Announcements
September 8, 2006
Population Biology
Lecture Objectives:
1. Learn the population characteristics that
determine population growth rate
2. Understand why the number of individuals
in a population may change over time.
3. Understand the different types of
population growth curves
4. Understand the difference between Kselected and r-selected species
Population Characteristics
Population – All organisms of the same kind
found within a specific geographic region
Populations can differ in:
*Birthrate
*Death rate
*Population growth rate
*Sex ratio
*Age distribution
*Density
Birthrate (Natality)
Birthrate (b)—Number of individuals added
through reproduction over a particular time
period.
- In many species, birthrate is influenced by
the amount of food available
- Sexually reproducing species must also
find mates
- In humans, expressed as # of babies/1000
individuals/year
Asexual Reproduction
Females that reproduce asexually do not
need to find a mate
New Mexico Whiptail
Daphnia
Mortality
Survivorship curve – a graph showing
the proportion of individuals likely to
survive to each age
 Three main types:
 Type
I
 Type II
 Type III
Type I survivorship curve
Survival is high until old age
Examples:
• Dall sheep
• Humans in
developed
countries
Type II survivorship curve
Mortality is spread evenly among all age
groups
Example:
•Many
species of
birds
Type III survivorship curve
Very high mortality among the young
Examples:
• Plants
• Animals
that
produce
many
offspring
Survivorship curves (continued)
One major factor determining the shape of the
survivorship curve is the degree of parental care
How long do parents care for the offspring after
they are born?
Population growth rate
Population growth rate (r) – the rate at which
the size of the population changes
r=b–d
Assumes no immigration or emigration—
when this happens population growth rate
is then more generally, (birth +
immigration) – (death + emigration)
Population growth rate
If r is positive, birthrate is higher than the
death rate and the population size increases
If r is zero, birthrate equals death rate and
the population size stays the same
If r is negative, birthrate is less than the
death rate and the population size gets
smaller
Example for a population of algae
r=0
r<0
(-)
r>0
(+)
Other factors influencing population growth rate
Dispersal

Dispersal: Movement of individuals

Emigration: leaving a population, often
from crowded areas or in response to
environmental change

Immigration: emigrating individuals
become immigrants in a new population
(joining a population)
Other factors influencing population growth rate
Sex Ratio
Sex Ratio—Relative number of males and
females in a population.
In many sexually reproducing species, with
separate sexes, sex ratio is 1:1
In other species (e.g., asexuals, social
insects) can have far more females than
males
Other factors influencing population growth rate
Age Distribution (Age Structure)
Age Distribution—Number of individuals of
each age in the population.
Greatly influences reproductive rate of a
population.
Summary of factors influencing population
growth rate
Birthrate
Death rate
Sex Ratio
Age distribution
Immigration
Emigration
How many individuals are in a population at any
given time?
How many individuals will be in the population one
generation later?
Which of these age distributions has a Type III
survivorship curve?
A
B
C
Population Density
Population Density (N)—Number of individuals per
unit area.
High population density may lead to increased
competition for resources, increased
emigration, increased death rate
Population 1
N = 4 flowers/m2
Population 2
N = 12 flowers/m2
How to combine number of individuals (N) and
population growth rate (r)?
Population Growth Curve

Biotic Potential—Inherent reproductive
capacity.
 Generally,
biotic potential is much above
replacement level.

Natural tendency for increase.
When not limited, populations tend to grow
exponentially
Exponential Growth
The number of individuals in
each generation is a multiple of
the previous generation
Note: This equation is provided only to
help. You do not need to memorize it. You
will not be asked about the equation on a
test.
Change
in
number
over
time
dN
 rN
dt
Number of
individuals
Population growth rate
Example of exponential growth
Populations do not grow exponentially forever
Population
Growth
Curve
Typical Growth Curve
Lag Phase—First portion of the curve; slow
population growth.
 Exponential Growth Phase—More
organisms reproducing causing accelerated
growth; continues as long as birth rate
exceeds death rate.
 Stable Equilibrium Phase—Death rate and
birth rate equilibrate; population stops
growing.

Carrying Capacity —Number of individuals of a species that
can be indefinitely sustained in a given area.
Environmental Resistance

Environmental Resistance—Any
factor (limiting factor) in the
environment influencing carrying
capacity. Four main factors:
 Raw
material availability
 Energy availability
 Waste accumulation and disposal
 Organism interaction
Logistic Equation
Note: This equation is provided only to
help. You do not need to memorize it.
You will not be asked about the
equation on a test.
dN
N

 rN 1  
dt
 K
Causes of population crashes
Population size can sometimes
overshoot carrying capacity
 Environmental resistance factors can
change, lowering carry capacity

When population > carrying capacity,
death rates > birth rates
Population crashes!!
Example 1: bacterial population crash due to
toxic waste accumulation
Example 2: Population cycling due to
organism interaction
Hare population:
birth rate limited by food
availability
death rate determined by
predators, food
Lynx population:
birth and death rate
determined by food
availability (hares)
Example 2: Population cycling due to
organism interaction
Start: both have low population density
 Hares – high food, low predators = pop increase
over next generations
 Lynx – as hares increase, more food = pop
increase
 Hares – when high pop density, increased
competition for food and increased predation =
low birth rate & high death rate = sharp pop
decrease
 Lynx – when high pop density and few hares,
low food = low birth rate & high death rate =
sharp pop decrease
 Back to start
Population Cycles
Remember: Several factors influence population
growth rate:
Birthrate
Death rate
Sex Ratio
Age Distribution
Immigration
Emigration
But, there are also additional things about the
individuals that influence how the population grows
Life history traits – characteristics of an individual
that influence survival and reproduction
Age at maturity
House Mouse
African elephant
2 months
11 - 20 years
Atlantic Salmon
3-6 years
Life history traits – characteristics of an individual
that influence survival and reproduction
Number of offspring produced
House Mouse
5-8 young every
month
African elephant
1 calf every
3-8 years
Atlantic Salmon
1,500 to 8,000
eggs once
Life history traits – characteristics of an individual
that influence survival and reproduction
Number of reproductive events
House Mouse
African elephant
~6-12
~3 - 10
Atlantic Salmon
1
Life history traits – characteristics of an individual
that influence survival and reproduction
Lifespan
House Mouse
~2 years
African elephant
60 - 70 years
Atlantic Salmon
3-6 years
How do the following life history traits affect the
rate of reproduction?
Age at
maturity
#
offspring
# reprod.
events
life span
2 mo
many
one
long
2 yr
few
many
short
2 mo
many
many
long
2 yr
many
many
long
Reproductive strategies and population fluctuations

K-strategists:
•long lived
•produce few
offspring
•parental care
Remember
that “K” is
Carrying
Capacity
Reproductive strategies and population fluctuations
K-strategists:
•stable populations at
carrying capacity (K)
•examples: humans,
deer, lions
–controlled by density-dependent limiting factors
factors more severe as population increases
example: food
Reproductive strategies and population fluctuations
r-strategists:
•small
•short life
•many offspring
Gypsy moth caterpillars
Remember that “r” is population growth rate
r=b-d
Reproductive strategies and population fluctuations
r-strategists:
•rarely reaches K
•little or no parental
care
Gypsy moth caterpillars
Reproductive strategies and population fluctuations
r-strategists:
•controlled by
density-independent
limiting factors:
weather, pond
drying
Gypsy moth caterpillars
–good at using temporarily abundant resources
Points to know:
Understand the three types of survivorship curves.
What factors influence population growth rates?
What are some life history traits?
Know the difference between K- and r- strategists and
what kind of limiting factors each has.