Wildlife, in the broadest scientific sense, refers to all organisms living in a wild state, meaning free from direct human management or domestication. That includes far more than the deer, eagles, and wolves that come to mind first. International scientific conventions and conservation organizations generally define wildlife to encompass animals, plants, and fungi that exist in their natural conditions. But the boundaries of the term are surprisingly contested, shaped by legal traditions, ecological context, and the growing realization that “wildness” itself is not a binary switch but a spectrum.
More Than Animals
Ask most people what wildlife means and they will describe vertebrates: bears, birds, maybe fish. The scientific and policy definition is considerably wider. A 2023 analysis of how legal systems around the world define the term found that international conventions and organizations tend to include all animals, plants, and fungi living in the wild under the wildlife umbrella.1Biological Conservation. What is wildlife? Legal definitions that matter to conservation The United Kingdom’s Countryside Access Regulations, for instance, define wildlife as “any species of animal and bird which is ordinarily resident, or plant that ordinarily grows, in England (and Wales) in a wild state.”1Biological Conservation. What is wildlife? Legal definitions that matter to conservation
Fungi are a good illustration of why this broad scope matters. Wild fungi decompose organic matter and maintain soil fertility, form mycorrhizal partnerships with plant roots that help trees absorb water and nutrients, and even enhance carbon storage in forests.2Conservation Letters. A way forward for wild fungi in international sustainability policy Excluding them from the definition of wildlife would mean ignoring organisms that hold ecosystems together underground.
Invertebrates present an even more dramatic case. In California, agricultural groups challenged the state’s authority to protect certain insect species under the California Endangered Species Act, arguing that when the law was drafted in 1970, insects were not specifically named. The legal resolution hinged on a creative piece of statutory language: the California legislature had defined “fish” to include “wild fish, mollusk, crustacean, invertebrate, amphibian, or part, spawn, or ovum of any of those animals,” and that definition applied to the entire Fish and Game Code.3American Entomologist. Terms of Art and Terms of Arthropods So insects were protected as “fish.” The episode reveals how legal language can lag behind scientific understanding, and how the public definition of wildlife is often narrower than the ecological reality.
What “Wild” Actually Means
If wildlife is defined by living in a wild state, you need a working idea of what “wild” means. The intuitive answer is “not domesticated,” but domestication and wildness are endpoints on a long continuum, not a clean either/or.
Domestication reshapes animals over generations through altered selection pressures. Brain gene-expression studies comparing domesticated animals with their wild counterparts, including dogs versus wolves, pigs versus wild boars, and domesticated versus wild rabbits, found that fewer than one percent of expressed genes differed between each pair. There was almost no overlap in which genes changed across different domestication events, suggesting that domestication does not follow a single genetic script.4PLoS Genetics. A Comparison of Brain Gene Expression Levels in Domesticated and Wild Animals In plants, researchers have identified specific “domestication genes” that were crucial to the transition from wild to cultivated forms, but animal domestication appears to have a more diffuse, polygenic background with no obvious single genes driving the process.5PubMed Central. Molecular genetic variation of animals and plants under domestication
Then there is feralization, when domesticated animals escape human management and begin living wild again. Feral horses, feral cats, feral pigs, and feral dogs occupy habitats worldwide. Are they wildlife? Ecologically, they function as wild organisms: they forage, reproduce, compete, and get eaten by predators. But domestication and feralization powerfully change selection pressures and population structure, and those changes can affect how well an animal actually functions in a natural environment.6PubMed Central. Maladaptation in feral and domesticated animals Feral animals may carry genetic legacies of domestication, such as reduced fear responses or altered metabolic profiles, that make them behave differently than truly wild populations. Whether you call them wildlife depends on whether your definition emphasizes genetics, behavior, or simply living outside human control.
Captivity and the Loss of Wildness
Even animals that were never domesticated can lose aspects of their wildness in captivity. Zoos, aquariums, and breeding programs house genetically wild species, but the captive environment changes what those animals are. Captive settings drastically alter the selection pressures an animal experiences: food arrives on schedule, predators are absent, space is limited, and social groups are artificially managed. These changes can reshape behavior, physiology, and even body shape within surprisingly few generations, creating fitness costs if the animals are later released into the wild.7PubMed. The phenotypic costs of captivity
One particularly revealing window into this process comes from gut microbiomes. A meta-analysis comparing the intestinal bacteria of wild and captive vertebrates found that captive animals pick up bacterial taxa commonly associated with the human gastrointestinal tract, genera like Oribacterium, Sarcina, and Subdoligranulum, while wild animals harbored more specialized microbes tied to their natural diets. Contact with humans, or simply living in a more human-influenced environment, seemed to shift the microbial communities. These human-associated bacteria did not become the dominant organisms in captive animals’ guts, but they were significant drivers of microbial turnover between wild and captive populations.8Scientific Reports. Diversity and compositional changes in the gut microbiota of wild and captive vertebrates: a meta-analysis In a sense, captive animals become microbially less wild even while remaining genetically wild. The finding raises an uncomfortable question for conservation breeding programs: if you change an animal’s internal ecosystem, have you changed the animal?
Urban Wildlife and the Myth of Pristine Nature
You do not need to visit a national park to find wildlife. Cities are functioning ecosystems, and the species living in them experience the full suite of biological processes, including evolution, that have occupied biologists for centuries in wilder settings. Urban environments provide intense and novel evolutionary pressures: artificial light, noise, pollution, fragmented habitat, new food sources, and altered predator communities.9PubMed Central. Adaptive evolution in urban ecosystems
Coyotes in Chicago, peregrine falcons nesting on skyscrapers, foxes in London, and bobcats in Los Angeles suburbs are all wildlife by any reasonable definition. They are genetically wild, reproduce without human management, and survive on their own. But their evolutionary trajectories are diverging from rural populations of the same species. Urban populations face different diseases, different diets, and different social structures. Some researchers now think of cities as massive, unplanned evolutionary experiments, generating novel selection pressures that produce measurable genetic and behavioral changes within decades rather than millennia.
The existence of thriving urban wildlife undermines a common assumption baked into the word “wild”: that wildlife belongs in wilderness, defined as land untouched by people. In practice, very little of Earth’s surface has been untouched by human activity for very long. If wildlife required pristine habitat to qualify, the category would be vanishingly small. The scientific understanding treats wildness as a property of the organism’s relationship to human management, not a property of the landscape it happens to stand on.
When Wild Meets Domestic at the Genetic Level
Hybridization between wild animals and their domestic relatives creates one of the sharpest definitional headaches in conservation. The Ethiopian wolf, one of the world’s rarest canids, hybridizes with domestic dogs that share its habitat in the Ethiopian highlands. The Scottish wildcat similarly interbreeds with domestic and feral cats to the point where genetically pure wildcats may no longer exist in parts of Scotland. In both cases, conservation assessments have concluded that the appropriate response is to eliminate or restrict the hybrids in order to protect the genetic integrity of the wild species.10PubMed. Evaluating hybrid speciation and swamping in wild carnivores with a decision-tree approach
This is a genuinely difficult area. A hybrid animal that looks, behaves, and lives like a wild organism but carries some percentage of domestic DNA occupies ambiguous territory. Is a cat that is 85 percent Scottish wildcat and 15 percent domestic still wildlife? What about 50/50? Conservation biology has had to develop decision frameworks for these cases, weighing the risk of genetic swamping, which can effectively erase a wild species, against the reality that hybridization is also a natural evolutionary process. The answer often depends less on biology and more on what you are trying to protect.
Wildlife Health Is Not Domestic Animal Health
One area where the distinction between wild and domestic animals has practical consequences is health management. In human medicine and veterinary science, the goal is straightforward: eliminate or reduce disease. For domestic livestock, disease management serves both productivity and public health. Wildlife health operates under fundamentally different logic.
Free-roaming wildlife in natural environments interact with pathogens as part of complex ecological networks. The goal of wildlife health management is not to eradicate disease from wild populations but to maintain natural disease dynamics that allow pathogens to play their regulatory role in ecosystems.11One Health. An operational framework for wildlife health in the One Health approach A wolf population occasionally thinned by canine distemper, or a deer herd partially checked by parasites, is not a sick population in the way we would describe a diseased livestock operation. It is a population functioning within its ecological context. Trying to eliminate all disease from wild populations would be ecologically nonsensical and practically impossible.
This distinction matters increasingly under the One Health framework, which recognizes that human health, domestic animal health, and wildlife health are interconnected. Disease spillover from wildlife to livestock or humans is a genuine concern, as recent decades of emerging infectious diseases have made abundantly clear. But the management response in the wildlife sphere is to understand and monitor natural disease circulation, not to impose the disease-elimination paradigm of domestic animal medicine onto wild systems.
Genome Editing and the Boundaries of “Natural”
Emerging biotechnology is pushing the definition of wildlife into genuinely new territory. Researchers are exploring genome editing tools to address conservation problems: engineering disease resistance into endangered amphibian populations, for instance, or using gene drives to control invasive species on islands. These applications could save species that are otherwise headed for extinction. They also create organisms that are wild in every sense except that their DNA has been deliberately modified by humans.
This poses a challenge not just philosophically but legally. Current nature protection laws and regulations governing genetically modified organisms were not written with wild animals in mind. Releasing a genome-edited animal into a natural ecosystem does not fit neatly into existing regulatory categories, and there is an urgent need to develop more adequate governance for conservation biotechnology.12PubMed Central. Emerging trends in genome editing of wild animals A genome-edited Hawaiian crow released into a forest to rebuild a wild population is neither a domesticated animal nor a GMO in the agricultural sense, but it is not quite what we have traditionally meant by wildlife either.
The Shifting Baseline Problem
Running beneath all of these definitional questions is a deeper issue: what counts as the “natural” state that wildlife is supposed to occupy? Conservation has traditionally worked with reference baselines, historical snapshots of what an ecosystem looked like before human disturbance. You define wildlife partly by imagining the ecosystem it belongs in, and then you try to restore or protect that ecosystem.
That approach is becoming harder to sustain. Rapid environmental change, growing awareness that past ecosystems were themselves shaped by Indigenous land management and other cultural traditions, and the ecological shift away from viewing nature as static have all undermined the idea of a single correct baseline. Some conservation thinkers have declared the whole baseline concept obsolete, arguing for a forward-looking approach that manages for resilience rather than restoration. Others have pushed the baseline further back, toward deep-time rewilding that imagines ecosystems before any human presence at all.13Environmental Values. Future Directions for Conservation
This matters for the definition of wildlife because the concept implicitly assumes a “natural” state from which organisms can depart. If that natural state is itself contested or unknowable, the line between wild and not-wild becomes less a fact about the organism and more a decision about values: what kind of nature do we want to protect, and which organisms count as belonging to it?
Indigenous and Non-Western Perspectives
The Western scientific definition of wildlife, with its emphasis on taxonomic classification and human-management status, is not the only way to think about wild organisms. Many Indigenous knowledge systems do not draw the same sharp boundary between wild and domestic, or between human communities and the rest of the living world. A literature review searching for Indigenous contributions in the animal sciences found that Indigenous perspectives have rarely been represented through empirical study in these fields, partly because the siloed structure of Western scientific disciplines and its hierarchy of methodology have excluded frameworks that treat humans and animals as part of interconnected systems rather than separate categories.
This is not simply a cultural nicety. Traditional Ecological Knowledge, accumulated over centuries of close observation and management of landscapes, often captures ecological relationships that Western science has only recently begun to document. Indigenous fire management, for example, created habitat mosaics that supported particular assemblages of species, blurring the line between “wild” and “managed” long before European colonization of many continents. When the baseline you use to define “natural” wildlife was itself shaped by generations of deliberate human stewardship, the clean separation between wildlife and human-managed nature starts to look like a specifically modern, Western assumption rather than a universal truth.
When Wildlife Lives at Your Doorstep
Synanthropic species, animals that live alongside humans without being domesticated, occupy a practical gray zone that tests any tidy definition. Rats, raccoons, pigeons, cockroaches, and coyotes thrive in human-modified environments. They are not pets, not livestock, and not managed by anyone, yet they depend on human infrastructure for food and shelter to varying degrees. Ecologically they function as wild organisms, engaging in predation, competition, and reproduction on their own terms.
The disease ecology of synanthropic animals illustrates their in-between status well. Studies of wild mammals living in peri-urban forest fragments, the patchy green spaces at the edges of expanding cities, have found remarkably diverse pathogen assemblages. Small mammals sampled in these zones carry protozoa, bacteria, and helminths spanning multiple transmission pathways, and coinfections are common. These animals serve as bridges between fully wild ecosystems and human communities, circulating pathogens that can move in both directions. Managing that disease risk requires treating them as wildlife, not as pests to simply exterminate, because their ecological roles and population dynamics follow wildlife patterns even when their addresses overlap with ours.
The question of whether synanthropic species are “real” wildlife may sound academic, but it has regulatory and funding consequences. Wildlife agencies in many countries focus their budgets and legal protections on species in remote or protected areas. Urban and peri-urban animals often fall through the cracks, managed by pest control rather than conservation. As human land use continues to expand, the fraction of wild species that live partly or entirely in human-dominated landscapes will only grow. A definition of wildlife that excludes them will become increasingly disconnected from ecological reality.