Carbon storage calculations don’t always take into account the effects of animals – when they eat, defecate and die, they help store lots of carbon.
Animals may enable many ecosystems to store two to three times as much carbon as they would without them, according to a new model. This suggests animals may play a much more important role in how much carbon is stored and released than previously thought.
 |
| Elephants boost carbon storage by trampling and grazing on small trees Art Konovalov/Shutterstock |
“Animals do a lot of different things,” says Matteo Rizzuto at Yale University. “They move around. They urinate. They defecate. They give birth. They die. And all of those things are part of the carbon cycle.”
However, most models of how carbon cycles through ecosystems still leave animals out. This is partly because animals store only a small amount of carbon in their bodies relative to the plants and microbes that dominate the planet’s biomass, says Rizzuto. Modelling the indirect effects of all their diverse activities is also full of complexity.
To capture these effects in a general way, Rizzuto and his colleagues created a simplified mathematical model of how nutrients like nitrogen and carbon move through an ecosystem across a wide range of plausible conditions. They then compared estimates of how much net carbon was stored in ecosystems when the model accounted for the influence of animals – for example, how much they eat, their population size and other factors – and when it didn’t.
Their model suggests that animals help ecosystems store an average of two to three times as much carbon as a scenario in which they are left out of the model, thanks to a “rewiring” of how nutrients move through the system. For instance, when animals are absent, available nitrogen is the controlling factor for carbon moving through ecosystems; when herbivores are present, other factors like nutrient recycling are more important.
However, in some cases, accounting for animals led to lower estimates of carbon storage – for instance, herbivores recycle a lot of nutrients by defecating and urinating, but these positive effects can be overwhelmed by how many plants they consume.
The upshot is that protecting and restoring wildlife could have major carbon storage benefits, alongside the advantages for biodiversity. Rizzuto says he also hopes the model will provide a mathematical framework for other researchers to study how animals affect the carbon cycle in more specific contexts. “We’re just now starting to have the computing power, field-based knowledge and mathematical tools to get into these things,” he says.
Hanqin Tian at Boston College says carbon cycle models that are calibrated with real measurements of nutrients in plants and soil may capture the influence of animals, even when they don’t explicitly model their effects. But he says models projecting changes to carbon storage in the future risk being off the mark without accounting for animal behaviour.
Journal reference:
JGR Biogeoscience DOI: 10.1029/2024JG008026