Animals are not decorative additions to landscapes; they are the working parts that keep ecosystems running. From the largest whales fertilizing ocean surface waters to earthworms tunneling through soil, animals drive nutrient cycling, shape plant communities, regulate disease, and even influence how much carbon the planet stores. Lose enough of those working parts, and the systems they maintain begin to unravel in ways that are sometimes obvious and sometimes eerily slow. The range of roles animals fill is broader than most people realize, and the evidence connecting animal activity to ecosystem health has grown dramatically in recent decades.
Predators and the Cascade That Flows Downward
When you remove a top predator from an ecosystem, the effects don’t stop at the next species down the food chain. They ripple through herbivores, plants, soil chemistry, and even the physical structure of the landscape. Ecologists call this a trophic cascade. Research in Australian ecosystems provides a clean example: where dingoes are rare, kangaroo populations swell, and the heavier grazing pressure those kangaroos exert measurably reduces vegetation cover and depletes soil carbon, nitrogen, and available phosphorus. Where dingoes are common, kangaroo numbers stay lower, vegetation stays healthier, and the soil nutrient pool stays intact.1PubMed Central. Removal of an apex predator initiates a trophic cascade that extends from herbivores to vegetation and the soil nutrient pool The finding that a predator’s presence can influence soil nutrients through this chain of effects illustrates just how far-reaching a single animal’s ecological role can be.
Network analyses of these cascading interactions confirm the pattern. Under strong apex predator influence, the web of species interactions in an ecosystem tends to be denser, more evenly distributed, and driven from the top down. Remove or suppress the predator, and the network frays: mesopredators (mid-level predators) and grazers start forming their own dominant modules, often with destabilizing effects on smaller species and plant communities.2Methods in Ecology and Evolution. Trophic cascades in 3D: network analysis reveals how apex predators structure ecosystems In short, top predators don’t just eat things. They organize the entire system.
Seed Dispersal and the Forests That Depend on It
Most tropical trees can’t just drop their seeds and hope for the best. They need animals to carry seeds away from the parent tree, often over considerable distances, to give seedlings a fighting chance at survival. Fruit-eating birds, primates, bats, and other vertebrates swallow fruit and deposit seeds elsewhere, sometimes kilometers away. In fragmented landscapes, where patches of forest are separated by farmland or cleared ground, these frugivores become the primary connectors between forest remnants. Research in fragmented tropical systems shows that larger, more mobile frugivores tend to dominate in the open matrix between forest patches, dispersing the seeds of taller, larger-seeded plant species across gaps that smaller animals would never cross.3PubMed Central. Frugivore-mediated seed dispersal in fragmented landscapes: Compositional and functional turnover from forest to matrix
The quality of seed dispersal also shapes the long-term recovery of degraded forests. In regenerating tropical forests, high connectivity between forest patches promotes animal movement and the arrival of large seeds and late-successional plant species from old-growth areas. Structurally complex forests support more diverse animal communities, and those diverse animal communities foster a more functionally diverse seed rain, meaning the incoming seeds represent a wider range of plant life strategies.4Journal of Ecology. Forest structure and connectivity drive the functional recovery of seed rain The implication is that forest recovery depends on functioning animal communities as much as it depends on the availability of seeds themselves.
When large seed-dispersing animals disappear from a forest, the genetic consequences show up quickly. Studies on palm species in defaunated forests have found that seedlings become more spatially clustered near parent trees, and the fine-scale genetic structure of the population tightens. Without animals carrying seeds away, offspring pile up in the same spot and genetic mixing declines.5PubMed Central. Defaunation Increases Clustering and Fine‐Scale Spatial Genetic Structure in a Small‐Seeded Palm Despite Remaining Small‐Bodied Frugivores Even when smaller frugivores remain in the system, they can’t fully compensate for the loss of the larger dispersers.
Seed dispersal isn’t limited to fruit-eaters. Scatter-hoarding rodents bury seeds in caches throughout the forest floor, and not every cache gets retrieved. The forgotten caches effectively plant new trees. Research on sympatric rodent species has shown that this behavior benefits both the rodents and the trees: cache owners recover their stored seeds at rates far exceeding pilferage, while the unclaimed seeds germinate and regenerate the forest.6PubMed Central. Does scatter-hoarding of seeds benefit cache owners or pilferers?
Pollination and the Diversity of the Pollinators That Provide It
Around three-quarters of the world’s flowering plants rely on animal pollinators to reproduce. When people think of pollination, they think of honeybees, but the reality involves a much wider cast: wild bees, hoverflies, beetles, butterflies, moths, bats, and birds all contribute. And the diversity of that pollinator community matters just as much as the number of individual visitors. Research on pollinator communities has shown that both fruit set and seed set increase with greater pollinator functional diversity. Social bees, for instance, visit flowers at roughly four times the rate of solitary bees or hoverflies, but solitary bees and hoverflies visit at different times of day and at different flower heights, filling niches the social bees miss.7PubMed Central. Diverse pollinator communities enhance plant reproductive success
This complementarity among pollinators has real consequences for food production. In agricultural systems, pollinator diversity directly increases both the quality and quantity of crop yield. In natural ecosystems, diverse pollinator communities help buffer plants against environmental and climatic disruptions, reducing pollen limitation even in bad years.8PubMed Central. Pollinator diversity benefits natural and agricultural ecosystems, environmental health, and human welfare Relying on a single managed pollinator species leaves both farms and wild landscapes vulnerable.
Moving Nutrients Across Land and Ocean
Animals are the primary vehicles for moving nutrients between ecosystems that would otherwise remain isolated from each other. Pacific salmon provide one of the best-documented examples. After spending years feeding in the ocean, salmon return to freshwater streams to spawn and die. Their carcasses deliver pulses of marine-derived nitrogen, phosphorus, and other nutrients into riverine and riparian ecosystems.9Ecosphere. Seasonal persistence of marine‐derived nutrients in south‐central Alaskan salmon streams Bears, eagles, and other scavengers drag carcasses into the surrounding forest, fertilizing soils and boosting the growth of streamside vegetation. Remote sensing studies have linked fluctuations in salmon abundance to changes in riparian forest productivity, suggesting that the effect is strong enough to detect from satellite imagery.10Ecosphere. Links between fluctuations in sockeye salmon abundance and riparian forest productivity identified by remote sensing
In the open ocean, sperm whales perform an analogous service. They feed on iron-rich prey in deep water and defecate iron-rich liquid feces near the surface, in the sunlit zone where phytoplankton grow. That iron fertilization stimulates new primary production and drives carbon export to the deep ocean.11PubMed Central. Iron defecation by sperm whales stimulates carbon export in the Southern Ocean The whales are acting as nutrient pumps, hauling limiting resources from one part of the ocean to another.
Zooming out to geological timescales reveals how much animal-driven nutrient transport has declined. Before the mass extinctions of large mammals during the late Pleistocene, the capacity of animals to move nutrients away from concentration patches was vastly greater than today. Current estimates suggest that animals now transport only about 8% of their pre-extinction capacity on land and roughly 5% in the oceans.12PubMed Central. Global nutrient transport in a world of giants The world’s nutrient distribution is, in a real sense, impoverished by the loss of its largest animals.
Engineers Underground and in the Water
Some animals reshape their physical environment so dramatically that ecologists call them ecosystem engineers. Beavers are the classic example. By damming streams, beavers create ponds that slow water flow, raise local water tables, filter sediment, improve water quality, and create wetland habitat used by dozens of other species. The hydrological, geomorphological, and ecological impacts are so profound that beaver reintroduction has become a restoration strategy across Europe and North America.13PubMed Central. Beaver: Nature’s ecosystem engineers
Underground, earthworms perform quieter but equally critical engineering. Anecic earthworm species that create deep, vertical burrows significantly increase water infiltration. One Vietnamese study found that the anecic species Amynthas zenkevichi increased water infiltration nearly 1.7-fold compared to soil without earthworms, thanks to vertical, well-connected burrow networks that extended through the entire soil column.14VNU Journal of Science: Earth and Environmental Sciences. Impact of Earthworms (Metaphire sp., Amynthas zenkevichi, and Amynthas robustus) on Soil Porosity and Water Infiltration Not all earthworm species have the same effect, though. Endogeic species that stay in the upper soil layers create shallower, more horizontal burrows that offer a smaller boost to infiltration, and that benefit largely disappears in compacted soils.15Pedobiologia. Decreased burrowing activity of endogeic earthworms and effects on water infiltration in response to an increase in soil bulk density The kind of earthworm matters as much as the presence of earthworms in general.
Dung beetles fill yet another engineering niche. By burying animal manure, they accelerate the breakdown of waste, reduce fly breeding habitat, and enrich deeper soil layers with potassium, phosphorus, and nitrogen. Controlled experiments with several dung beetle species have confirmed that they significantly increase soil nutrient content, though the magnitude varies by species.16Diversity. Contribution of Dung Beetles to the Enrichment of Soil with Organic Matter and Nutrients under Controlled Conditions In grazing systems, dung beetle populations are one of the unsung pillars of soil fertility.
Sanitation Services and Disease Regulation
Vultures may not win popularity contests, but they perform one of the ecosystem’s least glamorous and most vital jobs. By consuming carcasses rapidly and completely, vultures prevent the buildup of rotting animal tissue that would otherwise breed pathogens and attract less efficient scavengers like feral dogs and rats. Experts across the globe have reached consensus on the importance of vultures for waste recycling and disease control.17PubMed Central. The global contribution of vultures towards ecosystem services and sustainability: An experts’ perspective In regions where vulture populations have collapsed, the consequences have been tangible: carcass persistence times increase, and populations of substitute scavengers that carry rabies and other diseases expand.
Animal biodiversity also acts as a direct buffer against infectious disease through what ecologists call the dilution effect. When communities of potential hosts are diverse, the best hosts for a given pathogen make up a smaller fraction of the total. High-quality hosts for parasites and pathogens tend to persist when diversity is lost, so species-poor communities often become hotspots of transmission.18PubMed Central. Dilution effects in disease ecology A meta-analysis covering over 200 effect sizes across 61 parasite species found broad evidence that host diversity inhibits parasite abundance, through mechanisms like regulating populations of the most susceptible hosts and interfering with transmission pathways.19PubMed Central. Biodiversity inhibits parasites: Broad evidence for the dilution effect The practical takeaway is that biodiversity loss doesn’t just eliminate species; it can make the survivors sicker.
Grazing Animals as Fire Managers
Wildfire risk is shaped partly by how much fuel is available to burn, and large herbivores are remarkably effective at reducing that fuel. Across African savannas, grazing herbivores substantially reduce both standing herbaceous biomass and the area burned by fire. The relationship scales predictably with herbivore density: an increase in grazing herbivore biomass equivalent to about one zebra per square kilometer results in roughly a 53 kg per hectare reduction in standing grass and a measurable drop in burned area.20PubMed. Grazing herbivores reduce herbaceous biomass and fire activity across African savannas
This isn’t limited to Africa. A broad body of evidence confirms that large herbivores reduce fuel loads in ecosystems worldwide. A meta-analysis of thousands of records from mostly semi-arid and arid Australian ecosystems found that livestock reduced plant biomass by an average of about 40%. In mixed-conifer forests of the northwestern United States, understorey biomass was higher inside fenced areas that excluded ruminants than outside them.21PubMed Central. Can trophic rewilding reduce the impact of fire in a more flammable world? As agricultural land abandonment leads to shrub and tree encroachment in many parts of the world, the resulting buildup of flammable biomass increases wildfire risk. Targeted grazing by large herbivores is increasingly recognized as a management strategy to reduce fuel loads and mitigate wildfires.22Journal of Applied Ecology. Effects of large herbivores on fire regimes and wildfire mitigation
Animals, Carbon Storage, and Climate
The connection between animals and the global carbon budget is less intuitive than the other roles described above, but the evidence is striking. In tropical forests, many of the largest trees with the highest wood density depend on large vertebrates to disperse their seeds. Simulations of what happens when those large-seeded, animal-dispersed trees disappear from Atlantic Forest communities found that even the loss of a small proportion of such trees could significantly erode carbon storage.23PubMed Central. Defaunation affects carbon storage in tropical forests Because these trees tend to have denser wood, their loss isn’t compensated by the smaller-seeded species that replace them. Hunting and habitat loss that eliminate large frugivores can therefore indirectly undermine a forest’s climate value.
In the ocean, burrowing animals that live in soft sediments play their own role in carbon dynamics. Invertebrates that mix and oxygenate sediments influence whether organic carbon gets broken down and released back into the water or remains buried for long periods. Shifts in these benthic communities, driven by ocean warming, could alter the balance between carbon release and long-term storage.24Limnology and Oceanography Bulletin. Benthic Invertebrates on the Move: A Tale of Ocean Warming and Sediment Carbon Storage Ocean acidification and low oxygen conditions have also been shown to change how these sediment-dwelling communities process carbon, increasing organic carbon burial in some scenarios but disrupting normal biogeochemical cycling in others.25PubMed. Ocean acidification and hypoxia alter organic carbon fluxes in marine soft sediments
Coral Reefs and the Fish That Keep Them Alive
Coral reefs face a well-known threat from rising water temperatures, but one of the less discussed threats is the loss of herbivorous fish. Algae constantly compete with coral for space on a reef, and herbivorous fish are the main force keeping algae in check. Without them, reefs can undergo a phase shift from coral-dominated to algae-dominated systems, which are far less productive and support fewer species. A global assessment of coral reef herbivorous fish found that the browser functional group, the fish that consume macroalgae and can help prevent these phase shifts, appears to be the most susceptible to fishing pressure.26PubMed Central. Global assessment of the status of coral reef herbivorous fishes: evidence for fishing effects Overfishing these particular species can tip a reef past the point of recovery even if water conditions are otherwise favorable.
The Dollar Value of Pest Control by Bats
If the ecological arguments feel abstract, the economic ones sharpen the picture. Insectivorous bats provide pest control services that translate directly into money saved by farmers. In south-central Texas, Brazilian free-tailed bats prey heavily on agricultural pest moths, and research has confirmed that they play a vital role in protecting cotton crops from damage while reducing the costs of pesticide use. About 80% of the annual value of this pest control service accrues early in the growing season, before farmers would normally begin their first pesticide applications.27Frontiers in Ecology and the Environment. Economic value of the pest control service provided by Brazilian free-tailed bats in south-central Texas
In Australia, the numbers have been quantified even more precisely. Insectivorous bats consuming the pest moth Helicoverpa armigera provide an estimated benefit of roughly $100 to $125 per hectare in dryland cotton and $285 to $360 per hectare in irrigated cotton, amounting to about $64 million annually across the Australian cotton industry. Bat populations remove an estimated 77 to 119 tonnes of pest moths from cotton crops in an average growing season.28Ecosystem Services. Insectivorous bats provide significant economic value to the Australian cotton industry These figures make it clear that animals are providing services that human technology would struggle to replace at comparable cost.
Animals as Microbial Couriers
One of the more recently appreciated roles animals play is in dispersing microbial communities across landscapes. As animals move through their environment, they ingest soil and water microbes that pass through the selective filter of the gut. What comes out the other end is a blend of the animal’s own gut microbiota and environmentally acquired microbes, deposited into a new location. Researchers have proposed that animals function as “mobile bioreactors,” and the coalescence of animal and environmental microbiomes at deposition sites can alter nutrient cycling, organic matter decomposition, and the broader web of trophic interactions in ways that are only beginning to be mapped.29PubMed. Bioreactors on the Move: How Animals Contribute to Microbial Community Coalescence and Shape Ecosystem Function This means that even the seemingly incidental act of an animal defecating on a hillside is reshaping the invisible microbial landscape that underpins soil and plant health.
How Tropical Forests Lose Resilience Without Their Animals
The loss and degradation of tropical forests doesn’t just reduce species numbers. It changes which kinds of animals disappear first, and those losses are not random with respect to the traits that matter for ecosystem function. Research on tropical bird communities has found that habitat loss disproportionately eliminates species with functional traits linked to seed dispersal and insect herbivory regulation. The disruption of these trophic interactions has direct implications for forest structure and the capacity of human-modified tropical forests to bounce back from disturbance.30PubMed Central. Using avian functional traits to assess the impact of land-cover change on ecosystem processes linked to resilience in tropical forests An ecosystem with fewer seed dispersers regenerates more slowly. An ecosystem with fewer insectivores faces heavier herbivory on young plants. The result is a forest that looks intact on a satellite image but has quietly lost much of its ability to repair itself.