This intervention report reviews the evidence on supplemental feeding of wild animals. You can read a detailed literature review of the evidence in the paper “Feeding Wildlife As A Means of Promoting Welfare”
This paper uses the Five Domains/Five Freedoms framework, which is commonly used to assess the welfare of both domestic and wild animals. The Five Domains are as follows:
- Nutrition (e.g. food, water, dietary quality)
- Environment (e.g. temperature, odors, noises, light, level of environmental variety)
- Health (e.g. disease, injury, functional impairment)
- Behavior (e.g. sleep, sex, exploration, foraging, novel challenges, play, rearing young)
- Affective state (e.g. pleasure/pain, comfort/discomfort, anger/calmness, boredom/engagement, loneliness/sociality, exhaustion/energy).
Some species are far more likely to be studied than other species. Nearly all studies were performed on mammals and birds. Terrestrial animals and animals from temperate regions of the globe and the global north are overrepresented. Studies are more likely to be conducted on small mammals, hunted animals (particularly ungulates), animals fed for conservation purposes, and birds which use bird feeders. Overrepresentation of certain species is a significant limitation in this paper. In addition, there is likely to be considerable interspecies variation in many factors discussed: these should be considered broad generalizations. Since we rarely have detailed data on any specific species and since it is very difficult to provide supplemental food that is only used by one species, the average effects of supplemental feeding on species in general are relevant.
- 1 Introduction
- 2 Short-Term, Non-Aggregation Effects
- 3 Aggregation
- 4 Long-Term Effects of Feeding
- 5 Conclusions
Short-Term, Non-Aggregation Effects
Domain One: Nutrition
Supplemental feeding appears to improve body condition and growth rates, which are proxies for the animal’s nutritional status. However, in some cases, supplementally fed animals may already have adequate food intake; therefore, supplemental feeding may not be particularly helpful and may even lead to obesity.
Some evidence appears to suggest that supplemental feeding reduces foraging pressure and thus allows animals to eat more nutritious food (Brown and Cooper 2006; Murden and Risenhoover 1993; Plummer 2011).
Many humans feed animals inappropriate food. Some humans, particularly tourists, hunters, and backyard bird feeders, may feed animals obviously inappropriate food, such as candy, cake, bread, popcorn, or chips. Sometimes humans may make mistakes about what food is appropriate for animals: for example, deer who are fed a carbohydrate-rich low-fiber diet may have difficulty digesting it and die (Wobeser and Runge 1975). Other times, the food itself may be appropriate, but it may contain a contaminant such as aflatoxin.
However, the evidence suggests that if humans feed animals appropriate food, then the food improves the animals’ overall nutritional status. That is, if animals are supplementally fed with appropriate and non-contaminated food, in general they will be less likely to starve and more likely to have adequate nutrients.
Domain Two: Environment
Supplemental feeding does not appear to have any effects on the animals’ experience of their environment.
Domain Three: Health
Malnourished animals are more likely to get sick (Becker et al., 2015). If animals have to spend all their time foraging, they are less likely to spend time on grooming and other means of immune defense (ibid).
Supplemental feeding may decrease predation by allowing animals to engage in more anti-predator behavior (Robb, McDonald, Chamberlain, & Bearhop, 2008, p. 481).
In some cases, supplemental feeding may habituate animals to humans (Newsome & Rodger, 2013; Orams, 2002). Habituation spreads disease, makes animals more likely to approach humans who will harm them, makes some animals more hostile or dangerous to humans, and exposes animals to dangers they have not adapted to such as cars or fishing lines.
Supplemental feeding appears to decrease adult mortality and, with a few exceptions, juvenile mortality. Juvenile mortality may increase if supplemental feeding causes unfit adults to survive and have unfit offspring, adults to have more offspring than they can care for, or adults to spend less time taking care of offspring.
Domain Four: Behavior
In general, supplementally fed animals spend less time on foraging. Depending on the animal’s species, they may allocate the leftover time to a variety of other activities, including social behavior, grooming, resting, traveling, territorial defense, parenting, attempting to find additional mates, and preventing extra-pair copulation. While decreased time spent foraging is a change in the animal’s natural behavior, animals appear to allocate the time to activities that are equally or more rewarding.
Supplemental feeding may give animals more energy to be aggressive and engage in territorial defense.
Domain Five: Affective Experience
Supplementally fed birds appear to experience less stress.
Wildlife are attracted to sources of artificial food, which leads to abnormal concentrations of wildlife and closer contact between animals (Becker et al., 2015; Bradley & Altizer, 2007, p. 97; Campbell, Long, & Shriner, 2013; Dunkley & Cattet, 2003, p. 14; Newsome & Rodger, 2008, pp. 262–263). The negative effects of aggregation cut across domains three, four, and five.
Domain Three: Health
It has been conclusively established, using case studies of multiple diseases, that aggregation due to supplemental feeding increases the rate of disease transmission. Supplemental feeding leads to a variety of risk factors associated with disease transmission, including physical contact between infected and susceptible individuals, exposure to body secretions and aerosol droplets, and contact with contaminated surfaces (Inslerman et al., 2006, p. 5). It also increases disease risk by increasing density and encouraging prolonged and repeated presence at feeding sites (ibid: 5). Animals are attracted to artificial sources of food in higher density than occurs naturally, and competition for food increases contact rates among individuals (Dunkley & Cattet, 2003, p. 14). Stress from crowding reduces immunocompetence in some animals, increasing the likelihood of disease (ibid: 14-15). Provisioning may reduce host movement, leading to year-round pathogen exposure, as well as loss of connectivity with other groups such that pathogens go extinct on short timescales, eventually get reintroduced, and cause large outbreaks (Becker et al., 2015). Increased fecundity and survival of young animals may increase the population of susceptible hosts (ibid).
While one systematic review found that feeding generally increased pathogen prevalence (M. H. Murray et al., 2016), a meta-analysis found that there was no direct effect of feeding on pathogen prevalence, although there was significant heterogeneity– that is, while it affects some populations positively and some populations negatively, overall there is no effect (Becker et al., 2015).
Feeding wild animals may attract predators to the place where the animals are fed, a phenomenon called ‘hyperpredation’ (Newsome & Rodger, 2013; Orams, 2002; Oro et al., 2013, pp. 19–20. However, in some cases, crowding may reduce predation rates because there are more animals to watch for predators (Robb, McDonald, Chamberlain, & Bearhop, 2008, p. 481).
In some species, crowding may increase juvenile mortality due to disruptions in maternal behavior, lack of space, or both (Ozoga & Verme, 1982).
Domain Four: Behavior
For many species, crowding is itself an unnatural behavior. Species for which some amount of aggregation is a natural behavior may still crowd around feeding sites to an unnatural degree. Unlike changes in activity budget, discussed above, there is no reason to believe that animals deliberately choose to crowd together because of their increased resources, instead of being forced to crowd together in order to get food. Suffering linked to crowding is likely to be particularly strong for solitary species. Crowding may lead to other unnatural behaviors, such as disruptions in the normal spatial segregation of deer matrilines.
Supplementally fed animals are typically more aggressive. Across taxa, supplemental feeding may increase the risk of intraspecies aggression because of competition for food (Maréchal et al., 2016, p. 6; Newsome & Rodger, 2008; Orams, 2002) and increased density of animals (Dunkley & Cattet, 2003, p. 13). Otherwise solitary species may be particularly likely to experience increased aggression due to crowding.
On the positive side, increased density may decrease the need for any particular animal to be vigilant to avoid predators, which allows them to spend more time eating.
Domain Five: Affective Experience
Crowding causes stress, particularly in mammals.
Long-Term Effects of Feeding
In general, there are four ways we can expect supplemental feeding to affect population size. Based on a literature review, the following taxonomy is proposed.
A type-one population has sufficient food already; its population growth is limited by something other than food supply, such as predation, diseases, disturbances, or other biotic or abiotic factors. In that case, we wouldn’t expect supplemental feeding to increase population size, but we also wouldn’t expect it to have much effect on welfare. However, even if well-fed, animals may still crowd around feeding sites, which are an easy source of food. Thus, the animals may still face negative consequences from aggregation.
For a type-two population, provisioning reduces the variance of demographic parameters in response to harsh years (Oro et al., 2013). Thus, anthropogenic food would increase a population’s resilience against environmental perturbations and catastrophes, reducing the variance of population growth (ibid: 14). Instead of some years where many animals die and some years where many animals have offspring, the animals would have consistent birth and death rates. In this case, we would expect occasional, emergency supplemental feeding to improve wild-animal wellbeing. Some lines of evidence suggest that this is the case for many species; for example, for many birds, supplementation increases clutch sizes in bad years but not in good years, and never increases clutch size above the typical level in a good year. However, other researchers believe that in general food supplementation increases the amplitude of seasonal or multiannual fluctuations in mammal populations, but does not alter the overall pattern (Boutin, 1990, pp. 215–216)
Type-three populations, when fed, stabilize at a higher carrying capacity (Oro et al., 2013, p. 14). While animals may temporarily have positive psychosocial and physical health effects from supplemental feeding, they may live longer and have more offspring, which increases the population. The larger population requires more food, and all the supplemental food provided is needed merely to prevent the larger population from dying of starvation. In most cases, we expect supplemental feeding of type-three populations to have a short-term positive effect and a long-term neutral to negative effect. However, in some cases, it may have a positive effect, if causes of death at the new carrying capacity, such as disease or predation, are less painful than dying of starvation.
In a type-four population, supplemental feeding may lead to a phenomenon called overabundance, in which there are negative health consequences for animals because the population is above the ecosystem’s carrying capacity for a long period of time (Gortázar, Acevedo, Ruiz-Fons, & Vicente, 2006). Overabundance leads to poor body condition, increased parasite burdens, and increased infectious disease prevalence (ibid: 84).
It is difficult to estimate the effects of supplemental feeding on population sizes. Most studies take place over at most a few years, so they don’t last long enough to figure out which category the population is in. Many other studies are of animals fed for conservation purposes, the populations of which are often artificially low and well below the carrying capacity. While many large-scale, long-term feeding efforts exist, ranging from backyard bird feeding to supplemental feeding of deer, there is no control group for these efforts: no one randomized neighborhoods into ones which feed birds and ones which don’t.
Further, increased birth rates and decreased death rates are not the only way supplemental feeding affects a population. In general, animals are more likely to enter and less likely to leave an area with supplemental feeding. In the very short-term (a couple of months), much of the effect of supplemental feeding on population size is likely to be due to effects on immigration and emigration. For this reason, even studies that conclusively show that supplemental feeding increases abundance in the area in which animals are supplementally fed don’t show that supplemental feeding increases abundance overall.
Nevertheless, the evidence generally suggests that supplemental feeding increases both birth rate and population size. We generally expect most populations to be type-one, type-three, or type-four populations, and therefore expect supplemental feeding to have a neutral to negative effect in the long term.
It is probable that sufficiently long-term supplemental feeding has some sort of evolutionary effect. For example, supplemental feeding may allow more low-quality adults to survive (Blanco, 2006, p. 346) – e.g., provisioning blue tits with vitamin E allows lower-quality birds to survive to reproduce (Kate Elizabeth Plummer, 2011, p. 80). Relatively rapid evolution can result from evolutionary pressure exerted by humans, perhaps unintentionally (Blanco, 2006, p. 346). This may render the population maladapted for its natural environment (ibid: 346). There is also a risk of maladaptive genes spreading beyond the supplementally fed population. In addition, even if an animal breeds slowly and thus doesn’t have a short-term population response to increased food, consistent supplemental feeding may select for increased clutch or litter size, more reproductive bouts per reproductive season, or other traits which result in the animal having more offspring in the long run. The evolutionary effect of long-term supplemental feeding has been understudied.
Supplemental feeding favors some species over others. Supplemental feeding may favor the species which are capable of accessing the feeders over species which are not (D. N. Jones & James Reynolds, 2008, p. 5). In general, food provisioning favors more aggressive species and scavengers over other species (Newsome & Rodger, 2008, p. 264). Food provisioning favors larger species over smaller ones (Newsome & Rodger, 2008, p. 264; Oro et al., 2013). In cities, high populations and absence of predators lead to a lack of food even with supplemental feeding, which tends to favor very efficient foragers (Shochat et al., 2004).
The ecological effects of supplemental feeding are understudied, with the exception of the effects of supplemental feeding of deer on vegetation, which are very mixed. It is likely that the effects of supplemental feeding on other species and the environment may be the largest effects overall.
While a poorly-planned supplemental feeding program causes several kinds of harm to wild animals, such as habituation and the provision of inappropriate or contaminated food, a properly planned supplemental feeding program is likely to improve wild-animal welfare, ignoring the effects of aggregation.
Supplemental feeding causes animals to aggregate at the feeding site. Aggregation increases disease transmission rates, risk of predation, and aggression, and itself is stressful and unnatural.
Supplemental feeding may cause certain species to increase in population, both because of lower mortality and because they have more offspring. The population stabilizes at a higher carrying capacity, at which both supplemental and natural food are required to maintain the population. Since the food is just enough to support the new, higher population, supplemental feeding would no longer have positive effects, although animals would still experience the negative effects of supplemental feeding. If the supplemental food is no longer provided, however, many animals will starve to death. This line of argument is inherently speculative, because few studies have been conducted over a long enough time period to establish that this is how populations behave; however, from both theoretical models and the limited evidence available, we expect this outcome is likely.
In general, supplemental feeding should be discouraged. Supplemental feeding programs that benefit humans should be replaced with alternative programs whenever possible. In rare situations, a well-planned supplemental feeding program may be appropriate, such as to provide a vaccine or contraceptive, to direct animals away from a road or other hazard, for conservation purposes, or in an emergency when we expect emergency feeding not to increase populations to an unsustainable level. Effort should be made to reduce the harms of supplemental feeding when it must occur. Wild-animal advocates should not use supplemental feeding as an example of a possible intervention to prevent wild-animal suffering.