Animals are not passengers in the ecosystems they inhabit; they are structural forces that hold those systems together. From insects recycling dead plant matter in the soil to whales fertilizing the ocean surface with their waste, animals drive nutrient cycles, regulate climate, control pests, disperse seeds, and shape the physical structure of habitats in ways that directly sustain human food production, health, and economic activity. The scale of their influence is often invisible until they disappear, and the consequences of losing them are more severe than most people realize.
How Animals Structure Ecosystems
One of the clearest demonstrations of animal importance is what happens at the top of a food chain. When apex predators like wolves or dingoes are present and active, the web of species interactions below them becomes denser and more complex. Remove or suppress the top predator, and the network frays. Populations of mid-level predators balloon, prey species get hammered unevenly, and the whole community simplifies. Research modeling the influence of dingoes in Australian ecosystems found that under strong apex predator influence, ecological networks were more even and top-down driven, with tighter modules of interaction between species; under weak predator influence, the network became fragmented and dominated by unchecked mesopredators and grazers.1Methods in Ecology and Evolution. Trophic cascades in 3D: network analysis reveals how apex predators structure ecosystems That structural role is not something another species can easily fill once the predator is gone.
Animals also reshape their environments in physical, tangible ways. Beavers build dams that create wetlands. Burrowing mammals aerate soil and create microhabitats. Elephants knock down trees and open forest canopy, allowing grassland species to thrive. Ecologists call this “ecosystem engineering,” and a large meta-analysis of 122 studies found that animal ecosystem engineers increase species richness by about 25% on average across ecosystems globally.2PubMed. Ecosystem engineering effects on species diversity across ecosystems: a meta-analysis That is not a subtle statistical blip. It means ecosystems with active animal engineers support roughly a quarter more species than they would otherwise. The effect is overwhelmingly positive, making engineering one of the most broadly facilitative processes in ecology.
The Soil Workforce
Beneath the surface, animals are doing some of the most critical work in any terrestrial ecosystem. Earthworms, beetles, mites, springtails, and other soil invertebrates break down dead plant material, mix organic matter into deeper soil layers, and release nutrients that plants need to grow. For a long time, the assumption was that these creatures merely softened up dead leaves for microbes to finish off, but research has shown that many invertebrates produce their own enzymes capable of breaking down organic matter directly, making them true decomposers in their own right.3PubMed. The impact of invertebrate decomposers on plants and soil
The practical result is enormous. Earthworms and other microbial-feeding invertebrates speed up the release of plant-available nutrients like nitrogen and phosphorus into the soil solution, directly boosting plant growth.4Grass and Forage Science. The invertebrate fauna of grassland and its influence on productivity. III. Effects on soil fertility and plant growth Soil fauna also play a central role in the nitrogen cycle, influencing decomposition, mineralization, and even the conversion of nitrogen between different chemical forms that plants can or cannot absorb.5Geoderma. Uncovering the functional roles of soil fauna in nitrogen cycling and agricultural sustainability Without these animals, the nutrient economy of soil grinds to a crawl. Dead plant matter piles up, nutrients stay locked in forms plants cannot access, and fertility declines. Healthy soil is not just dirt with the right chemistry; it is an animal-maintained system.
Animals as Nutrient Highways
Before humans dramatically reduced populations of large animals, the planet had a far more active system for moving nutrients across long distances. Large marine mammals dove to the deep ocean to feed and then returned to the surface to breathe, defecate, and die, carrying nutrients like nitrogen and phosphorus upward. Seabirds and fish that migrate between ocean and freshwater carried those same nutrients onto land. Large terrestrial animals then spread nutrients inland, away from river corridors and coastlines, fertilizing continental interiors. This interconnected system recycled nutrients from the ocean floor to mountaintops.6PubMed Central. Global nutrient transport in a world of giants
The decline of large animals has disrupted this conveyor belt. The loss of megafauna from landscapes around the world has weakened nutrient redistribution at every scale, from local soil enrichment by grazing herds to the oceanic upwelling driven by whale populations. The cascading effects of losing large herbivores alone include altered vegetation structure, disrupted water cycles, changed fire regimes, and impoverished nutrient cycling.7PubMed Central. Collapse of the world’s largest herbivores Restoring these populations is not just a conservation ideal; it is a practical strategy for rebuilding ecological processes that human infrastructure cannot replicate at scale.
Carbon Storage and Climate Regulation
Animals contribute to climate regulation in ways that do not always make it into carbon policy discussions. One of the most compelling examples involves large fruit-eating animals in tropical forests. Many of the biggest, densest-wooded trees in the tropics depend on large vertebrates to disperse their seeds. When those animals are hunted out, the trees that replace them tend to be smaller and lighter-wooded, storing less carbon. In Atlantic Forest communities in Brazil, simulations showed that the local extinction of trees dependent on large fruit-eaters could significantly erode carbon storage, even when only a small fraction of large-seeded trees were lost.8PubMed Central. Defaunation affects carbon storage in tropical forests Similar work in Southeast Asian forests found that tree species dispersed by large-bodied frugivores accounted for roughly a third of total above-ground carbon in a study plot, and simulated defaunation reduced carbon storage by several percent.9Scientific Reports. Defaunation of large-bodied frugivores reduces carbon storage in a tropical forest of Southeast Asia A few percent of a tropical forest’s carbon stock, scaled across the tropics, is an enormous amount of CO₂ that stays in the atmosphere instead of in wood.
In the ocean, marine vertebrates contribute to carbon cycling through multiple pathways. Whales store carbon directly in their massive bodies, and when they die, their carcasses sink to the deep ocean floor, locking that carbon away for centuries. While alive, their iron- and nitrogen-rich excrement fertilizes surface waters and may stimulate phytoplankton growth, which captures atmospheric CO₂.10PubMed. Whales in the carbon cycle: can recovery remove carbon dioxide? More broadly, marine vertebrates influence the capacity of ocean ecosystems to fix, store, and transport carbon downward into sediments.11One Earth. Integral functions of marine vertebrates in the ocean carbon cycle and climate change mitigation Sediment analysis off the coast of Argentina found that up to about 7% of the organic carbon in coastal sediment cores carried isotopic signatures from the large marine vertebrates that inhabit the area, suggesting these animals make a real, measurable contribution to coastal carbon stocks.12Frontiers in Marine Science. Carbon and nitrogen stocks in sediment at Península Valdés Biosphere Reserve: novel insights into the potential contribution of large marine vertebrates to carbon sequestration
Back on land, large herbivores influence climate through a different mechanism: fire. By consuming grasses and shrubs that would otherwise dry out and become fuel, grazers reduce the intensity and spread of wildfires. Large vertebrates also alter vegetation density and the soil litter layer, all of which regulate fire frequency and severity.13PubMed Central. Can trophic rewilding reduce the impact of fire in a more flammable world? A review of the evidence concluded that herbivore-driven fuel reduction is a promising management strategy for wildfire mitigation, with practical applications for land managers looking for alternatives to mechanical clearing or prescribed burns.14Journal of Applied Ecology. Effects of large herbivores on fire regimes and wildfire mitigation In a warming world where fire seasons are lengthening, this is not a quaint ecological fact. It is a management tool.
Protecting Crops Without Chemicals
Animals provide pest control services worth staggering sums of money, often without farmers noticing. Insectivorous bats are among the most effective and least appreciated agricultural allies. In corn fields, bat exclusion experiments demonstrated that bats suppress herbivorous insect populations enough to reduce larval damage, and they also indirectly reduce the growth of pest-associated fungi and the toxic compounds those fungi produce. The estimated value of bat pest suppression on corn alone exceeds a billion dollars globally.15PubMed Central. Bats initiate vital agroecological interactions in corn
The benefits extend well beyond corn. In organic apple orchards, bat activity cut the number of trees damaged by codling moth by about a third and halved the weight of damaged apples per tree.16Journal for Nature Conservation. A bat a day keeps the pest away: Bats provide valuable protection from pests in organic apple orchards In tropical cacao agroforestry, excluding both bats and insectivorous birds led to a 31% drop in final crop yield, even though secondary predators like ants and spiders partially filled the gap.17PubMed. Bats and birds increase crop yield in tropical agroforestry landscapes When you hear debates about reducing pesticide use, animals like bats and birds are doing part of that work for free. Losing them means either accepting larger crop losses or spending more on chemical and mechanical alternatives, both of which carry costs that ripple through food systems.
Animals and Human Health
The relationship between animals and human health is more complex than most people assume, running in directions that seem contradictory until you look closely. On one hand, animals are the source of most emerging infectious diseases; the majority of new pathogens that jump to humans come from wildlife. On the other hand, biodiversity itself appears to buffer against disease transmission. In intact, species-rich ecosystems, the animals that tend to be efficient reservoirs for zoonotic pathogens make up a smaller share of the community, diluted by non-reservoir species. When biodiversity declines, those reservoir hosts become relatively more abundant, and human exposure to both new and established zoonotic pathogens increases.18PubMed Central. Impacts of biodiversity and biodiversity loss on zoonotic diseases In other words, the best defense against animal-borne disease is not fewer animals but a full, functioning community of them.
There is also evidence that early-life exposure to farm animals and the microbe-rich environments they create helps train the human immune system. Children raised on farms with regular animal contact show lower rates of allergies and asthma compared to non-farm children. Research has found that farm children have increased expression of genes involved in recognizing microbial compounds, and that contact with farm animals and their products contributes to this protective effect, which can persist into adulthood.19PubMed Central. 99th Dahlem Conference on Infection, Inflammation and Chronic Inflammatory Disorders: Farm lifestyles and the hygiene hypothesis The microbe-rich environment of farms, shaped in large part by the animals living there, appears to promote maturation of the lung and gut microbiome in early life, educating the immune system in ways that reduce the risk of allergic disease.20PubMed Central. The role of LPS and CpG in the farm effect against allergies, and beyond This is not a recommendation to go hug livestock for health reasons, but it does illustrate how profoundly animal presence shapes the microbial environment that our immune systems develop within.
Animals have also given medicine some of its most important drugs through an unlikely route: venom. Animal venoms contain highly potent, precisely targeted molecules that evolution has honed over millions of years, and pharmaceutical researchers have learned to repurpose them. The blood pressure drug captopril was developed from a compound found in the venom of a Brazilian pit viper. The painkiller ziconotide comes from cone snail venom. These are not curiosities; they are frontline medications, and ongoing research continues to explore venom-derived compounds for conditions ranging from blood clotting disorders to chronic pain.21PubMed Central. Animal Venom in Modern Medicine: A Review of Therapeutic Applications Every species extinction is, among other things, the permanent loss of a biochemical library that we have barely begun to read.
When Functional Redundancy Runs Out
A common assumption is that ecosystems have enough built-in backup that losing one species does not matter much because another can fill its role. This works up to a point. On large islands with rich communities, historical extinctions removed certain functional types of animals entirely, but enough similar species remained that the overall diversity of ecological roles did not collapse. Small islands were not so lucky. Some lost up to two-thirds of their native functional groupings, meaning whole categories of ecological work simply stopped being done.22PubMed Central. Defaunation and species introductions alter long-term functional trait diversity in insular reptiles
The troubling finding is that this redundancy is being eroded and not replenished. Introduced species, which now dominate many island faunas, do not fill the functional roles that native species occupied. The buffer is thinner than it used to be, and the next round of extinctions will hit harder. This pattern is not unique to islands; it applies anywhere animal communities are being simplified. Each lost species reduces the margin of safety, and at some point, the ecosystem crosses a threshold where a loss that would previously have been absorbed instead triggers functional collapse.
Engineering Borrowed From Animals
Beyond their ecological and health roles, animals have served as blueprints for human technology in ways both ancient and cutting-edge. The field of biomimicry takes solutions that evolution has refined over hundreds of millions of years and applies them to engineering problems. Marine animals have been a particularly rich source of inspiration because they have evolved to handle challenges that engineers also face: moving efficiently through fluid, sensing in low-visibility environments, and building structures that are both strong and lightweight. Researchers have drawn on marine biology to develop innovations ranging from lightweight skeletal structures modeled on sea sponges, to efficient swimming kinematics for underwater vehicles, to advanced sensing systems inspired by the lateral line organs of fish.23Ocean Engineering. Translating marine biology into engineering: A review of biomimicry and its applications These are not niche applications. Bio-inspired designs are entering fields from aerospace to medicine, and the catalog of species available to study is shrinking as biodiversity declines.
Putting a Dollar Value on Wildlife
Economists have tried to quantify what animals are worth in terms of the services they provide, and the numbers are large enough to reshape how people think about conservation spending. Animals deliver pollination, pest control, seed dispersal, and nutrient cycling services that would cost enormous sums to replace artificially.24Natural Resource Management and Policy. Animals and Ecosystem Services Some efforts have tried to assign dollar values to specific species. One analysis estimated that protecting African forest elephants, whose trampling and selective feeding maintain forest structure in ways that promote carbon-dense tree species, would be worth roughly 36 billion dollars across tropical Africa in carbon capture services alone.25Ecology and Society. The values of wildlife revisited
These valuations are imperfect and debated, but they serve an important function: they translate ecological processes into a language that policymakers and financial institutions understand. When a species can be linked to a quantifiable economic service, especially one with an existing market like carbon trading, the business case for conservation becomes legible to people who would otherwise treat it as a purely moral concern. The risk, of course, is that species without obvious economic value get deprioritized, but as the examples throughout this article suggest, the number of species with “no economic value” shrinks rapidly the more closely you look.
The One Health Framework
The realization that human health, animal health, and environmental health are deeply entangled has given rise to a formal approach known as One Health. The framework argues that you cannot effectively address challenges like pandemic preparedness, antimicrobial resistance, or food safety without simultaneously considering wild and domestic animal populations and the ecosystems they live in.26PubMed Central. One Health: A Holistic Approach to Tackling Global Health Issues It is an interdisciplinary model that aims to sustainably advance the health of all three domains together rather than optimizing one at the expense of the others.27PubMed. One Health
In practice, One Health means that veterinarians, ecologists, and physicians sit at the same table when governments make policy about disease surveillance, land use, or agricultural development. It is a direct institutional acknowledgment that animals are not a separate category from human welfare but are woven into the same fabric. Whether the conversation is about zoonotic disease spillover, soil fertility maintained by invertebrates, or carbon locked in forests by seed-dispersing primates, the underlying message is consistent: when animal populations are healthy and their ecological roles intact, the systems that humans depend on work better. When they are degraded, the costs show up in ways ranging from crop failures to pandemics, and fixing them after the fact is always harder and more expensive than preventing the damage.