The global carbon cycle describes how carbon moves between the atmosphere, oceans, and land. While large-scale processes like plant photosynthesis and ocean absorption are well-known, the animal kingdom, or fauna, plays a significant role in regulating carbon flow and storage. Plants pull carbon dioxide from the air, and the ocean absorbs vast amounts, storing it for centuries. Animals mediate the cycle through consumption, physical movement, temporary storage in their bodies, and the final return of carbon to the environment, influencing the speed and stability of carbon cycling.
Trophic Transfer through Consumption
The most immediate function of animals is transferring carbon fixed by primary producers up the food web. Herbivores, or primary consumers, ingest plant and algal biomass, which is rich in organic carbon captured through photosynthesis. This consumption moves carbon from the standing stock of plant matter into the mobile carbon pool of the animal population.
Once consumed, the carbon is either assimilated into the animal’s body tissue or metabolized for energy. The metabolism of these compounds results in the immediate release of carbon dioxide back into the atmosphere through respiration, completing a short-term biological loop. This process occurs across all trophic levels, as secondary and tertiary consumers eat other animals, continuing the flow of carbon through the ecosystem.
The speed of this transfer affects ecosystem-wide carbon storage. Herbivores can either reduce carbon storage by consuming plant biomass or stimulate plant regrowth, enhancing carbon uptake. The balance between carbon assimilated into biomass and carbon immediately respired dictates the short-term fate of carbon in the food web.
Carbon Sequestration in Animal Biomass
Animals also serve as temporary reservoirs, sequestering carbon within their body structures, known as biogenic carbon storage. The duration of this storage varies widely, from the short lifespans of insects to the decades-long lives of large terrestrial or marine species. Long-lived animals, such as elephants or whales, accumulate substantial amounts of carbon over their lifetimes, effectively delaying its return to the atmosphere.
Calcification and Geological Storage
In marine environments, this sequestration is particularly pronounced through calcification. Organisms like mollusks, corals, and foraminifera extract dissolved inorganic carbon from seawater to build their shells and skeletons, which are composed of calcium carbonate. When these organisms die, their carbon-rich shells sink to the ocean floor. There, they can become compacted and eventually form sedimentary rock like limestone, representing a pathway to long-term geological carbon sequestration.
Blue Carbon and Whale Falls
Deep ocean carbon storage associated with marine animals is often referred to as “blue carbon.” A single great whale can sequester an estimated 33 tons of carbon dioxide over its life. When a whale dies, its massive carcass sinks to the abyssal plain, taking the accumulated carbon with it, where it can remain buried in the sediment for hundreds or even thousands of years.
Physical Redistribution and Ecosystem Movement
Animals physically move carbon across landscapes and through the water column, linking different carbon reservoirs.
The Whale Pump
This movement is exemplified by the “Whale Pump,” where whales feed in the deep ocean but excrete nutrient-rich feces near the surface. This waste, containing iron and nitrogen, fertilizes surface waters, stimulating the growth of phytoplankton. Since phytoplankton absorb atmospheric carbon dioxide for photosynthesis, the whale pump indirectly enhances carbon uptake from the air.
Terrestrial Engineers
In terrestrial ecosystems, large herbivores move carbon through migration and waste deposition. Wildebeest, for example, shift carbon from above-ground plant biomass to the soil via their dung, promoting soil carbon storage.
Other animals act as ecosystem engineers, physically mixing carbon-rich organic matter into the soil. Earthworms, for instance, ingest surface litter and sub-surface soil, creating casts and burrows that mix organic matter deeper into the mineral layers. This process promotes the formation of stable soil aggregates, which stabilizes the carbon and improves long-term soil storage resilience.
Seed dispersal by animals is another crucial form of physical transport for regenerating carbon-storing forests. Tropical forests with healthy populations of seed-dispersing animals can absorb significantly more carbon, as these animals ensure the survival and growth of large, carbon-rich trees.
The Final Step: Decomposition and Return
The biological carbon cycle is completed when an animal’s organic matter is broken down after death through decomposition. This process is carried out by detritivores, such as insects, bacteria, and fungi, which break down the complex carbon compounds stored in the animal’s tissues and waste.
During this breakdown, the carbon is mineralized, converting it back into inorganic forms. The primary inorganic form released is carbon dioxide, which returns to the atmosphere or water. In conditions where oxygen is scarce, such as deep in saturated soils or sediments, decomposition can also release methane, a potent greenhouse gas.
A portion of the carbon from the decaying biomass is incorporated into the soil as stable organic carbon. This stable fraction resists rapid decomposition and contributes to the long-term carbon reservoir within the soil structure. The entire process of decomposition ensures that the carbon and nutrients stored in animal biomass are recycled, making them available for future generations of organisms.