How Do Animals Affect the Amount of Carbon in the Atmosphere?

Carbon levels in the atmosphere are influenced by both natural processes and human activities. While fossil fuel combustion is a well-known contributor, animals also regulate carbon through respiration, feeding, waste production, and ecosystem interactions.

Understanding how different animal groups impact carbon cycling provides insight into broader climate dynamics. From land to ocean environments, animals contribute to both carbon release and sequestration in ways that shape atmospheric composition.

Animal Respiration As A Carbon Emission Source

Respiration allows animals to convert oxygen and organic molecules into energy, releasing carbon dioxide (CO₂) as a byproduct. This process occurs across all animal taxa, from microscopic invertebrates to large mammals, contributing to atmospheric carbon levels. While individual organisms emit small amounts of CO₂, the cumulative effect of global animal respiration is substantial. Terrestrial and marine animals collectively release billions of metric tons of CO₂ annually, though this is largely offset by carbon uptake through photosynthesis in plants and algae.

The rate of CO₂ production varies by species, metabolic rate, and environmental conditions. Endothermic animals, such as mammals and birds, have higher metabolic demands and exhale more CO₂ per unit of body mass than ectothermic species like reptiles and amphibians. A resting human exhales approximately 0.9 kg of CO₂ per day, while a cow can emit over 1,000 kg annually due to its larger size and continuous metabolic activity. Cold-blooded animals, which rely on external heat sources, have lower respiratory carbon outputs due to reduced energy expenditure.

Population density and biomass also influence the overall contribution of animal respiration. Large herbivore populations, such as wildebeests in the Serengeti or bison in North America, collectively release significant amounts of CO₂. Similarly, dense aggregations of marine organisms, including fish schools and krill swarms, contribute to oceanic carbon fluxes. However, these emissions are typically offset by the carbon sequestration capacity of their ecosystems, particularly in regions with abundant vegetation or phytoplankton activity.

Role Of Decomposition In Carbon Cycling

When organisms die, their bodies decompose, a process essential to carbon movement through ecosystems. Microorganisms, including bacteria and fungi, break down organic matter, releasing CO₂ and methane (CH₄). The rate of decomposition depends on temperature, moisture, and oxygen availability, influencing whether carbon is quickly released into the atmosphere or stored in soil and sediment. In oxygen-rich environments, aerobic decomposition produces mostly CO₂, while anaerobic conditions, such as wetlands or deep ocean sediments, favor methane production, a greenhouse gas with a greater heat-trapping effect than CO₂.

Not all carbon released during decomposition enters the atmosphere; some becomes part of the soil as organic carbon, forming humus, which can persist for centuries. In forests, fallen leaves, decaying wood, and dead animals contribute to soil organic matter, enhancing nutrient cycling and supporting plant growth. In grasslands, the decomposition of roots and herbivore waste products enriches soil carbon pools. The extent to which decomposition leads to carbon sequestration or atmospheric release depends on microbial activity, which is shaped by climate and land-use changes.

Decomposition rates vary across ecosystems, with significant implications for global carbon cycling. In tropical rainforests, warm temperatures and high humidity accelerate microbial breakdown, leading to rapid carbon turnover. In contrast, boreal forests and tundra regions experience slower decomposition due to cold temperatures, resulting in the accumulation of organic material in peatlands and permafrost. These carbon-rich deposits store vast amounts of carbon for millennia. However, rising global temperatures threaten to accelerate decomposition in these regions, potentially releasing large quantities of previously trapped carbon and amplifying climate change effects.

Herbivory And Vegetation Related Carbon Storage

Herbivores influence carbon storage by consuming vegetation, altering plant biomass, growth patterns, and nutrient cycling. Grazing animals can reduce plant material accumulation, limiting carbon sequestration in leaves, stems, and branches. However, herbivory can also stimulate plant regrowth, increasing photosynthetic activity and potentially enhancing carbon uptake. Some grassland ecosystems, particularly those with grazing-adapted species, demonstrate a balance where moderate herbivory promotes denser root systems, increasing soil carbon retention.

Herbivore activity also affects plant community composition, favoring species resilient to grazing or capable of rapid regrowth. In savannas, selective feeding by elephants and antelope suppresses tree saplings, maintaining open landscapes dominated by grasses. This shift influences carbon storage, as trees store more carbon per unit area than grasses. Similarly, in boreal forests, browsing by deer and moose limits tree regeneration, affecting long-term forest carbon dynamics. The impact on carbon sequestration depends on herbivore density, plant diversity, and climate conditions.

Herbivores contribute to carbon cycling through their waste, which returns organic material to the soil. Dung and urine accelerate microbial activity and decomposition, influencing soil carbon storage. In some cases, this enhances soil fertility, supporting plant productivity and further carbon capture. In wetlands and floodplains, herbivores like hippos transport nutrients between terrestrial and aquatic ecosystems, affecting the carbon balance of both. Overgrazing, however, can degrade soil, reduce vegetation cover, and increase carbon loss through erosion and oxidation of exposed organic matter.

Marine Fauna And Carbon Uptake

The ocean plays a key role in regulating atmospheric carbon, with marine animals influencing carbon cycling through feeding, respiration, and waste production.

Large Marine Vertebrates

Whales, sea turtles, and other large marine vertebrates contribute to carbon sequestration through movement, feeding, and biological processes. When whales dive and surface, they mix ocean layers, redistributing nutrients that promote phytoplankton growth. These microscopic plants absorb CO₂ through photosynthesis, forming the base of the marine food web. Whale feces, rich in iron and nitrogen, further stimulate phytoplankton productivity, a phenomenon known as the “whale pump.” Research published in Nature Communications (2014) estimated that sperm whales alone help remove tens of thousands of metric tons of CO₂ annually by enhancing phytoplankton growth.

Beyond nutrient cycling, the bodies of large marine vertebrates act as carbon reservoirs. When whales die, their carcasses sink to the ocean floor in a process called “whale fall,” sequestering carbon in deep-sea sediments for centuries. Similar processes occur with other large marine species, such as sea turtles and sharks, whose remains contribute to long-term carbon storage.

Schooling Fish And Carbon Dynamics

Aggregations of fish, such as anchovies, sardines, and herring, influence oceanic carbon cycling through feeding and excretion. These fish consume plankton, incorporating carbon into their bodies, and later release it through respiration and waste. Their fecal pellets, which sink rapidly, transport organic carbon from surface waters to the deep ocean, a process known as the “biological carbon pump,” helping remove CO₂ from the atmosphere and store it in deep-sea environments.

Schooling fish also contribute to carbon sequestration through diel vertical migration, moving between surface and deeper waters daily. By feeding near the surface at night and descending during the day, they transfer carbon to the ocean interior. Mesopelagic fish, which inhabit depths between 200 and 1,000 meters, play a particularly significant role in this process, collectively transporting millions of metric tons of carbon annually. This movement enhances carbon export, reducing the likelihood of CO₂ returning to the atmosphere.

Contribution Of Microscopic Organisms

Microscopic marine organisms, including phytoplankton, zooplankton, and bacteria, are fundamental to oceanic carbon cycling. Phytoplankton absorb CO₂ through photosynthesis, converting it into organic matter that supports marine food webs. This process, known as the “biological carbon pump,” sequesters an estimated 10 gigatons of carbon annually, according to Global Biogeochemical Cycles (2021).

Zooplankton, which feed on phytoplankton, further influence carbon dynamics by producing fecal pellets that sink to the ocean floor, transporting carbon to deep-sea sediments. Microbial decomposition of organic material determines whether carbon is stored long-term or released back into the atmosphere. Some bacteria facilitate organic matter breakdown, while others contribute to carbon sequestration by forming stable compounds in deep waters. These microscopic organisms shape the ocean’s ability to regulate atmospheric CO₂ levels.

Invertebrates And Soil Carbon Processes

Soil invertebrates, including earthworms, insects, and fungi-associated arthropods, contribute to carbon cycling through feeding, burrowing, and waste production. These organisms break down organic matter, determining whether carbon is stored in soil or released as CO₂. Their activity influences soil aeration and nutrient mixing, promoting humus formation, a carbon-rich material that can persist for centuries.

Earthworms play a significant role by consuming organic material and excreting nutrient-rich casts. These casts bind minerals and organic matter, stabilizing carbon within soil aggregates. Studies show earthworm activity can increase soil organic carbon content by up to 20% in some ecosystems. Similarly, termites in tropical environments break down plant material, releasing CO₂ while also contributing to stable soil organic matter. Despite their role in carbon release, termite mounds can trap organic material, acting as carbon reservoirs.