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Biology LibreTexts

Formulas Summary

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Name

Formula

Interpretation

Fundamental equation of population growth

Nt+1=Nt+BtDt+ItEt

Nt = population size at time t

B = births; D = deaths; I = immigrants; E = emigrants

Net reproductive rate - average total number of female offspring per female over the course of her lifetime

R0=Ixmx

increasing pop: R0 > 1

stable population: R0 = 1

decreasing pop: R0 < 1

Generation time

T=xIxmxR0

T = average age of reproduction

Intrinsic/ per capita growth rate

r=Tln(R0)

increasing pop: r > 0

stable population: r = 0

decreasing pop: r < 0

Rule of 70 

t=70(100r)

t = time (in years) for population size to double

Fx

Fx=Sxmx+1

age-specific fecundity

Tx

Tx=xLx

Tx = years left to live

Ex

Ex=Txnx

Ex = life expectancy

Lx

Lx=(nx+nx+1)/2

Lx = number surviving

Geometric population growth - growth rate of population with pulsed (seasonal) reproduction patterns

λ=Nt+1/Nt

increasing pop: > 1

stable population: = 1

decreasing pop: < 1

Geometric growth model future population estimation

Nt=N0λt

Nt = population size at time t

Exponential population growth - growth by a population with continuous reproduction (rate of population size change over time)

dN/dt=rN

increasing population: r > 0

stable population: r = 0

decreasing population: r < 0

Exponential growth model future population estimation

Nt=N0ert

N = population size at time t

(e2.71828)

Logistic population growth - exponential growth limited by carrying capacity

dNdt=rN(KNt)K

increasing pop: N < K

stable population: N = K

decreasing pop: N > K

Bioenergetics model

S=IE(FE+UE)M

S = energy storage (growth and reproduction)

Lincoln-Peterson Index (Mark-Recapture Model)

N=(MS)R

N = population size estimate

M = # of animals marked and released

S = # of animals recaptured

R = size of sample on 2nd visit

Leslie Matrix formula

Nt+1=LNt

Nt+1 = population size at time t

L = Leslie Matrix

Nt = age-specific population at initial start time

Simpson’s Index of Diversity (measure of probability): the less diversity, the greater the probability that two randomly selected individuals will be the same

D=Si=1(niN)2

ni = number of individuals of species i

N = total number of individuals of all species

1 - D: if D is closer to zero, then less diversity; if D is closer to 1, then more diversity

Shannon-Wiener Diversity Index (measure of certainty): more common species, more uncertain which one will be selected

H=Si=1pilnpi

ni/N=pi = proportion of individuals of species i

H = 0 in the absence of diversity

Species richness

Hmax=ln(S)

Hmax = maximum number of different species H can reach

S = number of different species

Species evenness

J=HHmax

if J is closer to 0, then less evenness; if J is closer to 1, then more evenness

  1. alpha

  2. beta

  3. gamma

  1. average number of species

  2. gamma / alpha

  3. total # of species

  1. diversity within a specific habitat/ecosystem

  2. comparison of diversity within habitats

  3. measure of diversity at landscape level (in several habitats within a region)

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