POPULATION ECOLOGY
Population: a group of individuals
of the same species who live in
the same area
Measuring populations
• An ecologist
measures the
changing
populations by
COUNTING
INDIVIDUALS (N)
AT VARIOUS
TIMES OR IN
SAMPLE AREAS
• Similar to a census
4 processes change the size of
a population
1.
2.
3.
4.
Births (b)
Immigration (I)
Death (d)
Emigration (e)
∆N = [b+I] - [d+e]
Sample problem 1
• Suppose that, in a town of 1000 people,
there are 50 births and 30 deaths, and
no immigration or emigration in a year.
During this time interval, the growth rate
for the population is…
(50-30)/1000 = 0.02 or 2%
HUMAN POPULATION
GROWTH
1. The rate of global
population increase is
determined by the number
of daily births and deaths.
2. The global population
growth rate is about 1.1%
3. Human population has not
yet reached carrying
capacity. Current world
population is around 7
billion
Three most populous countries
in the world
• China: 1.3 billion
people; growth rate of
0.5%
• India: 1.2 billion
people; growth rate of
1.2%
• United States: 300
million people; growth
rate of 0.97%
Human carrying capacity?
• The English priest Thomas
Malthus said too many
mouths to feed but not
enough food would result in
famine, war and disease
• Human populations may be
limited by space, available
food and fresh water or
perhaps technology will
allow us to overcome all
limits to our growth?
Growth rate and technological
advances
• The industrial revolution and the
subsequent agricultural and
medical advances led to a
population boom. Vaccines, cures
for diseases, increased sanitation
and increased production of food
lowered the death rates.
• Recently, people with the greatest
access to technology are choosing
to have fewer children, resulting in
decreasing rates of population
growth in developed countries.
• Growth rates of non-industrialized
countries continue to increase.
Africa and South American
societies continue to have high
birth rates.
The rate of population growth
• The change in the
number of individuals
(∆N) in a population
over a specific time
frame (∆t), such as
annually, is known as
the growth rate (gr):
• Growth rates may also
be expressed as a
percentage of the
original population
Per capita growth rate
• To compare populations
that are different sizes the
per capita growth rate (cgr)
is used.
• This is the rate of change
per person
Cgr = ∆N/N
Change in the number
of individuals
divided by the
original number in
the population
Practice questions
1. A deer population increases in size from 2000
to 2300 individuals over one year. Calculate
the growth rate of the population during this
time interval.
2. In January 1997, the population of snails in
Banff National Park was estimated to be 3800.
Two years later, the population was estimated
to be 1800. What was the change in
population size? What was the growth rate?
How long will it take the population
to double its current size?
Practice questions
1. The United States has a population growth rate of
1%. In how many years will the population double
if that growth rate remains constant?
1. Compare the doubling time of the US to that of
Belize, which has a growth rate of about 2%?
1. Country X has a population of 50 million and an
annual growth rate of 3.5%. If the growth rate
remains constant, what will be the population of
country X in 40 years? 60 years?
1. dt= 70/r
dt= 70/1
dt= 70 years to
double
2. Dt (Brazil) = 70/2
= 35 years;
much shorter than the
US time since It has a
faster growth rate
3. dt=70/3.5= 20 years for country X to double in
population.
Starting population of 50 million will become 100
million in 20 years and 200 million in 40 years. In
another 20 years it will double again, so there
will be 400 million people after 60 years.
Managing Population growth
• Africa and Asian
countries are
expected to have
the greatest future
growth (China is the
exception because
of its long one-child
policy, even as it is
now lifted)
• Although 80% of the
Earth’s population lives in
less developed countries,
they consume only about
20% of the resources. The
impact of an society
depends on its population
size, affluence and use of
technology
Africa’s birth rate is high
• One reason for the high
birth rates is the need to
have more children to
replace those lost due to
high infant mortality.
• Other reasons are cultural
or religious or the belief
that many children will
assist the family through
child labor
Less Developed Countries (LDC)
are the fastest growing
AGE STRUCTURE
• One tool to predict
future population
demographics is the
age structure diagram
• It shows the relative
number of individuals
(males and females) of
each age in the
population
• PYRAMID shaped
diagrams are bottom
heavy, with many young
children
• Pyramids show rapid
future growth because all
those kids will one day
have kids of their own
(high population
momentum)
• Diagrams that are broader
on top predict more future
deaths than births as the
old people die
DEMOGRAPHIC TRANSITION
• In a stable human
population, birth rate
equals death rate (b=d)
• Assuming no immigration
or emigration takes place
• Lower br and dr take place
as a country moves toward
a state of greater economic
development and stability
to become an MEDC (more
economically developed
country)
•
Two possible ways
for a population to
become stable:
a) High birth rate= high
death rate
b) Low birth rate= low
death rate
The movement toward
low birth and death
rates is called
DEMOGRAPHIC
TRANSITION
TWO EXAMPLES
• GERMANY (MEDC)
– Low Birth rates and
death rates because…
– Industrialized country
– Increased quality of
health care, sanitation
– Increased education of
women which delays
pregnancies
– Higher availability of
contraceptives
– Families invest in the
education of a few
children rather than have
many
• KENYA (LEDC)
– High Birth rates and
death rates
– Developing country
– Decreased use of
contraceptives
– Lower quality of health
care, sanitation
– Families have more
children as kind of
“social security” or may
need them to work to
support the farm/family
Challenges to decreasing birth
rates are often cultural
http://www.nytimes.co
m/video/2012/04/14/
world/africa/100000
001488374/africaspopulation-peril.html
• In order to make the
transition to an MEDC a
country has to lower its
birth rates. This means
– offering family planning
(contraceptives);
– educational opportunities
for women (to delay
childbearing),
– encouraging later
marriages to delay child
bearing/starting a family
Memorize these…
• TFR (total fertility rate) is the number of
children a woman will have in her lifetime The
earlier a woman starts having children, the
higher the TFR. Education of women lowers
TFR since it puts off childbearing.
• REPLACEMENT fertility is the number of
children a couple must have in order to
replace themselves and keep the population
stable. Replacement fertility is 2.1 due to
infant mortality- not all babies born will survive
With demographic transition from
LEDC to MEDC, more women want
fewer children
• Government policies that
encourage the education of
women result in lower TFR
and therefore lower birth
rates
• Total fertility is the number
of children a woman will
have in her lifetime. The
more time spent in school,
the longer the delay of
pregnancies, and fewer
children are born
Demographic transition occurs in 4
stages. MEMORIZE THEM
Demographic transition in age
structure pyramids
Stage 1: PRE-TRANSITION-high birth rate
and high death rate, high infant mortality
Stage 2: TRANSITION-Country becomes
more economically developed, death
rates fall. Life expectancy increases due
to better medical care and food.The birth
rate is still high. Growth rate is high
Stage 3: INDUSTRIAL-More economic
opportunities, birth control and
education for women leads to falling
birth rates. But high life expectancy
causes continued population growth
even as birth rates decrease. Growth is
slowing
Stage 4: POST-INDUSTRIAL-the birth rate
and death rates are low; population
continues to have an older mean age.
Eventually reaches zero population
growth (ZPG) or may begin to see
negative growth rates
Practice questions
China’s One Child policy
In the late 1970s,
China experienced
population
momentum,
meaning that there
was already a large
population, most of
which was under
age 30.
• So even with two
children per couple, the
numbers were beyond
what could be
managed. In 1979,
China instituted a
mandatory policy that
there could only be 1
child per family without
loss of government
benefits (such as
subsidized housing or
schooling) and a very
high fine.
China’s one child policy- pros
and cons
• Pros (advantages)
– Total Fertility Rates,
TFR, in China was
reduced from 5.9 to
2.9
• Cons (disadvantages)
– Encouraged a preference for male
children, perhaps encouraging
abortion of female fetuses. Society
becomes unbalanced in the number
of males and females
– Increased economic burden on the
child to care for aging parents and
grandparents (4-2-1 conundrum).
China does not have an extensive
pension or social security program
so each child must care for 2
parents and 4 grandparents.
– Smaller population will mean fewer
tax payers and workers, lowering
the GNP
Practice question
Based on the history of human
populations in various countries, how
would you expect EACH of the following
to change as per-capita income
increases:
(a) birth rate (b) death rate ©average
family size (d) age structure of the
population? Explain your reasoning.
• The number of individuals in
a definite area (A) or volume
(V) is known as
POPULATION DENSITY
• Dp = N/A or N/V
• When it is not practical to
count every individual,
several large sample areas
are used to estimate the
actual population size. For
example, if among sheep the
average density is 1.2
sheep/m2 then there are 1.2
X 200 = 240 sheep in an
area that is 200m2.
PRACTICE QUESTION
• Suppose that a 200mL
sample of stagnant water
contains 54 mosquito
larvae. Calculate the
density of the sample.
Round your answer to the
nearest hundredth
This information can be used to estimate the
size of a mosquito population in an
aquatic community at a given time.
Since mosquitoes carry infectious
diseases, large numbers pose a health
risk. Based on the size of the population,
local authorities can consider
appropriate measures to control the
mosquitoes (IPM)
Limits to using density to
estimate population size
• Although one pond water
sample may contain numerous
larvae, other samples from the
same pond may not contain
any. The population dispersal
pattern affects the accuracy of
your samples.
• There are 3 theoretical
distribution patterns for nonhuman populations: CLUMPED,
UNIFORM AND RANDOM
FACTORS that affect
distribution patterns
• Distribution of
resources in a
habitat
• Interactions among
individuals in a
population or
community
Examples showing how random is
different from clumped or uniform
distribution
•
Random distribution can occur when resources are very abundant so
no competition exists between individuals. Random is very rare
•
More common is clumped distribution. Individuals congregate around
food, water or shelter. Mosquito larvae exhibit clumped distribution.
Also, plants that reproduce asexually by sprouting new trees from the
roots of older trees (like aspens) grow in clumps.
•
Artificial populations such as orchards or agricultural fields often show
uniform distribution, where individuals are evenly spaced over the area.
Also, birds of prey or animals with territories to defend will show
uniform distribution
PRACTICE QUESTIONS
1. What type of dispersal patterns do
human populations tend to show?
Explain your reasoning.
2. List two factors that influence the
distribution patterns of non-human
populations
FLUCTUATIONS IN
POPULATION GROWTH
1. When an organism is placed
in a new habitat that is
favorable, the population
size will INCREASE slowly
at first (lag phase) because
there are so few individuals
2. As reproduction continues
the number of individuals will
MULTIPLY RAPIDLY
(exponential phase) because
more of them are giving birth
at any given time
GROWTH WITH
RESTRICTIONS
• The rate of population increase will eventually
either level off or bust since ideal conditions
of food, water, oxygen, space and so on,
cannot be maintained forever.
• The leveling off is the CARRYING CAPACITY
(K) of the area and represents the maximum
number of individuals the area can support
REACHING CARRYING
CAPACITY
• The carrying capacity
is shown at the top of
the population growth
curve.
• It creates an S shaped
curve when best-fitted,
as it appears here in
blue
• For example, if the birth rate of trout in a certain lake were
higher than the death rate, the population will increase (the
overshoot)
• This increases competition for food, oxygen, space and so
on
• Many trout may then die of starvation or disease and
parasites spread more rapidly between them
• The death of some trout would bring the population density
back down towards carrying capacity
Biotic Potential, r
• Biotic potential is the
highest possible per capita
growth rate for a population.
The higher the biotic
potential, the faster the
growth rate. It is exponential
• This is the intrinsic rate of
growth that is possible
given unlimited resources
and ideal living conditions
A population growing at its biotic potential
would be expected to grow exponentially
• Exponential growth curves are shaped like the letter J and are
characterized by slow growth at first, followed by an explosion of very rapid
growth.
• This type of growth is characteristic of r-selected species ( r strategists)
Factors that determine a species’
biotic potential
1. The number of offspring
per brood or
reproductive period
2. The number of offspring
that survive
3. The age of reproductive
maturity
4. How long individuals
generally live (life span)
Practice questions
1. What is an
exponential growth
pattern? What is its
shape in a graph?
2. Explain how biotic
potential and
exponential growth
are related.
Growth is never exponential
indefinitely
• Competition for resources and
other limiting factors will slow
the rate of growth.
• Most populations show logistic
(S shape) growth. The initial J
shape is followed by a levelling
off of the curve over time. The
levelling off is the habitat’s
carrying capacity (K)- the max
population size the environment
can sustain
• Logistic growth is characteristic
Factors that limit a habitat’s
carrying capacity can be either
density-dependent or densityindependent
DENSITY DEPENDENT
FACTORS
• Any factor whose effect
increases when the
population size
increases is a density
dependent limiting
factor
• Factors such as starvation, parasitism, disease and predation
increase as the population gets larger.
•
More organisms die only when the size of the population increases. Density
dependent factors do not affect small populations
BOOM THEN BUST
• Once the population
density reaches its
peak, in some
cases, the
individuals begin to
die very rapidly
• This causes a huge
crash in the
population density
(BUST)
Life Strategies
• In an unstable
environment, it can be
advantageous to expend
energy to reproduce rapidly
while conditions are
favorable. The growth of
pests in a corn field is an
example. Species that
reproduce quickly, at an
early age and produce
large numbers of offspring
are r strategists.
• At the other end of the
spectrum are
populations that live
close to the carrying
capacity (K) of their
habitats. Species that
take a long time to
mature, live a long time
and have relatively few
offspring are K
strategists.
Practice question
• Scientists refer to populations with rselected life strategies as opportunistic
populations and populations with Kselected life strategies as equilibrium
populations. Use the characteristics of
each type of strategy to explain how
scientists came up with these terms
Another way to classify
populations
• Using how many
individuals survive at each
phase of life creates three
types (I, II and III)
• Type I have a high survival
rate through most of their
life (K-strategists)
• Type II have relatively
constant death throughout
their life
• Type III have high death
rate at the beginning of
their life (r strategists)