Fauna is the collective term for all animal life in a given region, habitat, or time period. It covers everything from microscopic worms living between grains of soil to blue whales crossing entire ocean basins. The word comes from the Roman goddess Fauna, a deity of fertility and the earth, and scientists have been using it since at least the eighteenth century to catalog the animal communities of particular places. While “flora” refers to plant life, fauna encompasses every kingdom Animalia member, and how researchers subdivide it tells you a lot about how ecosystems actually work.
How Scientists Sort Fauna Into Categories
There is no single classification scheme for fauna. Instead, ecologists slice the concept in several overlapping ways depending on what question they are trying to answer. The most common approaches group animals by where they live, how big they are, or what relationship they have with humans.
Habitat-based labels are probably the ones you will encounter most often. Avifauna means the birds of a region. Entomofauna refers to its insects. Ichthyofauna covers fish. Herpetofauna groups reptiles and amphibians together. Each of these terms simply narrows the spotlight to one slice of animal diversity in a place.
Size-based categories show up most frequently in soil and marine science, where body length determines which organisms pass through which mesh sizes in a sampling sieve. Microfauna are the smallest animals, typically under a fraction of a millimeter. Mesofauna are mid-range invertebrates like mites and springtails. Macrofauna include earthworms, beetles, and other creatures visible to the naked eye. And megafauna, in modern usage, usually refers to large-bodied vertebrates weighing more than about five kilograms.
A third approach classifies fauna by proximity to people. Synanthropic species are those that thrive alongside human settlements, such as rats, pigeons, and raccoons. Wild fauna live independently of human infrastructure, while domesticated fauna have been selectively bred over generations for agriculture, companionship, or labor.
Soil Fauna and Nutrient Cycling
Some of the most ecologically powerful fauna on the planet are ones you will never notice. The animals living in soil drive nutrient cycling, break down organic matter, and shape the physical structure of the ground beneath your feet.
At the smallest scale, soil microfauna and mesofauna move nutrients between soil layers in ways that matter for agriculture. Research has shown that soil fauna can facilitate the transfer of substantially more nitrogen from one soil layer to another, with one study measuring up to 26% more nitrogen transfer from a natural donor soil to agricultural soil when fauna communities were present.
1Elsevier. Soil faunal community transfers nutrient cycling functionality and plant-parasitic nematode suppression from different depths of a natural soil to an agricultural soilLarger soil animals play a different but equally critical role. Earthworms and termites act as ecosystem engineers, physically restructuring the soil by burrowing, mixing organic material, and creating pore networks that allow air and water to penetrate. A study of agricultural systems across Sub-Saharan Africa found that earthworms were the primary drivers of stable soil aggregation, with fallowing and conservation tillage enhancing biogenic soil structure. The resulting continuous pore systems and well-developed aggregates improved both the physical and chemical properties of the soil.
2ResearchGate. Soil macrofauna functional groups and their effects on soil structure, as related to agricultural management practices across agroecological zones of Sub-Saharan AfricaMarine Fauna and Bioturbation
Oceans host their own size-based fauna categories, and the distinction between infauna (animals that live within the sediment) and epifauna (animals living on its surface) turns out to be ecologically important. Infaunal creatures, the burrowing worms, clams, and crustaceans buried in the seafloor, drive a process called bioturbation: the mixing and displacement of sediment particles. This keeps marine sediments oxygenated and supports the chemical processes that recycle nutrients through ocean ecosystems.
3PubMed Central. Bioturbation in a declining oxygen environment, in situ observations from WormcamThe effects are measurable and specific. In laboratory experiments, two common burrowing species doubled the exchange of dissolved substances between the water column and the sediment, increasing oxygen consumption and nutrient release from the seafloor.
4Aquatic Microbial Ecology. Influence of bioturbation by three benthic infaunal species on microbial communities and biogeochemical processes in marine sediment In deeper waters, researchers studying recovery from a severe seabed disturbance found that oxygen consumption at the seafloor was linked more closely to the burrowing behavior of the fauna than to how many individuals were present or how much they weighed.
5Deep Sea Research Part I: Oceanographic Research Papers. Bioturbation and faunal-mediated ecosystem functioning in a deep-sea benthic community recovering from a severe seabed disturbanceFauna That Run on Chemistry, Not Sunlight
Most food webs trace back to photosynthesis. Deep-sea hydrothermal vents break that rule entirely. At these volcanic fissures on the ocean floor, fauna depend on chemosynthesis, where bacteria convert chemicals like hydrogen sulfide into energy instead of relying on sunlight. The animals clustered around vents feed on these bacteria, either directly or through symbiotic partnerships.
A study of the gastropod Parvaplustrum wareni at the Piip submarine volcano in the Bering Sea illustrates just how completely vent fauna have decoupled from surface productivity. The snail’s body chemistry showed almost no trace of phytoplankton-derived nutrition. Instead, over 70% of its fatty acids were bacterial in origin, dominated by compounds associated with chemosynthetic bacteria. The organic matter created by chemosynthesis provides the energetic foundation for the entire vent community.
6Marine Ecology. The Fatty Acid Profile of the Deep‐Sea Gastropod Parvaplustrum wareni Indicates a Dominant Role of Chemosynthesis in the Nutrition of the Hydrothermal Vent Ecosystem (Piip Volcano)At the other thermal extreme, cryofauna live inside and beneath sea ice in the Arctic and Antarctic. The highest concentrations of these ice-dwelling animals, including nematodes, rotifers, and tiny crustaceans, occur in the bottom centimeters of the ice, where they graze on algae growing within the frozen matrix. When the ice melts seasonally, these organisms sink to the seafloor, becoming a food source for the bottom-dwelling community below.
7PubMed. Adaptation of Arctic and Antarctic ice metazoa to their habitatCave Fauna and the Costs of Darkness
Subterranean environments present a different set of extreme conditions: permanent darkness, limited food, and stable but often low temperatures. Cave fauna, sometimes called troglobionts when they are fully cave-adapted, have evolved a recognizable suite of traits in response. These include the loss of eyes and pigmentation, elongated sensory appendages, and slower metabolic rates. Researchers studying the cavefish Astyanax mexicanus and cave crustaceans have documented both the regressive traits (like eye loss) and the constructive ones (like enhanced sensory abilities) that characterize cave adaptation, making these animals valuable for understanding how evolution works when a major environmental input disappears.
8PubMed Central. Evolution and development in cave animals: from fish to crustaceansFauna as Ecosystem Engineers
Some animals reshape their habitats so profoundly that they change conditions for every other species around them. Beavers build dams that create wetlands. Woodpeckers carve cavities that become nesting sites for dozens of other species. Burrowing mammals aerate soil and create shelter for reptiles and invertebrates. Ecologists call these species ecosystem engineers, and their effect on biodiversity is consistently positive. A global meta-analysis found that ecosystem engineers increase local species richness by about 25% on average, with the strongest effects coming from species that create entirely new habitats or microhabitats rather than simply modifying existing ones.
9PubMed. Ecosystem engineering effects on species diversity across ecosystems: a meta-analysisPredators, Prey, and Trophic Cascades
Large predators do more than just kill prey. By suppressing herbivore and mid-level predator populations, they create ripple effects that cascade down the food web and ultimately shape vegetation, nutrient cycles, and the abundance of species far removed from the predator’s diet. This is the concept of a trophic cascade, and it is one of the most studied phenomena in ecology.
Large predators commonly limit populations of prey and smaller predators in ways that enhance biodiversity more broadly.
10PubMed. Novel trophic cascades: apex predators enable coexistence Network analysis of ecosystems under strong apex predator influence has shown that food webs become denser, more complex, and more top-down driven, with predator effects reaching all the way to soil organisms.
11Methods in Ecology and Evolution. Trophic cascades in 3D: network analysis reveals how apex predators structure ecosystemsThe relationship is not always simple, though. Apex predators both suppress and facilitate different prey populations depending on context, and the strength of their regulatory role varies with their own abundance and with human disturbance in the landscape.
12Biological Conservation. Human and apex predators shape lower trophic levels through top-down controlInsect Fauna and Pollination
Entomofauna, the insect community of a region, delivers some of the most economically visible ecosystem services. Pollinating insects alone are responsible for the sexual reproduction of more than 75% of flowering plants and roughly 35% of global crop production.
13International Journal of Bioresource Science. Ecosystem Services by Insect Pollinators, Their Crisis and Climate Change Effects Bees do the bulk of the work, but butterflies, flies, beetles, and moths all contribute. Pollination maintains the genetic diversity of wild plant populations, ensures fruit and seed formation in agricultural systems, and underpins rural economies worldwide.
The vulnerability of this service has become a serious concern. Widespread losses of pollinating insects have been documented across Britain, where wild insect populations provide a substantial contribution to crop productivity and the seed set of wildflowers.
14Nature Communications. Widespread losses of pollinating insects in Britain These declines are not confined to one country; similar patterns have been reported on multiple continents, driven by habitat loss, pesticide exposure, disease, and climate shifts.
Seed Dispersal by Animal Vectors
Animals move seeds. This sounds trivial, but the entire structure of many forests depends on it. Birds are among the most prolific seed dispersers, carrying fruit in their guts across open ground between isolated woodland patches. Research has confirmed that avian seed dispersal within and across deforested landscapes is the norm rather than the exception, and these birds play a key role in maintaining the connectivity of fragmented forest patches.
15Journal of Ecology. Avian seed dispersal out of the forests: A view through the lens of Pleistocene landscapesMammals contribute too, and not just the obvious fruit-eaters. Carnivores disperse seeds through endozoochory, swallowing fruit and passing viable seeds in their feces. Studies of temperate and tropical dry forests have shown that carnivores deposit large quantities of seeds and that passage through the gut can scarify seed coats, promoting germination without destroying viability.
16PubMed Central. Seed dispersal by carnivores in temperate and tropical dry forests Ungulates function as dispersal agents too, and their feeding habits can act as a filter on which plant species get moved where. Red deer and wild boar, for instance, carry a disproportionate share of non-forest plant species in their guts, which allows plants from open habitats to colonize forested areas.
17Journal of Vegetation Science. Functional traits of seeds dispersed through endozoochory by native forest ungulatesFreshwater Fauna as Living Water Quality Monitors
If you want to know whether a stream is healthy, you could test its chemistry. Or you could look at what is living in it. Aquatic macroinvertebrates, the insect larvae, snails, crustaceans, and worms found on and in stream beds, are widely used as bioindicators of water quality because their presence or absence integrates conditions over weeks and months, not just the instant a water sample is taken.
18PubMed Central. Investigating interactions between macroinvertebrate indices, water quality parameters, and stream quality classifications in a Wisconsin agricultural watershedThese organisms stay put. Their relatively long life cycles and sedentary habits mean that the species composition at a given site reflects both past and present ecological conditions, making them a more reliable record of cumulative stress than any single chemical snapshot.
19Aquatic Life and Ecosystems. Benthic Macroinvertebrates as Bioindicators of Aquatic Health: A Review and Comparative Analysis of Diversity Indices for Water Quality Assessment Pollution-sensitive orders like mayflies and stoneflies disappear first when conditions deteriorate, while tolerant groups like certain midges and worms persist. The ratio between sensitive and tolerant groups forms the basis of biological water quality indices used by environmental agencies around the world.
Biogeographic Realms and Endemic Fauna
Fauna are not distributed evenly. Mountain ranges, ocean basins, deserts, and climate gradients divide the planet into biogeographic realms, each with a characteristic set of species. Within those realms, many species are endemic, found nowhere else. Understanding these patterns matters for conservation because protecting a region’s unique fauna requires knowing where uniqueness is concentrated.
On land, the picture is shaped by geology and climate history. A study of the Afro-Arabian region identified distinct biogeographic realms for each vertebrate group: three for amphibians, four for reptiles and birds, and five for mammals, with hotspots of endemism concentrated along the Mediterranean coast, the Ethiopian highlands, and the Red Sea mountains.
20PubMed Central. Classifying biogeographic realms of the endemic fauna in the Afro‐Arabian regionIn the ocean, an analysis of about 65,000 marine species distinguished 30 distinct marine realms, with an average of 42% of species unique to each realm. Eighteen of those realms were on continental shelves and twelve were in the offshore deep sea, reflecting the tendency of deep-sea and open-ocean species to have wider ranges than coastal ones.
21PubMed Central. Marine biogeographic realms and species endemicityWhat Happens When Fauna Disappear
Defaunation, the loss of animal species and populations from ecosystems, is sometimes called the “empty forest” problem. A forest can look structurally intact from a satellite photograph while having lost most of the large animals that once maintained its internal dynamics. The consequences extend far beyond the missing animals themselves.
The most studied downstream effect involves carbon storage in tropical forests. Most research shows that defaunation reduces forest carbon stocks by up to 26% in neotropical and Afrotropical forests, primarily because large-seeded, animal-dispersed tree species decline when their dispersers vanish. Extrapolated across whole ecosystems, those local losses add up: an estimated 1.6 billion tonnes of CO₂ equivalent across the Brazilian Atlantic Forest and 4 to 9.2 billion tonnes across the Amazon over a century.
22PubMed. Defaunation impacts on the carbon balance of tropical forestsA global meta-analysis confirmed that real-world vertebrate loss harms forest regeneration, with especially strong negative effects when primates and birds disappear and seed dispersal breaks down.
23Nature Communications. Quantifying the impacts of defaunation on natural forest regeneration in a global meta-analysis Beyond the forest, defaunation disrupts processes at every timescale, from immediate changes in pollination and seed dispersal to medium-term shifts in plant community structure and, over the long term, evolutionary changes in the traits of remaining species.
24Biological Conservation. Ecological and evolutionary consequences of living in a defaunated worldInvasive Fauna and Cascading Surprises
When an animal species establishes itself outside its native range, the effects on resident fauna can be dramatic and hard to predict. The cane toad invasion in Australia provides one of the best-documented examples. Cane toads are toxic to native predators that try to eat them, and three species of monitor lizard experienced population declines of 71 to 97% in the years after the toad’s arrival. With those top predators gone, populations of some smaller animals exploded: one small lizard species increased by roughly 500%. More than a decade later, the system had still not stabilized, and two of the three monitor species were no longer detectable in extensive surveys, suggesting local extinction.
25PubMed. Chronic effects of an invasive species on an animal communityInvasive species also interact with other human-caused changes in unexpected ways. In New Zealand, suppressing invasive predators and retiring land from grazing, both aimed at helping native species, sometimes backfired. Vegetation changes following land retirement, combined with the release of invasive mice from predator control, blocked the recovery of native lizard populations.
26PubMed. Invasive mammals and habitat modification interact to generate unforeseen outcomes for indigenous fauna The lesson is that managing fauna requires thinking about the whole web of interactions, not just one target species at a time.
Urban Fauna and the Filter of City Life
Cities are not lifeless. They are selective. Urbanization acts as an ecological filter, favoring animal species with certain traits and excluding others. Among mammals, species with high reproductive output tend to succeed in urban environments across all groups studied, likely because the high mortality rates of city life, from vehicles, domestic pets, and limited habitat, put a premium on producing many offspring quickly.
27PubMed Central. One strategy does not fit all: determinants of urban adaptation in mammalsSome species go beyond merely tolerating cities. Rapid evolutionary adaptation has been documented in a growing number of urban animal populations, from changes in body size and behavior to shifts in reproductive timing. This adaptive capacity has facilitated the success of some native species in urban areas, but it has also allowed human pests and diseases to spread more quickly.
28PubMed. Evolution of life in urban environments Urban ecology has moved well past the assumption that cities are ecological wastelands. They are, instead, novel ecosystems with their own selective pressures, and the fauna that thrive in them are adapting on timescales short enough for researchers to watch it happen.
Parasitic Fauna and the Hidden Wiring of Food Webs
When people think of fauna, parasites rarely come to mind first. But parasitic animals, from tapeworms and flukes to parasitoid wasps and blood-feeding flies, represent a staggering share of animal diversity and biomass. More than that, they shape the structure of food webs in ways that free-living animals alone cannot explain. Parasites add links to food webs by connecting hosts at different levels, and growing evidence suggests they alter food-web properties including chain length, the density of connections between species, and the overall stability of the network.
29PubMed Central. Parasites in food webs: the ultimate missing linksA food web mapped without its parasites is missing some of the most important links. Including parasites typically increases the number of species in a food web by a quarter to a third, and the number of links between species rises even more steeply because many parasites use multiple hosts across their life cycles. Ecologists increasingly view parasites not as peripheral players but as fundamental architects of the communities they inhabit.
Megafauna and Forest Structure
Large-bodied herbivores, the megafauna, have effects on ecosystems that smaller animals cannot replicate. By browsing, trampling, and moving seeds, terrestrial vertebrate herbivores above about five kilograms can have disproportionate direct and indirect effects on forest structure, function, and nutrient cycling.
30Ecography. Assessing the role of megafauna in tropical forest ecosystems and biogeochemical cycles – the potential of vegetation models Elephants, for instance, knock down trees and create clearings that alter light availability and promote different plant communities. Large ungulates compact soil, spread nutrients in their dung, and maintain open grasslands that would otherwise succeed to forest. The loss of megafauna, whether through prehistoric extinction or modern poaching, fundamentally alters the habitat that remains. This is one reason the defaunation effects discussed earlier hit tropical forests so hard: the largest dispersers move the largest seeds, and when they vanish, the tree species that depend on them follow.