Population dynamics

Population dynamics is the type of mathematics used to model and study the size and age composition of

dynamical systems

.

History

Population dynamics has traditionally been the dominant branch of

mathematical biology, which has a history of more than 220 years,^[1] although over the last century the scope of mathematical biology has greatly expanded.^{[citation needed}

]

The beginning of population dynamics is widely regarded as the work of

Malthus, formulated as the Malthusian growth model. According to Malthus, assuming that the conditions (the environment) remain constant (ceteris paribus), a population will grow (or decline) exponentially.^[2]^: 18 This principle provided the basis for the subsequent predictive theories, such as the demographic studies such as the work of Benjamin Gompertz^[3] and Pierre François Verhulst in the early 19th century, who refined and adjusted the Malthusian demographic model.^[4]

A more general model formulation was proposed by

Lotka–Volterra predator-prey equations are another famous example,^[6]^[7]^[8]^[9]^[10]^[11]^[12]^[13] as well as the alternative Arditi–Ginzburg equations.^[14]^[15]

Logistic function

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 of no external influence. Models can be more mathematically complex where "...several competing hypotheses are simultaneously confronted with the data."[16] 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:

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

where

N

is the total number of individuals in the specific experimental population being studied,

B

is the number of births and D is the number of deaths per individual in a particular experiment or model. The algebraic symbols

b

,

d

and

r

stand for the rates of birth, death, and the rate of change per individual in the general population, the intrinsic rate of increase. This formula can be read as the rate of change in the population (

dN / dT

) is equal to births minus deaths (

B - D

).^[2]^[13]^[17]

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

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.^[13]^[17]

Intrinsic rate of increase

The rate at which a population increases in size if there are no density-dependent forces regulating the population is known as the intrinsic rate of increase. It is

{\frac {dN}{dt}}=rN

where the derivative

dN/dt

is the rate of increase of the population,

N

is the population size, and

r

is the intrinsic rate of increase. Thus r is the maximum theoretical rate of increase of a population per individual – that is, the maximum population growth rate. The concept is commonly used in insect population ecology or management to determine how environmental factors affect the rate at which pest populations increase. See also exponential population growth and logistic population growth.^[18]

Epidemiology

Population dynamics overlap with another active area of research in mathematical biology:

mathematical epidemiology, the study of infectious disease affecting populations. Various models of viral spread have been proposed and analysed, and provide important results that may be applied to health policy decisions.^{[citation needed}

]

Geometric populations

Operophtera brumata populations are geometric.^[19]

The mathematical formula below can used to model

geometric populations. Geometric populations grow in discrete reproductive periods between intervals of abstinence, as opposed to populations which grow without designated periods for reproduction. Say that N denotes the number of individuals in each generation of a population that will reproduce.^[20]

N_{t+1}=N_{t}+B_{t}+I_{t}-D_{t}-E_{t}

where

N t

is the population size in generation

t

, and

N t +1

is the population size in the generation directly after

N t

;

B t

is the sum of births in the population between generations

t

and

t + 1

(i.e. the birth rate);

I t

is the sum of

emigrants

moving out of the population between generations.

When there is no migration to or from the population,

N_{t+1}=N_{t}+B_{t}-D_{t}.

Assuming in this case that the birth and death rates are constants, then the birth rate minus the death rate equals R, the geometric rate of increase.

{\begin{aligned}N_{t+1}&=N_{t}+RN_{t}\\N_{t+1}&=\left(1+R\right)N_{t}\\N_{t+1}&=\lambda N_{t}\end{aligned}}

where

λ = 1 + R

is the finite rate of increase.

At $t + 1$	$N t +1 = λN t$
At $t + 2$	$N t +2 = λN t +1 = λλN t = λ 2 N t$
At $t + 3$	$N t +3 = λN t +2 = λλ 2 N t = λ 3 N t$

Therefore:

N_{t}=\lambda ^{t}N_{0}

where

λ t

is the Finite rate of increase raised to the power of the number of generations (e.g. for

t + 2

[two generations] →

λ 2

, for

t + 1

[one generation] →

λ 1 = λ

, and for

t

[before any generations - at time zero] →

λ 0 = 1

Doubling time

G. stearothermophilus, E. coli, and N. meningitidis have 20 minute,^[21] 30 minute,^[22] and 40 minute^[23] 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 logistic

instead of geometric. Nevertheless, doubling times are applicable to both types of populations.

The doubling time ( $t d$ ) of a population is the time required for the population to grow to twice its size.^[24] We can calculate the doubling time of a geometric population using the equation: $N t = λ t N 0$ by exploiting our knowledge of the fact that the population ( $N$ ) is twice its size ( $2 N$ ) after the doubling time.^[20]

{\begin{aligned}N_{t_{d}}&=\lambda _{t_{d}}N_{0}\\2N_{0}&=\lambda _{t_{d}}N_{0}\\\lambda _{t_{d}}&=2\end{aligned}}

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

t_{d}\log _{2}(\lambda )=\log _{2}(2)=1\implies t_{d}={\frac {1}{\log _{2}(\lambda )}}

Or:

t_{d}\ln(\lambda )=\ln(2)\implies t_{d}={\frac {\ln(2)}{\ln(\lambda )}}={\frac {0.693...}{\ln(\lambda )}}

Therefore:

t_{d}={\frac {1}{\log _{2}(\lambda )}}={\frac {0.693...}{\ln(\lambda )}}

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: $N t = λ t N 0$ by exploiting our knowledge of the fact that the population ( $N$ ) is half its size ( $0.5 N$ ) after a half-life.^[20]

N_{t_{1/2}}=\lambda ^{t_{1/2}}N_{0}\implies {\frac {1}{2}}N_{0}=\lambda ^{t_{1/2}}N_{0}\implies \lambda ^{t_{1/2}}={\frac {1}{2}}

where

t 1/2

is the half-life.

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

t_{1/2}={\frac {1}{\log _{0.5}(\lambda )}}={\frac {\ln(0.5)}{\ln(\lambda )}}

Geometric (R) growth constant

R=b-d

{\begin{aligned}N_{t+1}&=N_{t}+RN_{t}\\N_{t+1}-N_{t}&=RN_{t}\\N_{t+1}-N_{t}&=\Delta N\end{aligned}}

where

Δ N

is the change in population size between two generations (between generation

t + 1

and

t

).

\Delta N=RN_{t}\implies {\frac {\Delta N}{N_{t}}}=T

Finite (λ) growth constant

1+R=\lambda

N_{t+1}=\lambda N_{t}\implies \lambda ={\frac {N_{t+1}}{N_{t}}}

Mathematical relationship between geometric and logistic populations

In geometric populations, $R$ and $λ$ represent growth constants (see 2 and 2.3). In logistic 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.^[25] However, both sets of constants share the mathematical relationship below.^[20]

The growth equation for exponential populations is

N_{t}=N_{0}e^{rt}

where

e

is

universal constant

often applicable in logistic equations, and

r

is the intrinsic growth rate.

To find the relationship between a geometric population and a logistic population, we assume the $N t$ is the same for both models, and we expand to the following equality:

{\begin{aligned}N_{0}e^{rt}&=N_{0}\lambda ^{t}\\e^{rt}&=\lambda ^{t}\\rt&=t\ln(\lambda )\end{aligned}}

Giving us

r=\ln(\lambda )

and

\lambda =e^{r}.

Evolutionary game theory

Evolutionary game theory was first developed by

The Genetic Theory of Natural Selection.^[26] In 1973 John Maynard Smith formalised a central concept, the evolutionarily stable strategy.^[27]

Population dynamics have been used in several control theory applications. Evolutionary game theory can be used in different industrial or other contexts. Industrially, it is mostly used in multiple-input-multiple-output (MIMO) systems, although it can be adapted for use in single-input-single-output (SISO) systems. Some other examples of applications are military campaigns, water distribution, dispatch of distributed generators, lab experiments, transport problems, communication problems, among others.

Oscillatory

Population size in

chaotic oscillations.^[28]

In popular culture

The

MMORPG Ultima Online, among others, tried to simulate

some of these population dynamics.

References

^ Malthus, Thomas Robert. An Essay on the Principle of Population: Library of Economics
^
S2CID 27090414
.

S2CID 145157003
.

^ Verhulst, P. H. (1838). "Notice sur la loi que la population poursuit dans son accroissement". Corresp. Mathématique et Physique. 10: 113–121.

JSTOR 23686557
. Retrieved 16 November 2020.

doi:10.4249/scholarpedia.1563
.

doi:10.1021/j150111a004
.

^ Goel, N. S.; et al. (1971). On the Volterra and Other Non-Linear Models of Interacting Populations. Academic Press.

Williams and Wilkins
.

^ Volterra, V. (1926). "Variazioni e fluttuazioni del numero d'individui in specie animali conviventi". Mem. Acad. Lincei Roma. 2: 31–113.

McGraw–Hill
.

ISBN 978-0-226-43728-6
.

^
JSTOR 1940005. Archived from the original
(PDF) on 2010-05-31.

doi:10.1016/s0022-5193(89)80211-5. Archived from the original
(PDF) on 2016-03-04. Retrieved 2020-11-17.

PMID 10884706
.

PMID 16701236. Archived from the original
(PDF) on 2011-06-11. Retrieved 2010-01-25.

^
ISBN 978-0-691-11440-8
.

doi:10.1603/0046-225X-34.4.938
.

JSTOR 4267
.

^ ^a ^b ^c ^d "Geometric and Exponential Population Models" (PDF). Archived from the original (PDF) on 2015-04-21. Retrieved 2015-08-17.

^ "Bacillus stearothermophilus NEUF2011". Microbe wiki.

PMID 1095767
.

PMID 20172999
.

^ Boucher, Lauren (24 March 2015). "What is Doubling Time and How is it Calculated?". Population Education.

^ "Population Growth" (PDF). University of Alberta. Archived from the original (PDF) on 2018-02-18. Retrieved 2020-11-16.

ISSN 1095-5054
. Retrieved 16 November 2020.

S2CID 82303195
.

^ ^a ^b ^c Altizer, Sonia; Dobson, Andrew; Hosseini, Parviez; Hudson, Peter; Pascual, Mercedes; Rohani, Pejman (2006). "Seasonality and the dynamics of infectious diseases". Reviews and Syntheses. S2CID 12918683
.

Further reading

ISBN 5-484-00414-4

Turchin, P. 2003. Complex Population Dynamics: a Theoretical/Empirical Synthesis. Princeton, NJ: Princeton University Press.

Smith, Frederick E. (1952). "Experimental methods in population dynamics: a critique". JSTOR 1931519
.

External links

The Virtual Handbook on Population Dynamics. An online compilation of state-of-the-art basic tools for the analysis of population dynamics with emphasis on benthic invertebrates.

v
t
e
Hierarchy of life

Biosphere > Biome > Ecosystem > Biocoenosis > Population > Organism > Organ system > Organ > Tissue > Cell > Organelle > Biomolecular complex > Macromolecule > Biomolecule

Authority control databases: National

Germany

Czech Republic

Retrieved from "https://en.wikipedia.org/w/index.php?title=Population_dynamics&oldid=1214773812"

[malthus-1] Malthus, Thomas Robert. An Essay on the Principle of Population: Library of Economics

[Turchin01-2] 
S2CID 27090414
.

[3] S2CID 145157003
.

[4] Verhulst, P. H. (1838). "Notice sur la loi que la population poursuit dans son accroissement". Corresp. Mathématique et Physique. 10: 113–121.

[5] JSTOR 23686557
. Retrieved 16 November 2020.

[scholarpedia-6] :10.4249/scholarpedia.1563
.

[7] :10.1021/j150111a004
.

[Goelmany-8] Goel, N. S.; et al. (1971). On the Volterra and Other Non-Linear Models of Interacting Populations. Academic Press.

[9] Williams and Wilkins
.

[10] Volterra, V. (1926). "Variazioni e fluttuazioni del numero d'individui in specie animali conviventi". Mem. Acad. Lincei Roma. 2: 31–113.

[11] McGraw–Hill
.

[12] ISBN 978-0-226-43728-6
.

[Berryman1992-13] 
JSTOR 1940005. Archived from the original
(PDF) on 2010-05-31.

[14] :10.1016/s0022-5193(89)80211-5. Archived from the original
(PDF) on 2016-03-04. Retrieved 2020-11-17.

[15] PMID 10884706
.

[Johnson04-16] PMID 16701236. Archived from the original
(PDF) on 2011-06-11. Retrieved 2010-01-25.

[Vandermeer03-17] 
ISBN 978-0-691-11440-8
.

[Jahn2005-18] :10.1603/0046-225X-34.4.938
.

[19] JSTOR 4267
.

[afrc.uamont.edu-20] "Geometric and Exponential Population Models" (PDF). Archived from the original (PDF) on 2015-04-21. Retrieved 2015-08-17.

[21] "Bacillus stearothermophilus NEUF2011". Microbe wiki.

[22] PMID 1095767
.

[23] PMID 20172999
.

[24] Boucher, Lauren (24 March 2015). "What is Doubling Time and How is it Calculated?". Population Education.

[25] "Population Growth" (PDF). University of Alberta. Archived from the original (PDF) on 2018-02-18. Retrieved 2020-11-16.

[26] ISSN 1095-5054
. Retrieved 16 November 2020.

[27] S2CID 82303195
.

[Infectious-Diseases-28] Altizer, Sonia; Dobson, Andrew; Hosseini, Parviez; Hudson, Peter; Pascual, Mercedes; Rohani, Pejman (2006). "Seasonality and the dynamics of infectious diseases". Reviews and Syntheses. S2CID 12918683
.

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