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 change over time and space. The term population ecology is often used interchangeably with population biology or population dynamics.

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. Although population ecology is a subfield of biology, it provides interesting problems for mathematicians and statisticians who work in population dynamics.

UN DESA continent population 1950 to 2100
The human population is growing at a logistic rate and has been affecting the populations of other species in return. Chemical pollution, deforestation, and population are very big problems. Irrigation are examples of means by which humans may influence the population ecology of other species. As the human population increases, its effect on the populations of other species may also increase.
Phanerozoic Biodiversity
Populations cannot grow indefinitely. Population ecology involves studying factors that affect population growth and survival. Mass extinctions are examples of factors that have radically reduced populations' sizes and populations' survivability. The survivability of populations is critical to maintaining high levels of biodiversity on Earth.


Terms used to describe natural groups of individuals in ecological studies[2]
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).

The most fundamental law of population ecology is Thomas Malthus' exponential law of population growth.[3]

A population will grow (or decline) exponentially as long as the environment experienced by all individuals in the population remains constant.[3]: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 (four demographic processes) 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 rate of change in the number of individuals in a population can be described as:

where N is the total number of individuals in the population, B is the raw number of births, D is the raw number of deaths, b and d are the per capita rates of birth and death respectively, and r is the per capita average number of surviving offspring each individual has. This formula can be read as the rate of change in the population (dN/dT) is equal to births minus deaths (B - D).[3][5]

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

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]

Geometric populations

Operophtera brumata (Winter moth) populations are geometric.[7]

The population model below can be manipulated to mathematically infer certain properties of geometric populations. A population with a size that increases geometrically is a population where generations of reproduction do not overlap.[8] In each generation there is an effective population size denoted as Ne which constitutes the number of individuals in the population that are able to reproduce and will reproduce in any reproductive generation in concern.[9] In the population model below it is assumed that N is the effective population size.[8]

Assumption 01: Ne = N

Nt+1 = Nt + Bt + It - Dt - Et

Term Definition
Nt+1 Population size in the generation after generation t. This may be the current generation or the next (upcoming) generation depending on the situation in which the population model is used.
Nt Population size in generation t.
Bt Sum (Σ) of births in the population between generations t and t+1. Also known as raw birth rate.
It Sum (Σ) of immigrants moving into the population between generations t and t+1. Also known as raw immigration rate.
Dt Sum (Σ) of deaths in the population between generations t and t+1. Also known as raw death rate.
Et Sum (Σ) of emigrants moving out of the population between generations t and t+1. Also known as raw emigration rate.
The general difference between populations that grow exponentially and geometrically
The general difference between populations that grow exponentially and geometrically. Geometric populations grow in reproductive generations between intervals of abstinence from reproduction. Exponential populations grow without designated periods for reproduction. Reproduction is a continuous process and generations of reproduction overlap. This graph illustrates two hypothetical populations - one population growing periodically (and therefore geometrically) and the other population growing continuously (and therefore exponentially). The populations in the graph have a doubling time of 1 year. The populations in the graph are hypothetical. In reality, the doubling times differ between populations.

Assumption 02: There is no migration to or from the population (N)

It = Et = 0

Nt+1 = Nt + Bt - Dt

The raw birth and death rates are related to the per capita birth and death rates:

Bt = bt × Nt

Dt = dt × Nt

bt = Bt / Nt

dt = Dt / Nt

Term Definition
bt Per capita birth rate.
dt Per capita death rate.


Nt+1 = Nt + (bt × Nt) - (dt × Nt)

Assumption 03: bt and dt are constant (i.e. they don't change each generation).

Nt+1 = Nt + (bNt) - (dNt)

Term Definition
b Constant per capita birth rate.
d Constant per capita death rate.

Take the term Nt out of the brackets.

Nt+1 = Nt + (b - d)Nt

b - d = R

Term Definition
R Geometric rate of increase.

Nt+1 = Nt + RNt

Nt+1 = (Nt + RNt)

Take the term Nt out of the brackets again.

Nt+1 = (1 + R)Nt

1 + R = λ

Term Definition
λ Finite rate of increase.

Nt+1 = λNt

At t+1 Nt+1 = λNt
At t+2 Nt+2 = λNt+1 = λλNt = λ2Nt
At t+3 Nt+3 = λNt+2 = λλNt+1 = λλλNt = λ3Nt
At t+4 Nt+4 = λNt+3 = λλNt+2 = λλλNt+1 = λλλλNt = λ4Nt
At t+5 Nt+5 = λNt+4 = λλNt+3 = λλλNt+2 = λλλλNt+1 = λλλλλNt = λ5Nt


Nt+1 = λtNt

Term Definition
λt Finite rate of increase raised to the power of the number of generations (e.g. for t+2 [two generations] → λ2 , for t+1 [one generation] → λ1 = λ, and for t [before any generations - at time zero] → λ0 = 1

Doubling time of geometric populations

G. stearothermophilus has a shorter doubling time (td) than E. coli and N. meningitidis
G. stearothermophilus has a shorter doubling time (td) than E. coli and N. meningitidis. Growth rates of 2 bacterial species will differ by unexpected orders of magnitude if the doubling times of the 2 species differ by even as little as 10 minutes. In eukaryotes such as animals, fungi, plants, and protists, doubling times are much longer than in bacteria. This reduces the growth rates of eukaryotes in comparison to Bacteria. G. stearothermophilus, E. coli, and N. meningitidis have 20 minute,[10] 30 minute,[11] and 40 minute[12] doubling times under optimal conditions respectively. If bacterial populations could grow indefinitely (which they do not) then the number of bacteria in each species would approach infinity (). However, the percentage of G. stearothermophilus bacteria out of all the bacteria would approach 100% whilst the percentage of E. coli and N. meningitidis combined out of all the bacteria would approach 0%. This graph is a simulation of this hypothetical scenario. In reality, bacterial populations do not grow indefinitely in size and the 3 species require different optimal conditions to bring their doubling times to minima.
Time in minutes % that is G. stearothermophilus
30 44.4%
60 53.3%
90 64.9%
120 72.7%
→∞ 100%
Time in minutes % that is E. coli
30 29.6%
60 26.7%
90 21.6%
120 18.2%
→∞ 0.00%
Time in minutes % that is N. meningitidis
30 25.9%
60 20.0%
90 13.5%
120 9.10%
→∞ 0.00%
Disclaimer: Bacterial populations are exponential (or, more correctly, logistic) instead of geometric. Nevertheless, doubling times are applicable to both types of populations.

The doubling time of a population is the time required for the population to grow to twice its size.[13] We can calculate the doubling time of a geometric population using the equation: Nt+1 = λtNt by exploiting our knowledge of the fact that the population (N) is twice its size (2N) after the doubling time.[8]

2Ntd = λtd × Nt

Term Definition
td Doubling time.

λtd = 2Ntd / Nt

λtd = 2

The doubling time can be found by taking logarithms. For instance:

td × log2(λ) = log2(2)

log2(2) = 1

td × log2(λ) = 1

td = 1 / log2(λ)


td × ln(λ) = ln(2)

td = ln(2) / ln(λ)

td = 0.693... / ln(λ)


td = 1 / log2(λ) = 0.693... / ln(λ)

Half-life of geometric populations

The half-life of a population is the time taken for the population to decline to half its size. We can calculate the half-life of a geometric population using the equation: Nt+1 = λtNt by exploiting our knowledge of the fact that the population (N) is half its size (0.5N) after a half-life.[8]

0.5Nt1/2 = λt1/2 × Nt

Term Definition
t1/2 Half-life.

λt1/2 = 0.5Nt1/2 / Nt

λt1/2 = 0.5

The half-life can be calculated by taking logarithms (see above).

t1/2 = 1 / log0.5(λ) = ln(0.5) / ln(λ)

Geometric (R) and finite (λ) growth constants

Geometric (R) growth constant

R = b - d

Nt+1 = Nt + RNt

Nt+1 - Nt = RNt

Nt+1 - Nt = ΔN

Term Definition
ΔN Change in population size between two generations (between generation t+1 and t).

ΔN = RNt

ΔN/Nt = R

Finite (λ) growth constant

1 + R = λ

Nt+1 = λNt

λ = Nt+1 / Nt

Mathematical relationship between geometric and exponential populations

In geometric populations, R and λ represent growth constants (see 2 and 2.3). In exponential populations however, the intrinsic growth rate, also known as intrinsic rate of increase (r) is the relevant growth constant. Since generations of reproduction in a geometric population do not overlap (e.g. reproduce once a year) but do in an exponential population, geometric and exponential populations are usually considered to be mutually exclusive.[14] However, geometric constants and exponential constants share the mathematical relationship below.[8]

The growth equation for exponential populations is

Nt = N0ert

Term Definition
e Euler's number - A universal constant often applicable in exponential equations.
r intrinsic growth rate - also known as intrinsic rate of increase.
Leonhard Euler 2
Leonhard Euler was the mathematician who established the universal constant 2.71828... also known as Euler's number or e.

Assumption: Nt (of a geometric population) = Nt (of an exponential population).


N0ert = N0λt

N0 cancels on both sides.

N0ert / N0 = λt

ert = λt

Take the natural logarithms of the equation. Using natural logarithms instead of base 10 or base 2 logarithms simplifies the final equation as ln(e) = 1.

rt × ln(e) = t × ln(λ)

Term Definition
ln natural logarithm - in other words ln(y) = loge(y) = x = the power (x) that e needs to be raised to (ex) to give the answer y.

In this case, e1 = e therefore ln(e) = 1.

rt × 1 = t × ln(λ)

rt = t × ln(λ)

t cancels on both sides.

rt / t = ln(λ)

The results:

r = ln(λ)


er = λ

r/K selection

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).[15] An r-selected species (e.g., many kinds of insects, such as aphids[16]) 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.[17][18]


Populations are also studied and conceptualized through the "metapopulation" concept. The metapopulation concept was introduced in 1969:[19]

"as a population of populations which go extinct locally and recolonize."[20]:105

Metapopulation ecology is a simplified model of the landscape into patches of varying levels of quality.[21] 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.[22]


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.


The first journal publication of the Society of Population Ecology, titled Population Ecology (originally called Researches on Population Ecology) was released in 1952.[23]

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

See also


  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. ^ Wells, J. V.; Richmond, M. E. (1995). "Populations, metapopulations, and species populations: What are they and who should care?" (PDF). Wildlife Society Bulletin. 23 (3): 458–462. Archived from the original (PDF) on November 4, 2005.
  3. ^ 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.
  4. ^ Johnson, J. B.; Omland, K. S. (2004). "Model selection in ecology and evolution" (PDF). Trends in Ecology and Evolution. 19 (2): 101–108. CiteSeerX 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 978-0-691-11440-8.
  6. ^ Berryman, A. A. (1992). "The Origins and Evolution of Predator-Prey Theory". Ecology. 73 (5): 1530–1535. doi:10.2307/1940005. JSTOR 1940005.
  7. ^ Hassell, Michael P. (June 1980). "Foraging Strategies, Population Models and Biological Control: A Case Study". The Journal of Animal Ecology. 49 (2): 603–628. doi:10.2307/4267. JSTOR 4267.
  9. ^ Holsinger, Kent (2008-08-26). "Effective Population Size".
  10. ^ "Bacillus stearothermophilus NEUF2011". Microbe wiki.
  11. ^ Chandler, M.; Bird, R.E.; Caro, L. (May 1975). "The replication time of the Escherichia coli K12 chromosome as a function of cell doubling time". Journal of Molecular Biology. 94 (1): 127–132. doi:10.1016/0022-2836(75)90410-6.
  12. ^ Tobiason, D. M.; Seifert, H. S. (19 February 2010). "Genomic Content of Neisseria Species". Journal of Bacteriology. 192 (8): 2160–2168. doi:10.1128/JB.01593-09. PMC 2849444. PMID 20172999.
  13. ^ Boucher, Lauren (24 March 2015). "What is Doubling Time and How is it Calculated?". Population Education.
  14. ^ "Population Growth" (PDF). University of Alberta.
  15. ^ 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.
  16. ^ Whitham, T. G. (1978). "Habitat Selection by Pemphigus Aphids in Response to Response Limitation and Competition". Ecology. 59 (6): 1164–1176. doi:10.2307/1938230. JSTOR 1938230.
  17. ^ MacArthur, R.; Wilson, E. O. (1967). "The Theory of Island Biogeography". Princeton, NJ: Princeton University Press.
  18. ^ Pianka, E. R. (1972). "r and K Selection or b and d Selection?". The American Naturalist. 106 (951): 581–588. doi:10.1086/282798.
  19. ^ Levins, R. (1969). Some demographic and genetic consequences of environmental heterogeneity for biological control. Bulletin of the Entomological Society of America. 15. Columbia University Press. pp. 237–240. doi:10.1093/besa/15.3.237. ISBN 978-0-231-12680-9.
  20. ^ Levins, R. (1970). Gerstenhaber, M. (ed.). Extinction. In: Some Mathematical Questions in Biology. AMS Bookstore. pp. 77–107. ISBN 978-0-8218-1152-8.
  21. ^ Hanski, I. (1998). "Metapopulation dynamics" (PDF). Nature. 396 (6706): 41–49. doi:10.1038/23876. Archived from the original (PDF) on 2010-12-31.
  22. ^ Hanski, I.; Gaggiotti, O. E., eds. (2004). Ecology, genetics and evolution of metapopulations. Burlington, MA: Elsevier Academic Press. ISBN 978-0-12-323448-3.
  23. ^ "Population Ecology". John Wiley & Sons.

Further reading

  • Kareiva, Peter (1989). "Renewing the Dialogue between Theory and Experiments in Population Ecology". In Roughgarden J., R.M. May and S. A. Levin (ed.). Perspectives in ecological theory. New Jersey: Princeton University Press. p. 394 p.
  • Odum, Eugene P. (1959). Fundamentals of Ecology (Second ed.). Philadelphia and London: W. B. Saunders Co. p. 546 p. ISBN 9780721669410. OCLC 554879.
  • Smith, Frederick E. (1952). "Experimental methods in population dynamics: a critique". Ecology. 33 (4): 441–450. doi:10.2307/1931519. JSTOR 1931519.

External links

Age class structure

Age class structure in fisheries and wildlife management is a part of population assessment. Age class structures can be used to model many populations include trees and fish. This method can be used to predict the occurrence of forest fires within a forest population. Age can be determined by counting growth rings in fish scales, otoliths, cross-sections of fin spines for species with thick spines such as triggerfish, or teeth for a few species. Each method has its merits and drawbacks. Fish scales are easiest to obtain, but may be unreliable if scales have fallen off the fish and new ones grown in their places. Fin spines may be unreliable for the same reason, and most fish do not have spines of sufficient thickness for clear rings to be visible. Otoliths will have stayed with the fish throughout its life history, but obtaining them requires killing the fish. Also, otoliths often require more preparation before ageing can occur.

Carrying capacity

The carrying capacity of a biological species in an environment is the maximum population size of the species that the environment can sustain indefinitely, given the food, habitat, water, and other necessities available in the environment.

In population biology, carrying capacity is defined as the environment's maximal load, which is different from the concept of population equilibrium. Its effect on population dynamics may be approximated in a logistic model, although this simplification ignores the possibility of overshoot which real systems may exhibit.

Carrying capacity was originally used to determine the number of animals that could graze on a segment of land without destroying it. Later, the idea was expanded to more complex populations, like humans. For the human population, more complex variables such as sanitation and medical care are sometimes considered as part of the necessary establishment. As population density increases, birth rate often increases and death rate typically decreases. The difference between the birth rate and the death rate is the "natural increase". The carrying capacity could support a positive natural increase or could require a negative natural increase. Thus, the carrying capacity is the number of individuals an environment can support without significant negative impacts to the given organism and its environment. Below carrying capacity, populations typically increase, while above, they typically decrease. A factor that keeps population size at equilibrium is known as a regulating factor. Population size decreases above carrying capacity due to a range of factors depending on the species concerned, but can include insufficient space, food supply, or sunlight. The carrying capacity of an environment may vary for different species and may change over time due to a variety of factors including: food availability, water supply, environmental conditions and living space.

The origins of the term "carrying capacity" are uncertain, with researchers variously stating that it was used "in the context of international shipping" or that it was first used during 19th-century laboratory experiments with micro-organisms. A recent review finds the first use of the term in an 1845 report by the US Secretary of State to the US Senate.

Density dependence

In population ecology, density-dependent processes occur when population growth rates are regulated by the density of a population. This article will focus on density-dependence in the context of macroparasite life cycles.


Fecundity, in human demography and population biology, is the potential for reproduction of an organism or population, measured by the number of gametes (eggs), seed set, or asexual propagules. Fecundity is similar to fertility, the natural capability to produce offspring. A lack of fertility is infertility while a lack of fecundity would be called sterility.

Human demography considers only human fecundity which is often intentionally limited through contraception, while population biology studies all organisms. The term fecundity in population biology is often used to describe the rate of offspring production after one time step (often annual). In this sense, fecundity may include both birth rates and survival of young to that time step. Fecundity is under both genetic and environmental control, and is the major measure of fitness. Fecundation is another term for fertilization. Superfecundity or retrofecundity refers to an organism's ability to store another organism's sperm (after copulation) and fertilize its own eggs from that store after a period of time, essentially making it appear as though fertilization occurred without sperm (i.e. parthenogenesis).Fecundity is important and well studied in the field of population ecology. Fecundity can increase or decrease in a population according to current conditions and certain regulating factors. For instance, in times of hardship for a population, such as a lack of food or high temperatures, juvenile and eventually adult fecundity has been shown to decrease (i.e. due to a lack of resources the juvenile individuals are unable to reproduce, eventually the adults will run out of resources and reproduction will cease).

Fecundity has also been shown to increase in ungulates with relation to warmer weather.In sexual evolutionary biology, especially in sexual selection, fecundity is contrasted to reproductivity.

In obstetrics and gynecology, fecundability is the probability of being pregnant in a single menstrual cycle, and fecundity is the probability of achieving a live birth within a single cycle.


Malacologia is a peer-reviewed scientific journal in the field of malacology, the study of mollusks. The journal publishes articles in the fields of molluscan systematics, ecology, population ecology, genetics, molecular genetics, evolution, and phylogenetics.The journal specializes in publishing long papers and monographs. The journal publishes at least one, sometimes two, volumes of about 400 pages per year, which may consist of 1 or 2 issues. According to the Journal Citation Reports, its 2010 impact factor is 1.024. This ranks Malacologia 66th out of 145 listed journals in the category "Zoology". The journal started publication in 1962.

Malthusian growth model

A Malthusian growth model, sometimes called a simple exponential growth model, is essentially exponential growth based on the idea of the function being proportional to the speed to which the function grows. The model is named after Thomas Robert Malthus, who wrote An Essay on the Principle of Population (1798), one of the earliest and most influential books on population.

Malthusian models have the following form:


The model can also been written in the form of a differential equation:

dP/dt = rP

with initial condition: P(0)= P0

This model is often referred to as the exponential law. It is widely regarded in the field of population ecology as the first principle of population dynamics, with Malthus as the founder. The exponential law is therefore also sometimes referred to as the Malthusian Law. By now, it is a widely accepted view to analogize Malthusian growth in Ecology to Newton's First Law of uniform motion in physics.

Malthus wrote that all life forms, including humans, have a propensity to exponential population growth when resources are abundant but that actual growth is limited by available resources:

A model of population growth bounded by resource limitations was developed by Pierre Francois Verhulst in 1838, after he had read Malthus' essay. Verhulst named the model a logistic function.


A metapopulation consists of a group of spatially separated populations of the same species which interact at some level. The term metapopulation was coined by Richard Levins in 1969 to describe a model of population dynamics of insect pests in agricultural fields, but the idea has been most broadly applied to species in naturally or artificially fragmented habitats. In Levins' own words, it consists of "a population of populations".A metapopulation is generally considered to consist of several distinct populations together with areas of suitable habitat which are currently unoccupied. In classical metapopulation theory, each population cycles in relative independence of the other populations and eventually goes extinct as a consequence of demographic stochasticity (fluctuations in population size due to random demographic events); the smaller the population, the more chances of inbreeding depression and prone to extinction.

Although individual populations have finite life-spans, the metapopulation as a whole is often stable because immigrants from one population (which may, for example, be experiencing a population boom) are likely to re-colonize habitat which has been left open by the extinction of another population. They may also emigrate to a small population and rescue that population from extinction (called the rescue effect). Such a rescue effect may occur because declining populations leave niche opportunities open to the "rescuers".

The development of metapopulation theory, in conjunction with the development of source-sink dynamics, emphasised the importance of connectivity between seemingly isolated populations. Although no single population may be able to guarantee the long-term survival of a given species, the combined effect of many populations may be able to do this.

Metapopulation theory was first developed for terrestrial ecosystems, and subsequently applied to the marine realm. In fisheries science, the term "sub-population" is equivalent to the metapopulation science term "local population". Most marine examples are provided by relatively sedentary species occupying discrete patches of habitat, with both local recruitment and recruitment from other local populations in the larger metapopulation. Kritzer & Sale have argued against strict application of the metapopulation definitional criteria that extinction risks to local populations must be non-negligible.Finnish biologist Ilkka Hanski of the University of Helsinki was an important contributor to metapopulation theory.

Mortality rate

Mortality rate, or death rate, is a measure of the number of deaths (in general, or due to a specific cause) in a particular population, scaled to the size of that population, per unit of time. Mortality rate is typically expressed in units of deaths per 1,000 individuals per year; thus, a mortality rate of 9.5 (out of 1,000) in a population of 1,000 would mean 9.5 deaths per year in that entire population, or 0.95% out of the total. It is distinct from "morbidity", which is either the prevalence or incidence of a disease, and also from the incidence rate (the number of newly appearing cases of the disease per unit of time).

In the generic form, mortality rates are calculated as:

where d represents the deaths occurring within a given time period, p represents the size of the population in which the deaths occur and is a conversion factor from fraction to some other unit (such as multiplying by to get mortality rate per 1,000 individuals).

Organizational ecology

Organizational ecology (also organizational demography and the population ecology of organizations) is a theoretical and empirical approach in the social sciences that is considered a sub-field of organizational studies. Organizational ecology utilizes insights from biology, economics, and sociology, and employs statistical analysis to try to understand the conditions under which organizations emerge, grow, and die.

The ecology of organizations is divided into three levels, the community, the population, and the organization. The community level is the functionally integrated system of interacting populations. The population level is the set of organizations engaged in similar activities. The organization level focuses on the individual organizations (some research further divides organizations into individual member and sub-unit levels).

What is generally referred to as organizational ecology in research is more accurately population ecology, focusing on the second level.


Overpopulation occurs when a species' population exceeds the carrying capacity of its ecological niche. It can result from an increase in births (fertility rate), a decline in the mortality rate, an increase in immigration, or an unsustainable biome and depletion of resources. When overpopulation occurs, individuals limit available resources to survive.The change in number of individuals per unit area in a given locality is an important variable that has a significant impact on the entire ecosystem.

Overshoot (population)

In population dynamics and population ecology, overshoot occurs when a population temporarily exceeds the long term carrying capacity of its environment. The environment usually has mechanisms in place to prevent overshoot. For example, plants are only able to regenerate and regrow a few times after being consumed before completely dying off. The consequence of overshoot is called a collapse, a crash or a die-off in which there is a decline in population density. The entire sequence or trajectory undergone by the population and its environment together is often termed 'overshoot-and-collapse'.

Overshoot can occur due to lag effects. Reproduction rates may remain high relative to the death rate. Entire ecosystems may be severely affected and sometimes reduced to less-complex states due to prolonged overshoot. The eradication of disease can trigger overshoot when a population suddenly exceeds the land's carrying capacity. An example of this occurred on the Horn of Africa when smallpox was eliminated. A region that had supported around 1 million pastoralists for centuries was suddenly expected to support 14 million people. The result was overgrazing, which led to soil erosion.

Physiological density

The physiological density or real population density is the number of people per unit area of arable land.

A higher physiological density suggests that the available agricultural land is being used by more and may reach its output limit sooner than a country that has a lower physiological density. Egypt is a notable example, with physiological density reaching that of Bangladesh, despite much desert.

Plant ecology

Plant ecology is a subdiscipline of ecology which studies the distribution and abundance of plants, the effects of environmental factors upon the abundance of plants, and the interactions among and between plants and other organisms. Examples of these are the distribution of temperate deciduous forests in North America, the effects of drought or flooding upon plant survival, and competition among desert plants for water, or effects of herds of grazing animals upon the composition of grasslands.

A global overview of the Earth's major vegetation types is provided by O.W. Archibold. He recognizes 11 major vegetation types: tropical forests, tropical savannas, arid regions (deserts), Mediterranean ecosystems, temperate forest ecosystems, temperate grasslands, coniferous forests, tundra (both polar and high mountain), terrestrial wetlands, freshwater ecosystems and coastal/marine systems. This breadth of topics shows the complexity of plant ecology, since it includes plants from floating single-celled algae up to large canopy forming trees.

One feature that defines plants is photosynthesis. Photosynthesis is the process of a chemical reactions to create glucose and oxygen, which is vital for plant life. One of the most important aspects of plant ecology is the role plants have played in creating the oxygenated atmosphere of earth, an event that occurred some 2 billion years ago. It can be dated by the deposition of banded iron formations, distinctive sedimentary rocks with large amounts of iron oxide. At the same time, plants began removing carbon dioxide from the atmosphere, thereby initiating the process of controlling Earth's climate. A long term trend of the Earth has been toward increasing oxygen and decreasing carbon dioxide, and many other events in the Earth's history, like the first movement of life onto land, are likely tied to this sequence of events.One of the early classic books on plant ecology was written by J.E. Weaver and F.E. Clements. It talks broadly about plant communities, and particularly the importance of forces like competition and processes like succession.The term ecology was coined by German biologist Ernst Hackel and term related to the study of animals in relation to both physical environment and other plants and animals with which they interacted.

Plant ecology can also be divided by levels of organization including plant ecophysiology, plant population ecology, community ecology, ecosystem ecology, landscape ecology and biosphere ecology.The study of plants and vegetation is complicated by their form. First, most plants are rooted in the soil, which makes it difficult to observe and measure nutrient uptake and species interactions. Second, plants often reproduce vegetatively, that is asexually, in a way that makes it difficult to distinguish individual plants. Indeed, the very concept of an individual is doubtful, since even a tree may be regarded as a large collection of linked meristems. Hence, plant ecology and animal ecology have different styles of approach to problems that involve processes like reproduction, dispersal and mutualism. Some plant ecologists have placed considerable emphasis upon trying to treat plant populations as if they were animal populations, focusing on population ecology. Many other ecologists believe that while it is useful to draw upon population ecology to solve certain scientific problems, plants demand that ecologists work with multiple perspectives, appropriate to the problem, the scale and the situation.


In biology, a population is all the organisms of the same group or species, which live in a particular geographical area, and have the capability of interbreeding. The area of a sexual population is the area where inter-breeding is potentially possible between any pair within the area, and where the probability of interbreeding is greater than the probability of cross-breeding with individuals from other areas.In sociology, population refers to a collection of humans. Demography is a social science which entails the statistical study of human populations.

Population in simpler terms is the number of people in a city or town, region, country or world; population is usually determined by a process called census (a process of collecting, analyzing, compiling and publishing data).

Population density

Population density (in agriculture: standing stock and standing crop) is a measurement of population per unit area or unit volume; it is a quantity of type number density. It is frequently applied to living organisms, and most of the time to humans. It is a key geographical term. In simple terms population density refers to the number of people living in an area per kilometer square.

Population dynamics

Population dynamics is the branch of life sciences that studies the size and age composition of populations as dynamical systems, and the biological and environmental processes driving them (such as birth and death rates, and by immigration and emigration). Example scenarios are ageing populations, population growth, or population decline.

Population growth

In biology or human geography, population growth is the increase in the number of individuals in a population.

Many of the world's countries, including many in Sub-Saharan Africa, the Middle East, South Asia and South East Asia, have seen a sharp rise in population since the end of the Cold War. The fear is that high population numbers are putting further strain on natural resources, food supplies, fuel supplies, employment, housing, etc. in some of the less fortunate countries. For example, the population of Chad has ultimately grown from 6,279,921 in 1993 to 10,329,208 in 2009, further straining its resources. Niger, Pakistan, Nigeria, Egypt, Ethiopia, and the DRC are witnessing a similar growth in population.

Global human population growth amounts to around 83 million annually, or 1.1% per year. The global population has grown from 1 billion in 1800 to 7.616 billion in 2018. It is expected to keep growing, and estimates have put the total population at 8.6 billion by mid-2030, 9.8 billion by mid-2050 and 11.2 billion by 2100.

R/K selection theory

In ecology, r/K selection theory relates to the selection of combinations of traits in an organism that trade off between quantity and quality of offspring. The focus on either an increased quantity of offspring at the expense of individual parental investment of r-strategists, or on a reduced quantity of offspring with a corresponding increased parental investment of K-strategists, varies widely, seemingly to promote success in particular environments.

The terminology of r/K-selection was coined by the ecologists Robert MacArthur and E. O. Wilson in 1967 based on their work on island biogeography; although the concept of the evolution of life history strategies has a longer history (see e.g. plant strategies).

The theory was popular in the 1970s and 1980s, when it was used as a heuristic device, but lost importance in the early 1990s, when it was criticized by several empirical studies. A life-history paradigm has replaced the r/K selection paradigm but continues to incorporate many of its important themes.

Species distribution

Species distribution is the manner in which a biological taxon is spatially arranged. The geographic limits of a particular taxon's distribution is its range, often represented as shaded areas on a map. Patterns of distribution change depending the scale at which they are viewed, from the arrangement of individuals within a small family unit, to patterns within a population, or the distribution of the entire species as a whole (range). Species distribution is not to be confused with dispersal, which is the movement of individuals away from their region of origin or from a population center of high density.

Food webs
Example webs
Ecology: Modelling ecosystems: Other components


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