Population dynamics and animal suffering

Most animals who ever live die shortly after they come into existence, often in painful or frightening ways. This happens because the predominant reproductive strategies result in most animals dying in infancy.

Population dynamics is the way in which, or the study of how and why, populations of living beings change over time, considering the factors that influence their growth and shifting composition. Understanding the interplay among these factors gives us a better picture of the total amount of suffering and wellbeing in different wild animal populations and allows us to effectively strategize the courses of action required to assist them.

Death and reproduction are two important factors in the study of population dynamics. They determine the growth, decline, or maintenance of any given wild animal population, and can significantly affect the wellbeing experienced by individual members of those populations. Population dynamics together with life history theory can help us find out how many animals die on average relative to how many survive at different life stages. Combining this information with the knowledge of how painful or frightening the types of death are can give us insight into the average quality of life of different wild animals.

Reproductive strategies and animal death

To assess how widespread animal suffering is in the wild, we can start by considering how populations vary depending on different factors. One factor is when individuals migrate from nearby regions to join a population, thereby increasing its overall size and energy requirements, while the populations these individuals migrated from are reduced in size.

However, population size is most influenced by the factors mentioned above: birth and death. For a population to be stable through time, the number of births must be matched by the number of deaths. Because there are limited resources like food and shelter, on average only one offspring per parent can survive to adulthood. This means that animals who have few offspring tend to have relatively low infant mortality, and animals who have large numbers of offspring typically have higher rates of infant mortality. In a stable population, most of the population at any one time is very young animals who were just born and are just about to die. This does not mean that the population is in decline.

Populations decline or grow over time due to changes to limiting factors in their environment, such as the availability of food or the presence of predators. A population might have a period of growth when some of these limitations change. For example, if a population of predators goes extinct, then individuals they would usually prey upon will have higher chances of survival – more than one offspring per parent can survive. This will lead to their population steadily increasing until they encounter another limitation that restricts their growth, such as availability of food. Even though fewer animals may die during periods of growth, future generations will bear more deaths when their growth is limited again, because there will now be both a larger number of adults and an increase in infant mortality rates – only one offspring per parent can survive again due to the resource limitations.

Some animals reproduce by having very few descendants and taking care of them. They may give birth to just one animal or lay just one egg each time they reproduce. They avoid high rates of mortality by investing more energy into traits that improve their odds of passing on their genes. Such traits might include parental care to protect and prepare infants for the risks they are likely to face, a longer lifespan that allows them to reproduce more than once, and greater mental faculties that increase their chances of overcoming the challenges they encounter.

Unfortunately, there are very few species of animals who follow this reproductive strategy. Some mammals such as great apes, cetaceans (whales, dolphins, seals, and porpoises), bears, elephants and other herbivores, and some birds such as albatrosses have this kind of reproductive strategy. However, the overwhelming majority of animals follow a different strategy, reproducing frequently and in large numbers.

There is a high cost to this strategy. Animals reproducing this way may not have many survival-enhancing traits that require a high energy investment if the trade-off is too high. For example, a trait that reduces the chances of reproducing may not be selected for, even if it provides some survival advantage. This is because the reproductive strategy maximizes reproduction, not the average survival of individuals. Because they reproduce in such high numbers and must make these trade-offs, most of these animals will have very short lives with little chance of escaping being eaten alive, starving to death, or other harms encountered by wild animals. Because they are likely sentient, they may mostly suffer during their short lives.

Examples of animals that exhibit this reproductive strategy include amphibians and reptiles whose clutch sizes range in the tens, hundreds, and, in the case of the common cane toad, exceeding 25,000.1 Certain species of fish like the Atlantic Salmon might produce close to 20,000 eggs per clutch, while other common species of salmon, cod, and tuna reproduce in the millions.2 Laying large numbers of eggs is also common among invertebrates. For instance, among crustaceans, crayfish can produce hundreds of eggs per brood,3 and among mollusks, octopuses can reproduce in the hundreds of thousands. Land-based invertebrates including many arthropods can lay hundreds, thousands, and in some cases millions of eggs at a time.4

Consequences for animal suffering

The predominance of reproductive strategies that result in large numbers of offspring has important consequences for the suffering of animals.5 There are strong reasons to believe that animals living in the wild experience much more suffering than positive wellbeing over the course of their short lives. Although some animals might experience little pain due to a quick death, many others suffer terribly from a prolonged death, and die when they are still very young. This means that they may not have the opportunity to have any significant positive experiences in their lives; in fact, they may have just a few experiences in addition to the terrible experience of dying.

Because their deaths are natural and a part of their life history, it might not seem like a moral issue. But if we think that we should help humans and domesticated animals when they are being harmed, it seems unreasonable to treat animals living in the wild differently just because of where they live. A lot of evidence shows that the way they experience harm is not so different from the way humans experience harm, and that it is equally morally relevant, as explained in Can animals in the wild be harmed in the same ways as domesticated animals and humans?

In addition, the fact that many animals start their lives very small and underdeveloped does not mean they aren’t sentient. For example, it has been shown that adult zebrafish respond to harmful stimuli in a way that indicates sentience, and that larval zebrafish respond in similar ways to adults.6 We know that most animals who ever live will die shortly after coming into existence because, for most animals, there isn’t room or resources for most of their offspring to survive. Consequently, we can conclude that, in nature, negative states like pain and distress prevail over positive states like happiness and interest satiation.

This doesn’t mean that the few animals who live to adulthood are automatically happy and don’t need assistance. In many cases, these individuals will have lives that consist of prolonged suffering due to factors like disease, malnutrition and thirst, weather conditions, parasitism and predation, injuries, and psychological stress. Thus, even if an animal survives past their infancy, their life might still consist of more suffering than enjoyment. But even if adult animals did have good lives, a population’s experiences of suffering would still outweigh the positive experiences, because of the disproportionate number of offspring who don’t survive and who have horrible lives.

All animal populations face significant suffering and death

Animals who belong to species with high survival rates in infancy still often die before reaching maturity. Even if they give birth to only one offspring per reproductive season, the frequency of their reproduction means they can have many offspring over the course of their lifetime. Regardless of their reproductive habits, for a population to remain stable, an average of only one offspring per parent will survive to pass on their genes through reproduction.

It is often said that only old and sick animals die in the wild, while young and healthy animals have happy lives. This is considered positive because the death of old and sick animals relieves them of the pain and distress they would otherwise experience from disease or other age-related harms. However, evidence suggests this is not the case. Listed below are some examples that show that young animals who survive infancy are more likely to die than older ones.

In the central Superior National Forest in Minnesota, 209 white-tailed deers were observed from 1973 through the winter of 1983-1984; over one-third of the deers died during this time, and for both males and females, the deers in this study who were most likely to die were the youngest deers, those under one year old.7

Another study analyzed 439 Isle Royale moose deaths between 1950 and 1969. Calf deaths accounted for 45% of total deaths.8

There is documented research of the huge number of deaths that occur during the winter when the population density of a group of Soay sheep in Scotland rises above 2.2 per hectare. More than 90% of lambs and 70% of yearlings die under these conditions, compared with 50% of adults.9

This has also been noticed with birds. One study found that the death rate of yellow-eyed juncos is highest in their first year.10

Of course, these studies only provide data for a handful of cases about infant versus adult mortality in wild animal populations. Our analysis of the problem of wild animal suffering is based on the inevitable number of premature deaths due to predominant reproductive strategies and the likelihood of the deaths being painful or frightening, with case studies being useful to exemplify this problem.

Further readings

Barbault, R. & Mou, Y. P. (1998) “Population dynamics of the common wall lizard, Podarcis muralis, insouthwestern France”, Herpetologica, 44, pp. 38-47.

Bjørkvoll, E.; Grøtan, V.; Aanes, S.; Sæther, B. E.; Engen, S. & Aanes, R. (2012) “Stochastic population dynamics and life-history variation in marine fish species”, The American Naturalist, 180, pp. 372-387 [accessed on 25 November 2019].

Boyce, M. S. (1984) “Restitution of r- and K–selection as a model of density-dependent natural selection”, Annual Review of Ecology and Systematics, 15, pp. 427-447 [accessed on 15 February 2014].

Clarke, M. & Ng, Y.-K. (2006) “Population dynamics and animal welfare: Issues raised by the culling of kangaroos in Puckapunyal”, Social Choice and Welfare, 27, pp. 407-422.

Cody, M. (1966) “A general theory of clutch size”, Evolution, 20, pp. 174-184 [accessed on 13 March 2014].

Coulson, T.; Tuljapurkar, S. & Childs, D. Z. (2010) “Using evolutionary demography to link life history theory, quantitative genetics and population ecology”, Journal of Animal Ecology, 79, pp. 1226-1240 [accessed on 14 October 2019].

Dawkins, R. (1995) “God’s utility function”, Scientific American, 273, pp. 80-85.

Dempster, J. (2012) Animal population ecology, Amsterdam: Elsevier.

Horta, O. (2010) “Debunking the idyllic view of natural processes: Population dynamics and suffering in the wild”, Télos, 17, pp. 73-88 [accessed on 13 January 2013].

Horta, O. (2015) “The problem of evil in nature: Evolutionary bases of the prevalence of disvalue”, Relations: Beyond Anthropocentrism, 3, pp. 17-32 [accessed on 6 November 2015].

Jenouvrier, S.; Péron, C. & Weimerskirch, H. (2015) “Extreme climate events and individual heterogeneity shape life‐history traits and population dynamics”, Ecological Monographs, 85, pp. 605-624.

Leopold, B. D. (2018) Theory of wildlife population ecology, Long Grove: Waveland.

Lomnicki, A. (2018) “Population ecology from the individual perspective”, in DeAngelis, D. L. & Gross, L. J. (eds.) Individual-based models and approaches in ecology, New York: Chapman and Hall, pp. 3-17.

Murray, B. G., Jr. (2013) Population dynamics: Alternative models, Amsterdam: Elsevier.

Ng, Y.-K. (1995) “Towards welfare biology: Evolutionary economics of animal consciousness and suffering”, Biology and Philosophy, 10, pp. 255-285.

Parry, G. D. (1981) “The meanings of r- and K- selection”, Oecologia, 48, pp. 260-264 [accessed on 15 February 2013].

Phillips, B. L.; Brown, G. P. & Shine, R. (2010) “Life‐history evolution in range‐shifting populations”, Ecology, 91, pp. 1617-1627.

Pianka, E. R. (1970) “On r- and K-selection”, The American Naturalist, 104, pp. 592-597 [accessed on 20 February 2013].

Pianka, E. R. (1972) “r and K selection or b and d selection?”, The American Naturalist, 106, pp. 581-588 [accessed on 11 December 2013].

Reznick, D.; Bryant, M. J. & Bashey, F. (2002) “r-and K-selection revisited: The role of population regulation in life-history evolution”, Ecology, 83, pp. 1509-1520.

Rockwood, L. L. (2015 [2006]) Introduction to population ecology, 2nd ed., Hoboken: Wiley-Blackwell.

Roff, D. A. (1992) Evolution of life histories: Theory and analysis, Dordrecht: Springer.

Rolston, H., III (1992) “Disvalues in nature”, The Monist, 75, pp. 250-278.

Royama, T. (2012) Analytical population dynamics, Dordrecht: Springer.

Sæther, B. E.; Coulson, T.; Grøtan, V.; Engen, S.; Altwegg, R.; Armitage, K. B.; Barbraud, C.; Becker, P. H.; Blumstein, D. T.; Dobson, F. S. & Festa-Bianchet, M. (2013) “How life history influences population dynamics in fluctuating environments”, The American Naturalist, 182, pp. 743-759 [accessed on 11 July 2019].

Sagoff, M. (1984) “Animal liberation and environmental ethics: Bad marriage, quick divorce”, Osgoode Hall Law Journal, 22, pp. 297-307 [accessed on 12 January 2016].

Schaffer, W. M. (1974) “Selection for optimal life histories: The effects of age structure”, Ecology, 55, pp. 291-303.

Schmickl, T. & Karsai, I. (2010) “The interplay of sex ratio, male success and density-independent mortality affects population dynamics”, Ecological Modelling, 221, pp. 1089-1097.

Stearns, S. C. (1976) “Life history tactics: A review of the ideas”, Quarterly Review of Biology, 51, pp. 3-47.

Stearns, S. C. (1992) The evolution of life histories, Oxford: Oxford University Press.

Tomasik, B. (2013) “Speculations on population dynamics of bug suffering”, Essays on Reducing Suffering, Jun 11 [accessed on 13 April 2019].

Tomasik, B. (2015a) “The importance of wild-animal suffering”, Relations: Beyond Anthropocentrism, 3, pp. 133-152 [accessed on 20 November 2015].

Tomasik, B. (2015b) “Estimating aggregate wild-animal suffering from reproductive age and births per female”, Essays on Reducing Suffering, Nov 28 [accessed on 5 July 2019].

Tuljapurkar, S. (2013) Population dynamics in variable environments, Dordrecht: Springer.

Vandermeer, J. H. & Goldberg, D. E. (2013 [2003]) Population ecology: First principles, 2nd ed., Princeton: Princeton University Press.

Notes

1 Rastogi, R. K.; Izzo-Vitiello, I.; Meglio, M.; Matteo, L.; Franzese, R.; Costanzo, M. G.; Minucci, S.; Iela, L. & Chieffi, G. (1983) “Ovarian activity and reproduction in the frog, Rana esculenta”, Journal of Zoology, 200, pp. 233-247.

2 Zug, G. R. (1993) Herpetology: An introductory biology of amphibians and reptiles, San Diego: Academic Press. Junk, W. J. (1997) The Central Amazon floodplain: Ecology of a pulsing system. Berlin: Springer. Tyler, M. J. (1998) Australian frogs, London: Penguin.

3 Baum, E. T. & Meister, A. L. (1971) “Fecundity of Atlantic Salmon (Salmo salar) from two Maine rivers”, Journal of the Fisheries Research Board of Canada, 28, pp. 764-767. Hapgood, F. (1979) Why males exist, an inquiry into the evolution of sex, New York: Morrow. Hinckley, S. (1987) “The reproductive biology of walleye pollock, Theragra chalcogramma, in the Bering Sea, with reference to spawing stock structure”, Fishery Bulletin, 85, pp. 481-498. Boyle, P. & Rodhouse, P. (2005) Cephalopods: Ecology and fisheries, Oxford: Blackwell. Kozák, P.; Buřič, M. & Policar, T. (2006) “ The fecundity, time of egg development and juvenile production in spiny-cheek crayfish (Orconectes limosus) under controlled conditions ”, Bulletin français de la pêche et de la disciculture, 380-381, pp. 1171-1182 [accessed on 14 November 2019]

4 Brueland, H. (1995) “Highest lifetime fecundity”, in Walker, T. J. (ed.) University of Florida book of insect records, Gainesville: University of Florida, pp. 41-43 [accessed on 16 November 2019].

5 Overall, the distinction between these two strategies of emphasis on survival versus maximal reproduction has traditionally been referred to as K-selection and r-selection, although these terms are not used that much today. The reason for this terminology of K-selection and r-selection is that in the common equations used to estimate how populations vary through time, the variable referring to the number of offspring is usually named “r,” while the variable that refers to the carrying capacity of the environment, that is, how many individuals can survive in the ecosystem, is usually named “K”. Accordingly, “r” stands for “rate”, while “K” stands for the German word “Kapazität” (capacity). In a simple form, the equation can be put thus: dN/dt=rN (1- N/K), where N stands for the initial number of individuals of the population and t stands for the time at which we measure how the population varies. Verhulst, P.-F. (1838) “ Notice sur la loi que la population poursuit dans son accroissement ”, Correspondance mathématique et physique, 10, pp. 113-121. One reason why these terms are no longer used much is because they are associated with a wider theory that made other claims concerning how the lives of the animals classified as K-strategists and r-strategists are, especially concerning their life histories. According to this wider theory, r-strategists would tend to live short lives, be generalists, have small sizes, reproduce at an early age, prevail in unstable ecosystems, and have density-independent survival rates, among other features; while K-strategists would tend to live long lives, be specialists, have large sizes, reproduce at an older age, prevail in stable ecosystems, and have density-dependent mortality rates. There is contradictory evidence to some of the claims of the theory.

7 The annual survival rate for deers under 1 year old was 0.31, for females between 1 and 2 years old it was 0.80, for males between 1 and 2 years old it was 0.41, for females older than 2 years it was 0.79, and for males over 2 years it was 0.47. Nelson, M. E. & Mech, L. D. (1986) “Mortality of white-tailed deer in Northeastern Minnesota”, Journal of Wildlife Management, 50, pp. 691-698.