How do we model population
dynamics as growth slows?
Logistic Growth (Transitional Phase)
•
Logistic growth (transition phase)
occurs when population growth
slows or stops following a period
of exponential growth
•
Creates an “S” shaped
curve
•
Population growth may slow or
stop
for a number of reasons:
Birthrate ↓
death rate ↑
immigration ↓ emigration ↑
At some point, the growth of a
population will level off
•
•
Comparing Exponential and Logistic
Growth
Carrying Capacity
Carrying Capacity (K)
•
The maximum number of
individuals that a
particular habitat can
support (i.e. sustain,
without depleting or
degrading the resources)
• For most species, carrying
capacity is fairly easy to
calculate
• It is complicated for
humans
The classical ‘Sigmoid’ Population
growth curve
Most plant and animal
populations follow
this type of
growth
Classic Lodkik-Volterra
predator-prey modelling
Population fluctuations
Population fluctuations
Birds in decline: How do they work that
out?....
What is happening in this
population graph?
Some formulae (you don’t need to
know these!)
(Reference:
http://www.nature.com/scitable/knowledge/li
brary/how-populations-grow-theexponential-and-logistic-13240157
• Exponential growth:
•
Logistic Growth (Transitional phase):
Homework : Zebra Mussel
Webquest (Population Study)
Zebra mussel population Dynamics
Webquest
A
B
A
5.3.2, 5.3.3
K = carrying
capacity
A
r = rate of
reproduction
N0 = starting
population
B
A
Let’s Review:
What are the four factors that affect populations?
The study of human population
dynamics: Demography
•
•
•
•
How has the human population changed
over time?
Why do population growth rates vary in
countries throughout the world?
What is the ‘demographic transition’ and
why should I care?
What is the ‘carrying capacity’ of the
biosphere for humans?
Historical Overview
•
Welcome to the Anthropocene!....
World population, 500 BC – 2010 AD
•
Until the 1800s the world's
population grew slowly for
thousands of years.
• In 1820 the world's population
reached one billion.
• In the early 1970s, the world's
population reached three billion.
• In 1999, less than 30 years later,
the population doubled to six
billion.
• The global rate of population
growth is now one billion every
13 years.
Historical Overview
•
Human Population growth
Why was the ‘Lag Phase’ of the
human population so prolonged?
The present…and the future?
http://www.bbc.co.uk/news/magazine-15449959 7.12.12
Human Population Dynamic
predictions
Here are the United
Nations Predictions for
population change over
the next 100 yearsUN
World population
prospects
(Image:
http://esa.un.org/unpd/wpp/PWPP/htm/PWPP_Total-Population.htm)
7.12.12
Watch this video and
write down the reasons
for the growth in the
world’s population
•
BBC report on the 7th
Billion...
Why did the human population
begin to rise...
Increased survival rate (i.e. decreased death rate)
•Improved agricultural technology and techniques
•Industrial Revolution
•Stability of food supplies
•Improved sanitation
•Improved medical knowledge
•Improved healthcare
•High birth rate
Birth/Death Rates
Is population growth really a problem?
Some say NO:
• People can find or
manufacture additional
resources to keep
pace with population
growth.
• Nations become
stronger as their
populations grow.
Some say YES:
• Not all resources can
be replaced.
• Even if they could,
quality of life suffers.
• Nations do not
become stronger as
their populations grow.
Modelling population and its consequences
Some models show population growth leading to
resource depletion, which can result in declining food
production, industrial output, and population
The Demographic Transition
model
Explaining the demographic
transition model
•
Over the past 100
years, population
growth in ‘economically
developed countires’
has slowed dramatically
• The model to explain
this change is called the
Demographic Transition
Model
• It has 5 key stages
Stage 1
•
•
Total population is low but balanced – high
birth and high death rates
‘Pre-industrial’
Stage 2
•
•
•
Death rates fall as
sanitation and
health care improve
Birth rates remain
high
Population
increases
Stage 3
•
•
•
Total population
rises rapidly
Birth rates slow as
contraception is
made available and
fewer children
needed to work
Death rates still
falling
Stage 4
•
•
Total population high but balanced by low
birth rate and low mortality
Desire for smaller families
Stage 5
•
•
•
Total population decreasing
Desire for smaller families
People have children later in life
Where is your country?
•
Learning about the 'New Demography'
Where is your country?
•
•
Most wealthy countries are in Stage 4
Several European countries are in stage 5:
•
•
•
•
Russia has lost 5.7 M since the breakup of
the Soviet Union
Germany, Italy: population is decreasing
Most developing countries are in Stage 2
Most population growth in the world is
contributed by only 10 countries, with
China and India leading
Population profiles and agestructure diagrams
Demographers predict future growth using
models called ‘pyramids’/age-structure
diagrams
• let's look at population pyramids!
Mozambique, 2000: Stage 1 of the
demographic transition model
Mozambique, 2025: Stage 2 of the
demographic transition model
UK, 2000: Stage 4 of
demographic transition
UK, 2025: Stage 5 of
demographic transition
What does human demography mean
for the future of our planet?
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•
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Will today’s emerging nations pass
through the demographic transition?
Do you believe that national governments
should implement policies, subsidies, or
other programs to reduce birth rates?
UN data...