Understanding
POPULATIONS
Chapter 20
Definition
• A population is a group of
organisms that belong to the
same species & live in a
particular place at a particular
time, using same resources.
Examples:
• All of the bass in a pond
• All of the blue jays in Washington County
• All of the red maples in Western PA
Examples of populations:
All the bacteria of one
kind on this agar plate
All the gorillas of 1
species on this
mountain
Population ecology
• is concerned with
fluctuations in population
size and the factors that
regulate populations.
Properties of Populations
May differ from properties of the individual
1. Size
2. Density
3. Dispersion
4. Population dynamics;
A. Growth rate
B. Age structure
C. Survivorship.
1. Population Size
• Is the # of individuals
• Can be difficult to measure directly
• How is counting a tree species different
than counting a fish population?
• Ways to find size:
–Counting individuals (might be
impossible for some large, mobile, widespread
populations)
–Estimate
2. Population Density
- Measures
how crowded a
population is.
• Expressed as number of individuals
per unit or volume.
• Example:
pop. density of humans in USA is 30people/ sq km.
Estimating Density
• 1. Count
all individuals in a
sample plot. (Estimates become more
accurate as sample plots increase in size.)
• 2. Estimate by indirect indicators
(such as number of nests or droppings or tracks.)
• 3. Mark-recapture method.
(animals trapped in boundaries, marked & later
retrapped. )
To Estimate Population Size
• Instead of laboriously
counting everyone, just count
organisms in 1 area & multiply
Example- If 25 oaks in 1 Km2, then estimate
250 in 10 km2.
(This assumes even distribution, or there will
be an error in #)
3. Population Dispersion
• Clumped-most frequent pattern of
distribution. Individuals clustered together
in groups in response to uneven
distribution of resources.
• Even - members maintain a minimum
distance from one another, generally
indicates strong competition
• Random - spacing is the least
common pattern.
4. Population Dynamics
• All populations change in size
and composition over time.
A. Growth Rate includes:
• Birth rate = # births
• Death (mortality) rate = #deaths
• Life expectancy = average life span
B. Age Structure
• The distribution of ages in a
population.
• Important to know ages of
individuals.
• Example- older populations do not
reproduce but a young population may
have rapid growth.
Age structure represented by
graphs.
users.rcn.com/.../ P/Populations.html
C. Patterns of Mortality
• Survivorship curvesShow the likelihood of
survival at different ages for
an organism.
• Data tends to fall into 3
types
3 types of Survivorship curves
–Type I
- likelihood of dying is
small until old age
–Type II
(humans, elephants)
- likelihood of dying is
even over lifetime. (birds)
–Type III
- likelihood of surviving
youth is small (many die young) but if
live- then may live to be old (insects, fish)
Charles Darwin
Calculated that a single pair of elephants
could increase to a population of 19 million
individuals in 750 years.
Since the earth is NOT overrun with
elephants, this is evidence that factors in the
environment limit population growth
Charles Darwin (1809-1882)
His Life and Work
*Born to a wealthy family
*Studied medicine - quit.
*Then studied to enter the
clergy.
*Spent a lot of time collecting
biological specimens.
* Quit religion studies to
become a NATURALIST.
Voyage on the H.M.S.
Beagle
• studied many things on the
islands passed by boat.
• In part because he was so
seasick he wanted to get off the
boat.
• Basis for his life work.
Darwin’s Evolutionary Theory
(which he liked to call “natural selection”
not “evolution”)
States that:
-over
many generations,
natural selection causes
characteristics of
populations to change.
-natural selection is responsible for evolution.
What is survival of the fittest?
• It is NOT- “King of the Jungle”with the meanest, biggest, most muscular
animal killing all the smaller ones.
• Example- 2 dogs- One is the perfect specimenshiny fur, wins shows, smart, etc. & the other is
small, ugly, mangy, living in the alley.
The ugly dog visits every female dog in the
county and reproduces many times, but the
perfect dog never has puppies-
• which dog will pass on his traits?
Measuring Populations
1. Growth rate
2. Exponential Model
3. Logistic Model
4. Population Regulation
1. Growth Rate
• Definition:
–The amount by which a
population’s size
changes over time.
• 4 Processes that affect growth:
–Birth
–Death
–Immigration (individuals moving in)
–Emigration (individuals moving out)
• Usually assume Immigration &
Emigration are zero
Growth Rate
• Describe & graph growth rate
mathematically
• Divide large populations into groups
of 1000.
• Growth = Birth rate – Death rate
Example population
• If 32 Births & 10 Deaths per 1000 per year
of individuals, & total population is 40,000.
– Birth rate is
– Death rate is
0.032
0.010
–Growth rate is
0.022
• To find the # of new individuals, multiply
growth rate by total population:
–40,000 X 0.022 = 880 - increase in 1 year
2. Exponential Model
• Describes a population that
increases rapidly after only a few
generations.
• The larger it gets, the faster it
grows.
• Assumes birth/death rate are
constant
• Assumes unlimited resources (like
food).
http://www.agen.ufl.edu/~chyn/age2062/lect/lect_27/39_05.GIF
Limiting factors
• In reality, populations cannot
grow indefinitely because
– Resources become depleted
– Waste accumulates
• As Competition increases
–Birth rate declines
–Mortality increases
3. Logistic Model
• Builds on Exponential Model
• Accounts for limiting factors
(environmental resistance)
•Carrying Capacity
–The number of individuals the
environment can support
over a long period of time.
http://www.stockton.edu/~cromartw/populationbio/logistic.JPG
Population Regulation
• Density–independent factors
–Weather, fire, etc
–Reduces population regardless of
size
• Density–dependent factors
–Resources shortages- like food,
nesting sites- triggered by increase
in population
Populations may fluctuate in response
to predator- prey relationships
Small Populations
• Decreased genetic variations due
to small gene pool.
• Inbreeding (mating with close
relatives)
• May reduce species ability to adapt
to changes in the environment
• May be due to human activities
• Examples: in wild
– Less than 200 Siberian tigers left
– Only 9 California condors in wild 1980s
Human Population History
• For most of Homo sapiens existence,
population grew very slowly
• Small Nomadic Hunter gather groups
• Agricultural Revolution- about 10,000
years ago- Human population grew
moderately.
• Population Explosion – Middle ages
• Times of decrease from disease, wars.
• Population grew from 3 to 5 billion
people between 1960 & 1987.
Growth today
• Developed Countries
–Leveling off or decreasing
• Developing Countries
–Still growing rapidly
Demographic Transition Model
Questions:
1. Which of the following is a population?
A. all the fish in a pond
B. all the birds in New York City
C. all the members of a family of humans
D. all the fish of the same species in a lake
2. Which of the following refers to the population size
that can be sustained by an environment over
time?
A. bell curve
B. allele frequency
C. carrying capacity
D. exponential growth
d. c.
More questions:
3. Which of the following is true in the exponential model of
population growth?
a. Population growth continues indefinitely.
b. Population growth stops at the carrying capacity.
c. Population growth increases and then decreases.
d. The immigration rate falls with increasing population
size.
4. Which of the following is a density-dependent factor for a
population of deer in a forest?
a. a drought
b. a landslide
c. a period of freezing weather
d. the number of cougars in the forest
a. d.