Population Biology
Population = all members of same species
(interbreeding organisms) within an
ecosystem.
Important terms
• Population: a group of individuals of the
same species that live in the same
geographical area
• Population growth; is the change in the
size of a population with time
• Population size: is the number of
individuals in a population
• Population density; the number of
individual per unit of volume or area
Population Growth
• The size of a population changes due to Births and
immigration (the movement of individuals into a
population) and death and emigration ( the movement of
individuals out of a population).
• For example
• Birth rate is the rate at which reproduction adds new
individuals(eg. 300 births into population of 10000
• Birth rate= 300/1000 x100% or 3%immigration is the
movement of organism into a new area
• Death rate is the rate at which organisms die
• Emigration is the m movement of organisms out of an area
Conditions that affect population
size!
Population growth is limited by "environmental resistance”
Density - Dependent Factors (tend to be biotic and
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involve living things)
Limited resources (food, space, light for photo synthesizers)
Competition interspecific (different species and Intraspecific
(same species)
Predation - increased prey means increased predation
Parasitism - spread more easily in high density pops
Disease- close contact at high density, can spread by physical
contact
Environmental quality- the conditions affecting the habitat of
organisms
Density- Independent Factors (tend to be
abiotic)
Factors that affect populations regardless of
their of their density
• Weather (e.g. plants, insects sensitive to
extreme hot, cold)
• Natural disasters - fire, hurricanes,
earthquake, volcanos
Biotic Potential
• Maximum rate at which a population can increase in ideal
conditions.
• Ex. A house flies lays 100 eggs in which ½ of the flies are
female that can reproduce after 1 month and then die. After
7 generations there will be over 15 billion flies produced.
But organisms reach their biotic potential because of such
limiting factors as availability of food and space
Biotic Potential is affected by the organism’s
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Lifespan
Age at first reproduction
Frequency of reproduction
Clutch size (how many offspring produced)
• Length of reproductive capability
Populations can grow
exponentially...
...If each organism has
multiple offspring.
For Example
• 1 fly lays about 120 eggs
• In one year, that one fly has
about 5 x 1012 great, great,
great, great grandchildren.
• This fly population is
meeting its Biotic Potential
because it is increasing at
the maximum rate possible.
Environmental resistance
• The sum of all the limiting factors to
prevent a population from reaching its
biotic potential is environmental resistance.
Exponential growth of 2 organisms
with different biotic potentials
• Bacteria divide every 20
minutes; it takes 220
minutes to reach a
population of about 2000.
• Eagles reproduce once a
year starting at age 4 (red)
or age 6 (green). It takes
about 23 years (red) or 32
years (green) to reach
2000 individuals.
Some populations expand until they
reach equilibrium at their limit
• Exponential growth under
favorable conditions: food,
space available, little to no
predation, parasitism or
competition.
• Once the population size
matches the carrying
capacity of the ecosystem, its
growth slows and reaches
equilibrium.
Carrying Capacity
•Is the maximum population size that can be supported by an
ecosystem over the long term
•Is typically limited by the resources available in that ecosystem
Population growth rate curves
• These are graphs of the rate of increase in
size of a population with time
• Examples of population growth curves
– Exponential
– Logistic or s shaped
– Predator prey
Exponential growth or j shaped
curve
• A graph or population size vs time that can
be used to show population growth in an
unlimited or ideal environment. A
population if left unchecked would grow to
an infinitely high number. The curve rises
slowly at first (ie. Slow growth or lag
phase) and then shoots up rapidly and keeps
on going indefinitely(rapid growth or
exponential growth)
What happens if a population exceeds
carrying capacity? (J-Shaped Curve –
Exponential Growth)
• Some populations
grow too fast…
• Population overshoots
resources…
• Population crashes
• E.g. Gypsy Moth
caterpillars can defoliate
the trees they live on so
quickly that their larvae
have nothing to feed on!
Sometimes they overshoot but are able to
stabilize (S-Shaped (Logistical) Curve)
Phases of Population Growth
• Lag – Initial Phase of Population Growth.
Initially slow due to limited starting point
• Exponential Growth – Period of Rapid Expansion.
Population is growing rapidly due to a lack of
competition and limits.
• Equilibrium – Population growth slows
dramatically as the carrying capacity is
approached. Populations do not fluctuate as rate
of increase approximates the rate of decrease.
Logistic growth or shaped
• A graph of population size vs time that can show
population change that occur in limited
environments.
• A population increases slowly at first(slow
growth or lag phase) rate of change increases
quickly(rapid growth or exponential growth) and
eventually the population becomes so large that
it stops growing altogether(no growth or
equilibrium phase-the birth rate and death rate
are equal).
• The population is near carrying capacity
Predator - Prey relationships can
cause cyclical population curves
• When prey populations increase, more predation occurs
because- (1) predators encounter prey more often and
(2) more prey support a bigger predator population.
• When predators get too numerous, they reduce the prey
population, thus depleting their food supply.
• A change in the prey population illicits change in the
predator population and vice versa.
Predator-Prey population trends
Human Population Growth:
When will we hit carrying capacity?
Advances have increased the earth’s carrying capacity and pop size
Human population growth
• It should be noted that real populations
grow exponentially for a short period of
time until environmental resistance sets
growth limits. For human populations it is
presently growing at a very rapid rate
Factors that limit Human
Population Growth
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Disease
War
Famine
Space
Food
Poverty
Etc….
• For our planet there is an upper limit for population size
that the earth can support. More industrialized nations
have used technology to raise the carrying capacity of
their environments
• Less industrialized nations have reached or exceeded
their carrying capacity for their environments. An
increase in population growth puts stress on
environmental life support systems. We must curb or
control, expanding global human populations
• More industrialized have increased carrying capacity but
there is still a limit
• Less industrialized nations have reached their carrying
capacity due to depletion of water, soil resources
Demographic Transition
• The demographic transition model seeks to
explain the transformation of countries from
having high birth and death rates to low
birth and death rates. In developed countries
this transition began in the eighteenth
century and continues today. Less
developed countries began the transition
later and are still in the midst of earlier
stages of the model.
• The model is based on the change in crude birth rate (CBR) and crude
death rate (CDR) over time. Each is expressed per thousand
population. The CBR is determined by taking the number of births in
one year in a country, dividing it by the country's population, and
multiplying the number by 1000. In 1998, the CBR in Canada is 14 per
1000 (14 births per 1000 people) while in Kenya it is 32 per 1000. The
crude death rate is similarly determined. The number of deaths in one
year are divided by the population and that figure is multiplied by
1000. This yields a CDR of 9 in the U.S. and 14 in Kenya.
Demographic Transition Model
Stage 1
• Prior to the Industrial Revolution, countries in Western
Europe had a high BR and DR. Births were high because
more children meant more workers on the farm and with
the high death rate, families needed more children to
ensure survival of the family. Death rates were high due to
disease and a lack of hygiene. The high BR and DR were
somewhat stable and meant slow growth of a population.
Occasional epidemics would dramatically increase the DR
for a few years (represented by the "waves" in Stage I of
the model.
Stage 2
• In the mid-18th century, the death rate in Western European countries
dropped due to improvement in sanitation and medicine. Out of
tradition and practice, the birth rate remained high. This dropping
death rate but stable birth rate in the beginning of Stage II contributed
to skyrocketing population growth rates. Over time, children became
an added expense and were less able to contribute to the wealth of a
family. For this reason, along with advances in birth control, the BR
was reduced through the 20th century in developed countries. BR> DR
Populations still grew rapidly but this growth began to slow down.
• Many less developed countries are currently in Stage II of the model.
For example, Kenya's high BR of 32 per 1000 but low DR of 14 per
1000 contribute to a high rate of growth (as in mid-Stage II).
Stage 3
• In the late 20th century, the BR and DR in developed
countries both leveled off at a low rate. In some cases the
BR is slightly higher than the DR (as in the U.S. 14 versus
9) while in other countries the BR is less than the DR (as
in Germany, 9 versus 11). BR=DR leads to stable
population. This is known as zero population growth
• Immigration from less developed countries now accounts
for much of the population growth in developed countries
that are in Stage III of the transition. Countries like China,
South Korea, Singapore, and Cuba are rapidly approaching
Stage III
Model - Summarised
• As with all models, the demographic transition model has
its problems. The model does not provide "guidelines" as
to how long it takes a country to get from Stage I to III.
Western European countries took centuries through some
rapidly developing countries like the Economic Tigers are
transforming in mere decades. The model also does not
predict that all countries will reach Stage III and have
stable low birth and death rates. There are factors such as
religion that keep some countries' birth rate from dropping.
Model - Summarised
• Though this version of the demographic
transition is composed of three stages, you'll
find similar models in texts as well as ones
that include four or even five stages. The
shape of the graph is consistent but the
divisions in time are the only modification.
Model - Summarised
• An understanding of this model, in any of
its forms, will help you to better understand
population policies and changes in
developed and less developed countries
around the world.
• More industrialized nations-transition to
the third stage-our society
• Less industrialized-in second stage- a
demographic trap