Demography Behind the Population

By Rob Salguero-Gómez^1,2,3,4 & Alden Griffith⁵

¹ Department of Animal & Plant Sciences, University of Sheffield, Alfred Denny Building, Western Bank, Sheffield, S10 2TN. UK.
² Australian Research Council Centre of Excellence for Environmental Decisions, The University of Queensland, School of Biological Sciences, Queensland 4072, Australia.
³ Evolutionary Biodemography Laboratory, Max Planck Institute for Demographic Research. Konrad-Zuse-Straße 1, Rostock, DE-18057, Germany.
⁴ School of Natural Sciences, Department of Zoology & Trinity Centre for Biodiversity Research, Trinity College Dublin, Dublin 2, Ireland.
⁵ Environmental Studies Program, Wellesley College, Wellesley, MA 02481, USA.

Okay, so here goes your daily quota of “productive procrastination”^[1]: go to Google and look up “Population Ecology”. Likely, the very first hit will be brought to you by Nature Education and it will state something along the lines of “Population ecology is the study of … what factors affect population and how and why a population changes over time. Population ecology has its deepest historic roots, and its richest development, in the study of population growth, regulation, and dynamics, or demography…”. We will admit it… this is a bit of a boring definition, and rather limiting too, as we have recently argued (Griffith et al. 2016). However, this definition rightfully highlights what’s behind centuries of arduous work by generations of researchers.

Recently, we published the cross-journal special feature “Demography Beyond the Population” (Griffith et al. 2016) following a BES homonym symposium and workshop that took place in Sheffield just over a year ago. This special feature highlights exciting venues of research in which demographic approaches are being successfully applied, with disciplines as different as conservation biology, forest dynamics, evolution, functional ecology, infectious diseases, life history strategies, etc. Up until this week, the special feature had been unique in its nature in that it brought together for the first time all journals of an ecological society – the British Ecological Society in this case – to pitch the idea that demography is an ideal currency of scientific exchange across sub-disciplines in ecology and evolution. After all, any organism, be a nematode, Bristlecone pine or human, goes about making a living in similar ways: it must survive, age (or grow or develop) and, if lucky, reproduce. These are the fundamental individual-level processes (or vital rates, as we fancy calling them, us silly demographers) that reside at the core of demography.

Joining this special feature is a newly produced cross-journal virtual issue, “Demography Behind the Population”. The goal of this virtual issue is to honor the long, deep roots of demography by offering a repertoire of outstanding demographic research that has been published by all BES journals in recent years.

This new and exciting cross-journal virtual issue leads to the natural question “where is the origin of population ecology?” You may think of C. S. Elton (1900-1991), whose work helped formalised the turning of natural history in ecology, with a strong focus on animals (1927, 1958). Maybe E. R. Pianka (1939-…), whose seminal work on the determinants of r- vs. k-strategies (1970) is still very active nowadays (Oli 2004, Salguero-Gómez et al. 2016). Or perhaps J. L. Harper (1925-2009), who re-invigorated plant population ecology by advocating experimental approaches to vegetation science (Full virtual issue details here), and whose work arguably innitiated comparative plant demography (Saruhkan & Harper 1973). Or could it be R. H. MacArthur (1930-1972) and E. O. Wilson (1929-…), whose treaty on island biogeography (1967) remains today one of the most widely cited pieces of ecological research. Very important contributions indeed… but we can go a bit further back! How about A. J. Lotka (1880-1949)? After all, among many remarkable contributions, his work led to the so-called Lotka-Euler equation (building on previous work from L. Euler (1707-1783)) for age-structured populations, which constitutes one of the most widely used principles of population ecology. Farther back?! Okay, and how bout T. R. Malthus (1766-1834) who, in his work “An Essay on the Principle of Population” (1798) laid down the foundations and implications of exponential growth of a population under ideal conditions? Now you are starting to get to the root of it… mathematics is central root at the base of population ecology, as with many other scientific disciplines.

As an example, in 1744, the French mathematician A. Deparcieux (1703-1768) examined in his work “Essay on the probabilities of the human lifespan” (1746) mortality rates among 2,045 nuns and monks born between 1580 and 1640, with the expectation that they should live longer given the sinless of their lives. His work resulted in one of the earliest life tables, a tool that has now taught in any intro ecology course around the globe. Much to the surprise of Deparcieux, the opposite result was found: monks and nuns lived shorter than married individuals – though the pattern seems to have reversed in more modern times. In the same vein, in 1858, epidemiologist William Farr (1807-1883), considered by some as the father of demography, found that married couples lived longer than singles, and that widowed people lost life: a man whose wife just died would double its mortality probability the next year. Providing a more explicitly ecological contextualisation of human demography, G. Buffon (1707-1788) examine how our own mortality rates relate to climate, geological formation, elevation and settlement density. This is one of the earlier modern demographic treatments of ecological questions. Fascinating, isn’t it? By the way, let us not treat humans apart from all other organisms here, shall we!?

The reality is that population ecology, which is strongly influenced on demography and mathematics as briefly demonstrated above, can be informally tracked back several hundreds, and even thousands of years. Ever since humans have been able to count and interact with the environment, population ecology has likely existed in some form – or so we argue. What evidence do we have for such a claim? Well, for one, old documents reminiscent of demography have been found in ancient Greek, Roman, Indian and Chinese cultures. For instance, the so widely and currently  discussed “caloric restriction” theory, which states that a decrease in caloric intake (without reaching starvation) results in the prolongation of lifespan (e.g. Colman et al. 2014), was already proposed by Aristotle back in 350 B.C. He, in his treatise “On Youth and Old Age, on Life and Death, on Breathing”,  discussed what makes for a long, prosperous life: one where pretty much caloric intake is not abused and one exercises mildly but frequently. In another one of his works, “On Longevity and Shortness of Life”, Aristotle stressed the need to distinguish external vs. internal causes of mortality to best understand longevity (Ross 2012), a topic that is still active focus of research (e.g. Koons 2014).

The 25 manuscripts of the cross-journal virtual issue “Demography Behind the Population” are a testament to how active those connections between classical works and current questions are in the field of population ecology. They range from applications of age- and or size-structure models (Bassar et al. 2015; Chu & Adler 2014; Merow et al. 2014; Metcalf et al. 2013), to classical life history questions (Tachiki et al. 2015 – recent Harper Prize winner, by the way!; Acker et al. 2014; Scheiner 2014), to predator-prey dynamics (Ujvari et al. 2016), transient dynamics (Gaoue 2016; Iles et al. 2015), effects of harvest (Gaoue 2016; Herández-Barrientos et al. 2015), demographic treatment of imperfect data (Sanz-Aguilar 2016; Adams et al. 2015), or evolution of senescence (Giaimo 2014; Pardo et al. 2014), among others. This virtual issue is packed with demographic pearls, and reviewing them all here would (a) turn this “short-ish” blog into a Quijote-length script, and (b) would not do justice to their contributions. We sincerely encourage the readers of the British Ecological Society journals to take a closer look at these fantastic articles. What’s even cooler about these contributions, is that many of them have been led by early career population ecologists (Metcalf, Merow, Plard, Chu, Iles, Tachiki, Martin, Gaoue, Sanz-Aguilar, Adams, and many more). The new cohort of age-structured population ecologists is doing rather well, securing a bright future in the field.  Long live population ecology!

Acknowledgements: We thank Dr Andrew Beckerman for the suggestion to elaborate on a much needed retrospective look at population studies within the context of the British Ecological Society, following our recent prospective take.

Cited references:

Acker, P., Robert, A., Bourget, R. & Colas, B. (2014) Heterogeneity of reproductive age increases the viability of semelparous populations. Functional Ecology, 28, 458–468.

Adams, V.M., Petty, A.M., Douglas, M.M., Buckley, Y.M., Ferdinands, K.B., Okazaki, T., Ko, D.W. & Setterfield, S.A. (2015) Distribution, demography and dispersal model of spatial spread of invasive plant populations with limited data. Methods in Ecology and Evolution, 6, 782–794.

Bassar, R.D., Heatherly, T., Marshall, M.C., Thomas, S.A., Flecker, A.S. & Reznick, D.N. (2015) Population size-structure-dependent fitness and ecosystem consequences in Trinidadian guppies. Journal of Animal Ecology, 84, 955–968.

Colman, R. J., Beasly, T. M., Kemnitz, J. W., Johnson, S. C., Weindruch, R. & Anderson, R. M. (2014) Caloric restriction reduces age-related and all-cause mortality in rhesus monkeys. Nature Communications, 5, 3557

Chu, C. & Adler, P.B. (2014) When should plant population models include age structure? Journal of Ecology, 102, 531–543.

Elton, C. S. (1927) Animal Ecology. First edition. Sidgwick and Jackson, London. The University of Chicago Press, ISBN 0-226-20639-4.

Elton, C. S. (1958) The Ecology of Invasions by Animals and Plants. Methuen, London. The University of Chicago Press. ISBN 0-226-20638-6.

Gaoue, O.G. (2016) Transient dynamics reveal the importance of early life survival to the response of a tropical tree to harvest. Journal of Applied Ecology, 53, 112–119.

Giaimo, S. (2014) Evolution of aging through reduced demographic stochasticity – an extension of the pleiotropy theory to finite populations. Ecology and Evolution, 4, 167–173.

Hernández-Barrios, J.C., Anten, N.P.R. & Martínez-Ramos, M. (2015) Sustainable harvesting of non-timber forest products based on ecological and economic criteria. Journal of Applied Ecology, 52, 389–401.

Iles, D.T., Salguero-Gómez, R., Adler, P.B. & Koons, D.N. (2016) Linking transient dynamics and life history to biological invasion success. Journal of Ecology, 104, 399–408.

Koons, D. N., Gamelon, M., Gaillard, J-M., Aubry, L. M., Rockwell, R. F., Klein, F., Choquet, R. & Gimenez, O. (2014) Methods for studying cause-specific senescence in the wild. Methods in Ecology and Evolution  5: 924-933.

MacArthur, R. H. & Wilson, E. O. (1967). The Theory of Island Biogeography. Princeton, New Jersey: Princeton University Press.

Malthus T.R. 1798. An Essay on the Principle of Population. Chapter 1, p 13 in Oxford World’s Classics reprint.

Metcalf, C.J.E., McMahon, S.M., Salguero-Gómez, R. & Jongejans, E. (2013) IPMpack: an R package for integral projection models. Methods in Ecology and Evolution, 4, 195–200.

Oli, K. M. (2004) The fast-slow continuum and mammalian life-history patterns: an empirical evaluation. Basic and Applied Ecology, 5, 449-463.

Pardo, D., Barbraud, C. & Weimerskirch, H. (2014) What shall I do now? State-dependent variations of life-history traits with aging in Wandering Albatrosses. Ecology and Evolution, 4, 474–487.

Pianka, E. R. (1970) On r and K selection. American Naturalist, 104, 592-597.

Ross, G. R. T. (2012) Translation “Aristotle (350 BC) On Youth and Old Age, on Life and Death, on Breathing”. The University of Adelaide.

Salguero-Gómez, R., Jones, O. R.,  Jongejans, E., Blomberg, S., Hodgson, D., Mbeau Ache, C., Zuidema, P. A., de Kroon, H. & Buckley, Y (2016) The fast-slow continuum and reproductive strategies structure plant life history variation worldwide. Proceedings of the National Academy of Sciences of the USA, 113, 230-235.

Sanz-Aguilar, A., Igual, J.M., Oro, D., Genovart, M. & Tavecchia, G. (2016) Estimating recruitment and survival in partially monitored populations. Journal of Applied Ecology, 53, 73–82.

Sanz-Aguilar, A., Igual, J. M., Oro, D., Genovart, M. & Tavecchia, G. (2016) Estimating recruitment and survival in partially monitored populations. Journal of Applied Ecology, 53, 73–82.

Saruhkan, J. & Harper, J. L. (1973) Studies on plant demography: Ranunculus repens L., R. bulbosus L. and R. acris L. I. Population flux and survivorship. Journal of Ecology, 61, 675-716.

Tachiki, Y., Makita, A., Suyama, Y. & Satake, A. (2015) A spatially explicit model for flowering time in bamboos: long rhizomes drive the evolution of delayed flowering. Journal of Ecology, 103, 585–593.

Ujvari, B., Brown, G., Shine, R. & Madsen, T. (2016) Floods and famine: climate-induced collapse of a tropical predator-prey community. Functional Ecology, 30, 453–458.

^[1] This text was written in a transcontinental productive procrastination session.