# Population ecology

Population ecology is a sub-field of ecology that deals with the dynamics of species populations and how these populations interact with the environment.[1] It is the study of how the population sizes of species living together in groups change over time and space.

The development of population ecology owes much to demography and actuarial life tables. Population ecology is important in conservation biology, especially in the development of population viability analysis (PVA) which makes it possible to predict the long-term probability of a species persisting in a given habitat patch, such as a national park. Although population ecology is a subfield of biology, it provides interesting problems for mathematicians and statisticians who work in population dynamics.

## Fundamentals

The most fundamental law of population ecology is Thomas Malthus' exponential law of population growth.[2]

Terms used to describe natural groups of individuals in ecological studies[3]
Term Definition
Species population All individuals of a species.
Metapopulation A set of spatially disjunct populations, among which there is some immigration.
Population A group of conspecific individuals that is demographically, genetically, or spatially disjunct from other groups of individuals.
Aggregation A spatially clustered group of individuals.
Deme A group of individuals more genetically similar to each other than to other individuals, usually with some degree of spatial isolation as well.
Local population A group of individuals within an investigator-delimited area smaller than the geographic range of the species and often within a population (as defined above). A local population could be a disjunct population as well.
Subpopulation An arbitrary spatially delimited subset of individuals from within a population (as defined above).

A population will grow (or decline) exponentially as long as the environment experienced by all individuals in the population remains constant.[2]:18

This principle in population ecology provides the basis for formulating predictive theories and tests that follow.

Simplified population models usually start with four key variables including death, birth, immigration, and emigration. Mathematical models used to calculate changes in population demographics and evolution hold the assumption (or null hypothesis) of no external influence. Models can be more mathematically complex where "...several competing hypotheses are simultaneously confronted with the data."[4] For example, in a closed system where immigration and emigration does not take place, the per capita rates of change in a population can be described as:

$\frac{dN}{dT} = B - D = bN - dN = (b - d)N = rN,$

where N is the total number of individuals in the population, B is the number of births, D is the number of deaths, b and d are the per capita rates of birth and death respectively, and r is the per capita rate of population change. This formula can be read as the rate of change in the population (dN/dT) is equal to births minus deaths (B - D).[2][5]

Using these techniques, Malthus' population principle of growth was later transformed into a mathematical model known as the logistic equation:

$\frac{dN}{dT} = aN \left( 1 - \frac{N}{K} \right),$

where N is the biomass density, a is the maximum per-capita rate of change, and K is the carrying capacity of the population. The formula can be read as follows: the rate of change in the population (dN/dT) is equal to growth (aN) that is limited by carrying capacity (1-N/K). From these basic mathematical principles the discipline of population ecology expands into a field of investigation that queries the demographics of real populations and tests these results against the statistical models. The field of population ecology often uses data on life history and matrix algebra to develop projection matrices on fecundity and survivorship. This information is used for managing wildlife stocks and setting harvest quotas [5][6]

## r/K selection

At its most elementary level, interspecific competition involves two species utilizing a similar resource. It rapidly gets more complicated, but stripping the phenomenon of all its complications, this is the basic principle: two consumers consuming the same resource.[5]:222

An important concept in population ecology is the r/K selection theory. The first variable is r (the intrinsic rate of natural increase in population size, density independent) and the second variable is K (the carrying capacity of a population, density dependent).[7] An r-selected species (e.g., many kinds of insects, such as aphids[8]) is one that has high rates of fecundity, low levels of parental investment in the young, and high rates of mortality before individuals reach maturity. Evolution favors productivity in r-selected species. In contrast, a K-selected species (such as humans) has low rates of fecundity, high levels of parental investment in the young, and low rates of mortality as individuals mature. Evolution in K-selected species favors efficiency in the conversion of more resources into fewer offspring.[9][10]

## Metapopulation

Populations are also studied and conceptualized through the "metapopulation" concept. The metapopulation concept was introduced in 1969:[11]

"as a population of populations which go extinct locally and recolonize."[12]:105

Metapopulation ecology is a simplified model of the landscape into patches of varying levels of quality.[13] Patches are either occupied or they are not. Migrants moving among the patches are structured into metapopulations either as sources or sinks. Source patches are productive sites that generate a seasonal supply of migrants to other patch locations. Sink patches are unproductive sites that only receive migrants. In metapopulation terminology there are emigrants (individuals that leave a patch) and immigrants (individuals that move into a patch). Metapopulation models examine patch dynamics over time to answer questions about spatial and demographic ecology. An important concept in metapopulation ecology is the rescue effect, where small patches of lower quality (i.e., sinks) are maintained by a seasonal influx of new immigrants. Metapopulation structure evolves from year to year, where some patches are sinks, such as dry years, and become sources when conditions are more favorable. Ecologists utilize a mixture of computer models and field studies to explain metapopulation structure.[14]

## History

The older term, autecology (from Greek: αὐτο, auto, "self"; οίκος, oikos, "household"; and λόγος, logos, "knowledge"), refers to roughly the same field of study as population ecology. It derives from the division of ecology into autecology—the study of individual species in relation to the environment—and synecology—the study of groups of organisms in relation to the environment—or community ecology. Odum (1959, p. 8) considered that synecology should be divided into population ecology, community ecology, and ecosystem ecology, defining autecology as essentially "species ecology."[1] However, for some time biologists have recognized that the more significant level of organization of a species is a population, because at this level the species gene pool is most coherent. In fact, Odum regarded "autecology" as no longer a "present tendency" in ecology (i.e., an archaic term), although included "species ecology"—studies emphasizing life history and behavior as adaptations to the environment of individual organisms or species—as one of four subdivisions of ecology.

## Journals

The first journal publication of the Society of Population Ecology, titled Population Ecology (originally called Researches on Population Ecology) was released in 1952.[15]

Scientific articles on population ecology can also be found in the Journal of Animal Ecology, Oikos and other journals.

## References

1. ^ a b Odum, Eugene P. (1959). Fundamentals of Ecology (Second ed.). Philadelphia and London: W. B. Saunders Co. p. 546 p. ISBN 9780721669410. OCLC 554879.
2. ^ a b c Turchin, P. (2001). "Does Population Ecology Have General Laws?". Oikos 94 (1): 17–26. doi:10.1034/j.1600-0706.2001.11310.x
3. ^ Terms and definitions directly quoted from: Wells, J. V.; Richmond, M. E. (1995). "Populations, metapopulations, and species populations: What are they and who should care?". Wildlife Society Bulletin 23 (3): 458–462.
4. ^ Johnson, J. B.; Omland, K. S. (2004). "Model selection in ecology and evolution.". Trends in Ecology and Evolution 19 (2): 101–108. doi:10.1016/j.tree.2003.10.013. PMID 16701236
5. ^ a b c Vandermeer, J. H.; Goldberg, D. E. (2003). Population ecology: First principles. Woodstock, Oxfordshire: Princeton University Press. ISBN 0-691-11440-4
6. ^ Berryman, A. A. (1992). "The Origins and Evolution of Predator-Prey Theory". Ecology (Ecology, Vol. 73, No. 5) 73 (5): 1530–1535. doi:10.2307/1940005. JSTOR 1940005.
7. ^ Begon, M.; Townsend, C. R.; Harper, J. L. (2006). Ecology: From Individuals to Ecosystems (4th ed.). Oxford, UK: Blackwell Publishing. ISBN 978-1-4051-1117-1
8. ^ Whitham, T. G. (1978). "Habitat Selection by Pemphigus Aphids in Response to Response Limitation and Competition". Ecology (Ecology, Vol. 59, No. 6) 59 (6): 1164–1176. doi:10.2307/1938230. JSTOR 1938230.
9. ^ MacArthur, R.; Wilson, E. O. (1967). The Theory of Island Biogeography. Princeton, NJ: Princeton University Press
10. ^ Pianka, E. R. (1972). "r and K Selection or b and d Selection?". The American Naturalist 106 (951): 581–588. doi:10.1086/282798.
11. ^ Levins, R. (1969). "Some demographic and genetic consequences of environmental heterogeneity for biological control". Bulletin of the Entomological Society of America (Columbia University Press) 15: 237–240. ISBN 978-0-231-12680-9.
12. ^ Levins, R. (1970). Gerstenhaber, M., ed. Extinction. In: Some Mathematical Questions in Biology. AMS Bookstore. pp. 77–107. ISBN 978-0-8218-1152-8.
13. ^ Hanski, I. (1998). "Metapopulation dynamics". Nature 396 (6706): 41–49. doi:10.1038/23876.
14. ^ Hanski, I.; Gaggiotti, O. E., eds. (2004). Ecology, genetics and evolution of metapopulations.. Burlington, MA: Elsevier Academic Press. ISBN 0-12-323448-4.
15. ^