ABOVE: Indirect killing occurs when non-target animals die from accidental poisoning associated with use of the drugs, pesticides and herbicides used to protect animals and plants or as bycatch in traps intended for damage-causing animals. Photo: Emerson Begnini
Killing animals has been a ubiquitous human behaviour throughout history, yet it is becoming increasingly controversial and criticised in some parts of contemporary human society.
Over a three-part series, researchers from around the globe review 10 primary reasons why humans kill animals, discuss the necessity or not of these forms of killing and describe the global ecological context for human killing of animals.
The article can be viewed in its entirety at sciencedirect.com/science/article/pii/S0048969723039062
Humans historically and currently kill animals either directly or indirectly for the following reasons:
- Wild harvest or food acquisition
- Human health and safety
- Agriculture and aquaculture
- Urbanisation and industrialisation
- Invasive, overabundant or nuisance wildlife control
- Threatened species conservation
- Recreation, sport or entertainment
- Mercy or compassion
- Cultural and religious practice
- Research, education and testing.
The first five of those reasons are discussed here.
Fig. 1. Three ecological mechanisms of animal killing behaviour by humans, showing (1) predation (carnivory or meat-based diets), (2) interference competition (when animals are behaviourally deprived of access to shared resources or fear effects) and (3) exploitative competition (herbivory or plant-based diets, or consumption of shared resources). Solid lines = direct effects, broken lines = indirect effects.
1. Wild harvest or food acquisition
Many omnivorous and carnivorous predators – from insects to whales – hunt and kill wild animals for food.
This behaviour is known as predation and is a process integral to the proper functioning and maintenance of ecosystems.
Predation can, and often does, cause great harm and suffering to the individual animal being killed.
Some predators are specialists that target a narrow range of prey species and others are generalists that target a wider range of prey species.
Humans, and their ancestors and relatives, are generalists – omnivorous mammals that have hunted, killed and harvested a wide variety of animals for approximately 2-4 million years.
Wild harvest is the most ancient form of predation by humans.
Moreover, the evolution of humans’ proportionately larger brain size is hypothesised to have occurred because of the fats and proteins acquired by eating animals, and therefore killing and eating animals was essential for the very emergence of humans.
Humans on or in the waters around all continents still harvest wild animals for food today, including people from developed and developing countries and those practicing traditional and contemporary lifestyles.
Many types of sentient and non-sentient animals are harvested, including echinoderms, molluscs, crustaceans, insects, fish, reptiles, birds and mammals.
Wild harvest of mammals, reptiles and birds is often characterised by low-volume or opportunistic hunting, such as the acquisition of bushmeat.
Other forms of high-volume or intensive harvesting are also practiced, such as the many fisheries in operation around the world or the commercial kangaroo harvest in Australia.
Wild harvest of animals cannot be practiced without killing animals.
Wild harvest, predation or directly killing animals for food can be avoided by adopting plant-based lifestyles (for example, herbivory or veganism), but doing so cannot avoid all the indirect forms of animal killing associated with such lifestyles (see reasons 3 and 4).
This type of indirect killing is known as competition, which can also lead to prolonged animal suffering, death and eventual extinction over time.
Herbivory leads to competition-induced animal killing when humans eat plants that would otherwise be utilised by other animals, that is exploitative competition.
Competition-induced animal killing also occurs when fear effects behaviourally deprive animals of otherwise available resources, that is interference competition.
Hence, the wild harvest of both animals and plants results in animal killing – the primary difference is that one is direct killing and the other is indirect killing (see Fig. 1).
Human carnivory and herbivory are forms of wild harvest that are ubiquitous across trophic levels, ecosystems and epochs.
All forms of wild harvest cause harm to animals and there are no viable alternatives to these forms of animal killing if we are to continue feeding the 8 billion plus humans currently on the planet.
Directly killing animals for food can often be done in ways that cause no or negligible amounts of pain or harm (see Fig. 2).
When done in these ways, it can give animals a more humane or painless death than the alternatives they would otherwise experience from large-scale plant or animal-based agriculture or through natural causes such as disease, starvation or intraspecific fighting.
Fig. 2. Examples of direct and indirect animal killing by humans, showing (A) two springbok wild-harvested for meat in South Africa (reason 1), (B) land clearing for peanut and sugar cane crops in Australia with a feral pig shot to alleviate damage to the crops (reason 3), (C) small-scale urbanisation illustrated by a group of rondavels under trees in Botswana (reason 4) and (D) a feral cat in Australia trapped to protect threatened fauna from cat predation (reason 5). Photos: Benjamin Allen
2. Human health and safety
Killing animals in self-defence or to protect human health and safety is also one of the most ancient forms of animal killing by humans.
It is done proactively when an animal is killed to prevent a possible threat or reactively to eliminate a present threat.
Examples of proactively killing animals for human health and safety reasons include killing large carnivores (e.g. lions, saltwater crocodiles or great white sharks) in the vicinity of human settlements or controlling populations of smaller mammals (such as racoons, feral dogs or black rats) to prevent the spread of zoonoses including rabies or leptospirosis.
Reactive killing for human health and safety reasons occurs when any animal attempts to harm or kill a human and the humans kill the animal in self-defence.
Examples include killing Asian elephants, cougars or eastern brown snakes that had attacked humans.
Killing animals for traditional medicinal use is another expression of killing for human health reasons practiced in many parts of the world, and the raising and killing of genetically modified pigs to provide a source of replacement organs for xenotransplantation into humans represents an emerging form of killing animals for human health reasons.
Proactive and reactive forms of animal killing (such as control of rodents in impoverished neighbourhoods) may also improve human mental health and wellbeing by reducing anxiety over both food security and disease risk.
In many, or perhaps most, cases there may be less harmful or even nonlethal ways to eliminate human health and safety risks from animals, which might eliminate the need to kill animals, especially contemporary proactive forms of animal killing (see reasons 3 and 5).
This could include vaccinating animals and humans against zoonoses, installing animal exclusion fencing around human communities, sealing buildings and grain silos to exclude grain-destroying birds and rodents or managing risk-enhancing human behaviours.
It might also be possible to reduce the need for reactive forms of animal killing by increasing tolerance of perceived threats, or by taking appropriate measures to prevent an incident or animal attack from arising, including avoidance of areas with high densities of large carnivores or other dangerous animals.
Such nonlethal practices might also include maintaining strong biosecurity systems to prevent zoonotic diseases or their animal vectors from invading new areas (see reason 5), chasing or relocating dangerous animals away from vulnerable humans, or adoption of plant-based traditional medicines or modern manufactured medicines rather than animal-based traditional medicines where culturally appropriate.
Refraining from killing animals to protect human health and safety might be possible for some humans to avoid, particularly those in affluent circumstances.
But because of human inequality and poverty across much of the world, refraining from this form of animal killing will be largely impossible at broader societal scales without compromising human welfare, ignoring cultural sensitivities and losing human lives.
Within only a 10-year period, the national conservation status of koalas has deteriorated from being unlisted to listed as vulnerable in 2012, and then to endangered in 2022, which is intrinsically linked to feeding and housing a rapidly urbanising population. Photo: Klub Boks
3. Agriculture and aquaculture
Agriculture and aquaculture are associated with the most globally prevalent forms of animal killing.
Agriculture has been practiced by humans for at least 11,000 years and enabled humans to establish themselves as the dominant vertebrate on Earth.
Agriculture includes the production and protection of both plants and animals in both small (that is subsistence farming) and large (that is commercial farming) quantities.
Agriculture and aquaculture are forms of optimal foraging behaviour, whereby animals and humans obtain food resources in ways that minimise risk and optimise energy expenditure.
These practices are also analogous to caching behaviour or food storage given that a live animal can convert biomass inedible to humans into edible protein that can be consumed later at times of seasonal shortage of other plant-based foods.
Humans farm and kill a wide variety of mammals (domestic cattle, sheep, goats and pigs), birds (for example domestic chickens, ducks, geese, turkeys, pigeons and ostriches), fish (such as Atlantic salmon, common carp and bluefin tuna) and other animals (prawns, oysters or turtles) for their meat.
Animals are also farmed and killed for other reasons, such as obtaining milk and eggs (for example killing male dairy cattle or male egg-breed chickens) or feathers, fur, skins or leather (ostriches, crocodilians or American mink).
Beyond the direct killing and use of farmed animals for food or fibre, wild predators and competitors of farmed animals and plants negatively affect the production of farmed species in many cases and are also intentionally killed to mitigate the agricultural production losses they would otherwise cause.
Examples include the killing of canids, felids or mustelids to mitigate their predation on farmed animals or the killing of ungulates, macropods, birds or rodents to mitigate their competition for farmed plants.
Other examples include killing infected domestic and wild animals to stem disease outbreaks that could harm and kill vast numbers of livestock and wild animals (for example biosecurity activities).
Such diseases include foot-and-mouth disease, rabies, tuberculosis, anthrax, avian influenza, African swine fever and many others.
Indirect killing occurs when non-target animals die from accidental poisoning associated with use of the drugs, pesticides and herbicides used to protect animals and plants or as bycatch in traps intended for damage-causing animals.
Though it is not often viewed as a source of animal killing, the establishment and harvesting of crops (for example land clearing and tilling) required and still requires the direct and indirect killing and displacement of animals (that is interference competition) at enormous scales, as does the protection of crops following establishment (Fig. 2, see also reason 4).
For example, red-billed quelea are killed in their millions to protect grain crops.
Demand for soybeans and palm oil has also been a major driver of deforestation in South America and Southeast Asia, causing the displacement and death of innumerable animals through the destruction of the natural habitat they relied on.
Many but not all of such crops or their by-products might also be used for industrial non-food purposes such as biofuels or livestock feeds.
Approximately one-third of crops grown across the world also require animal manure for fertilisation, which inherently requires livestock farming to accumulate manure for later dispersal, causing consequent displacement and death of other animals.
Whether animals are killed to be eaten or worn or because almost all animals have been eliminated from land where we now grow biofuel crops or food crops for ourselves or our livestock (see also ourworldindata.org/soy), animal killing is an indisputable and unavoidable component of both the plant and animal food production systems that support human life.
Engaging in animal and plant agriculture and aquaculture in this way enables a greater amount of food to be obtained for humans than would otherwise be attainable through wild harvest (reason 1).
It is of course possible to produce livestock and crops in ways that minimise both the direct and indirect impacts on wild animal lives, but generating food on such large scales to feed a large and growing global human population is impossible without killing animals.
Killing animals for agriculture is a critical human food security endeavour, and many humans will die if humans do not kill animals to produce and protect animal-based and plant-based agriculture and aquaculture.
Associate Professor Benjamin Allen, research co-author and wildlife management and research team leader at the Institute for Life Sciences and the Environment at the University of Southern Queensland.
4. Urbanisation and industrialisation
Perhaps the most universal form of animal killing occurs when humans construct houses, factories, mines, power stations, roads, railways and other industries and infrastructure needed to support sedentary human populations.
In ecological terms, urbanisation might be better thought of as mass irreversible habitat destruction that has resulted in some of the highest rates of decline and local extinction of a range of fauna worldwide.
Urbanisation thus kills animals in ways similar to intensive agriculture (reason 3), which is intrinsically linked to feeding a rapidly urbanising human population.
Furthermore, because urban areas are typically situated in places that were once biodiversity hotspots, the impacts on flora and fauna are more severe for urbanisation than for most other human activities.
Urbanisation represents competition for the critical resource of space and results in the killing and expulsion of countless other animals whenever it occurs at either large or small scales.
For example, koalas are arboreal dietary specialists distributed along the entire east coast of Australia, where most of the Australian human population lives in multiple cities.
Within only a 10-year period, the national conservation status of koalas has deteriorated from being unlisted to listed as vulnerable in 2012, and then to endangered in 2022 – almost exclusively through the ongoing direct and indirect effects of urbanisation, vehicle collisions and tree clearing, or removing both the food and refuge of this iconic species.
Though far smaller in scale, the establishment of rondavels under trees in the Okavango Delta of Botswana similarly displaces the wild animals that would otherwise live there (see Fig. 2).
Thus, every living human on the planet contributes to the displacement and death of animals in this way, and/or has and is benefitting from the proceeds of such animal killing in the past (reasons 1-3 and 5).
Continued animal killing through urbanisation seems inevitable so long as global human population growth remains positive and the current trend of migration towards urban nodes continues.
Directly killing medium and large-sized animals may be avoidable when construction is undertaken carefully and affected individual animals are captured and translocated.
However, the subsequent displacement and indirect forms of animal killing (for example lack of food, exposure to predation, diseases or pollutants) associated with urbanisation are largely unavoidable.
The number of animals killed in this way may be reduced to some degree when urbanisation is directed upwards and not outwards, or when water and waste are recycled sustainably.
However, increasing human populations will still place ever increasing demands on natural resources and the associated industry and infrastructure required to support sedentary populations, which are almost always permanent.
5. Invasive and overabundant native animal control
Killing exotic, non-native, extralimital or overabundant native animals is practiced widely.
However, the motivation for this type of killing is distinct from other forms of animal killing.
Animals might be killed by humans simply because they are exotic or ‘not from here’, but they are usually killed because their invasive characteristics and traits raise concern that they will cause subsequent issues that will require further and otherwise avoidable animal killing.
These concerns include the protection of human health and safety (reason 2), agricultural production (reason 3), threatened species protection (reason 6) or the prevention of ecosystem collapse or shifts characterised by the mass killing and loss of many local animals.
Many invasive and overabundant animals create real and perceived undesirable impacts on the environment, human economies and on social or cultural values.
These impacts include the harm, killing and death of relatively large numbers of other animals that could otherwise be alleviated and avoided by killing relatively small numbers of invasive and overabundant native animals.
Killing invasive animals typically aims to prevent, for example, any potential negative impacts on agriculture, native species, wilderness areas or human health.
Directly and indirectly, killing invasive and species may be avoidable, but doing so will often yield unavoidable adverse consequences for both humans and animals.
Though it may sometimes be possible to undertake invasive animal control in ostensibly non-lethal ways, such as trap-neuter-release or translocation, these practices are often ineffective and typically cause greater harm to animals than simply killing them.
Attempted ‘non-lethal’ exclusion of invasive animals by creating landscapes of fear can create serious animal welfare issues, in addition to indirect killing.
So, while restoring ecosystems through restoration of native carnivores and herbivores might ‘naturally’ eliminate invasive and overabundant animals, this does not evade animal killing given subsequent predation and competition – it merely outsources animal killing from humans to animals or other ecological processes (for example predation, starvation, disease).
Sometimes it is simply impossible to remove all invading animals without killing at least some of them.
Humans do not need to kill or exclude invasive or overabundant invasive animals like they need to eat or protect themselves (see reasons 1-3), but past experience indicates that allowing invasive and overabundant native animal populations to grow unchecked usually results in ecosystem degradation, including widespread harm and death to many other animals and to the agricultural products that humans rely on for food (reason 3).
Next month, Part 3 will cover the remaining five reasons as to why humans kill animals and why we can’t avoid it.
Ben Allen
University of Southern Queensland
