s not simple, and we may have more allies than we think, if only we can understand them.
