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Lecture 15: Population Structure & Growth

â€¢ Introduction

o Why study populations

â–ª Key to saving endangered species

â–ª Helps us control pests

â–ª Managing fish and game

populations

â–ª Study Diseases/Parasites

â–ª Population response to humans

o Characteristics of populations

â–ª Size or abundance (number of

individuals)

â–ª Growth

â–ª Reproductive rate

â–ª Age distribution

â–ª Immigration/emigration

â–ª Spacing

â€¢ Population Growth

o Factors that influence population growth:

â–ª factors leading to Increases in pop size

â€¢ number of births in the entire population (B)

â€¢ number of immigrants in the entire population (I)

â–ª factors leading to decrease in pop size

â€¢ number of deaths in the entire population (D)

â€¢ number of emigrants in the entire population (E)

â–ª Population size is N = B + I - D - E

o Equation to estimate population size (N) at time t+1

â–ª Nt+1 = Nt + B + I - D - E

o If the population size in 2012 = 1000 what will the population size be in 2013 if there

are 20 births, 5 deaths and there is no immigration or emigration.

â–ª Nt = N2012 = 1000

â–ª B = 20

â–ª D= 5

â–ª Nt+1 = 1000 + 20 â€“ 5 =1015

â–ª From this point forward we assume I = 0 and E = 0, a closed population

â–ª (WILL HAVE A PROBLEM LIKE THIS ON THE EXAM - AND MANY OTHERS)

o Equation to estimate change in population size (population growth) over a specific time

period

â–ª Nt+1-Nt = B - D

â–ª Î”N = B â€“ D

o Now we want to understand how N changes over time and to estimate N at any point

in time.

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â€¢ Models of population growth

o Exponential growth - continuous addition of new

individuals

o Exponential growth equation

â–ª Nt = Noert

â–ª Nt = number individuals at time t (size at time t)

â–ª No = initial number of individuals

â–ª r = per capita growth rate

â–ª e = base of natural log (2.72)

o Understanding r

â–ª r = per capita (per individual) growth rate

â–ª r = b â€“ d

â–ª b = per capita birth rate (average births per individual) number of births per

individual per unit time units are births/(individual * time)

â–ª d = per capita death rate number of deaths per individual per unit time units are

deaths/(individual * time)

â–ª r = births/(individual *time) â€“ deaths/(individual *time)

o Average number of births (b) = 2 births per person per year,

o Average number of deaths (d) = 0.5 deaths per person per year

o r = 2 births/(person* year) â€“ 0.5 deaths/(person*year) = 1.5 individuals added per

person per year

o Can r be less than 0?

â–ª Yes, the case would be; population growth rate is declining.

o Now that we understand r we can find Nt, the population size at any point in time

â–ª Nt = Noert (KNOW EQUATIONS)

â–ª Nt = number individuals at time t (size at time t)

â–ª No = initial number of individuals

â–ª r = per capita growth rate

â–ª e = base of natural log (2.72)

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