The global carbon cycle describes the continuous movement of carbon atoms between the atmosphere, oceans, land, and living organisms. While discussion often focuses on massive carbon reservoirs held by plants and the deep ocean, animals are significant, interconnected agents in this cycle. They directly and indirectly govern how carbon is stored, transferred, and released across terrestrial and aquatic ecosystems.
The Direct Output: Carbon Dioxide from Respiration
Animals contribute to the carbon cycle most immediately through cellular respiration. This fundamental metabolic activity breaks down organic carbon compounds—primarily sugars derived from food—to release energy. The byproduct of this energy conversion is carbon dioxide (CO2), which is exhaled directly into the atmosphere.
This release represents a rapid return of carbon that was recently fixed from the atmosphere by plants through photosynthesis. For instance, a grazing herbivore consumes plant biomass, and a fraction of that carbon is quickly respired back into the air.
Trophic Transfer: Moving Carbon Through Ecosystems
Animals function as biological conduits, transferring organic carbon from one trophic level to the next. When a primary consumer, such as an insect or a deer, eats a plant, the carbon stored in the plant’s tissues is incorporated into the animal’s biomass. This action temporarily stores the carbon in the animal’s body structure, delaying its return to the soil or atmosphere.
The efficiency of this transfer is low, with approximately ten percent of the carbon biomass transferred from one trophic level to the next. The majority of the remaining carbon is lost through respiration or expelled as waste.
Beyond direct consumption, the presence of animals can indirectly influence carbon storage through complex food web interactions known as trophic cascades. In some ecosystems, the presence of predators can limit herbivore populations, resulting in increased plant growth and greater retention of carbon in the plant biomass.
Trophic Cascades and Soil Carbon
This top-down control can lead to greater storage of carbon in the ecosystem, with some studies showing up to a 1.4-fold increase in plant carbon retention when carnivores are present. Furthermore, the diversity of large animals in terrestrial environments, such as the Amazon, has been correlated with higher concentrations of carbon sequestered in the soil. This suggests that varied feeding and movement patterns enhance the distribution of organic material, affecting where carbon settles within the landscape.
Returning Carbon to Reservoirs: Waste and Decomposition
Carbon stored in animal bodies and waste products eventually returns to major environmental reservoirs through two primary pathways: waste excretion and decomposition after death. Feces and urine represent a rapid injection of partially processed organic carbon and nutrients into the soil or water column. In grazing systems, animals such as wildebeest shift carbon from standing vegetation to the soil via their dung, promoting carbon storage underground.
The addition of animal waste, like manure, is a mechanism for increasing soil organic carbon content. Manure provides a balanced supply of carbon, nitrogen, and phosphorus, which stimulates the growth of microbial communities in the soil. These microbes convert unstable organic material into stable soil organic matter, a long-term form of terrestrial carbon sequestration.
When an animal dies, the carbon content of its biomass becomes available for decomposition. Bacteria and fungi break down the complex organic molecules, releasing some carbon as CO2 back into the atmosphere through their own respiration. The remaining fraction of the carcass is integrated into the soil structure, contributing to the pool of soil organic matter.
Specialized Roles of Marine Fauna
Marine animals play a governing role in the oceanic carbon cycle, which is distinct from terrestrial processes due to the unique chemistry of seawater.
Calcification and Sequestration
One specialized mechanism is calcification, where organisms like corals, mollusks, and plankton form shells or skeletons from calcium carbonate (CaCO3). These organisms extract dissolved carbon, primarily bicarbonate, from the water to build their hard structures, fixing the carbon into a solid form. When these organisms die, their CaCO3 shells sink, accumulating on the seabed to form deep-ocean sediments. This sinking process is a form of long-term carbon sequestration, removing the carbon from active exchange with the atmosphere for potentially millions of years.
The Biological Carbon Pump
The other major mechanism is the biological carbon pump, the biologically mediated export of organic carbon from the surface to the deep ocean. Zooplankton and larger swimming animals engage in diel vertical migration, feeding on phytoplankton in the sunlit surface waters at night and descending to deeper waters during the day. As they descend, they excrete dense fecal pellets that rapidly sink. These pellets, along with sinking carcasses, form a carbon-rich material known as “marine snow.” This fast transport mechanism bypasses the upper ocean layer where most carbon is recycled, pumping organic carbon to depths below 500 meters, sequestering it from the atmosphere for centuries.