Transcript 5-1 How Populations Grow

How Populations Grow
Chapter 5
Section 1
Let’s Review!
 What


is a population?
A group of similar organisms that can breed
and produce fertile offspring
Species or not a species?
Characteristics of Populations


1.
Populations often change due to a variety
of influential factors
There are three important characteristics
of a population.
Geographic
distribution –
the range of area
inhabited by a
population
Characteristics of Populations
2.
Population density – the number of
individuals per unit area
Population Density

Can vary tremendously depending on the
species and its ecosystem
A field of sunflowers
A flock of birds
A few saguro cactus
Turtles grouped to sun
Population Density
What are some benefits of living close
together or in a clumped population?

1.
2.
3.
Help finding food
Protection from predators
or wind
Easier to find species
PopulationDensity
What are some drawbacks (negatives)
of living in a clumped population?

1.
2.
3.
Crowded living areas
Battles over food/shelter
Easier for disease to spread
Characteristics of Populations
3.
Growth rate- the amount by which a
population’s size changes in a given
time
Population Growth

1.
2.
3.
Three factors affect population size
Number of births
Number of deaths
Number of individuals
that enter
or leave
Population Growth




Migration- periodic movements of
populations away from and back to their
place of origin
(not what we are talking about here)
Irregular movements of some individuals
out of an area, into a new area, with no
return - affects population size
Immigration – movement of individuals
into an area
 Population increases in size
Emigration – movement of individuals
out of an area
 Population decreases in size
Population Growth
 Why
would an animal immigrate to or
emigrate from a population?


Competitive pressures on some individuals
To find food, better living condition, or a mate
Exponential Growth
 If
a population has plenty of food and
space and is protected from predators
and disease, what will happen to the size
of the population?

It will increase!
Exponential Growth
Bacteria  reproduce in ~20
minutes by splitting in half
 Example:

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
1 bacteria after 20 min = 2 bacteria
2 bacteria after 40 min = 4 bacteria
4 bacteria after 60 min = 8 bacteria
In 180 min (3 hours) = 512 bacteria
In one day = 4,720,000,000,000,000,000,000 bacteria
Exponential Growth
 Exponential
growth occurs when the
individuals in a population reproduce at a
constant rate
This is what a graph


Creates a “J” shaped
curve
Under ideal conditions
with unlimited resources
and no predation, a
population will grow
exponentially
of bacterial growth
would look like if the
growth went
unchecked
Logistic Growth
 Do
bacteria cover the planet?
 Exponential growth does not continue in
populations very long.
 As resources become less available, the
growth of a population slows or stops
 What might cause population growth to
stop or slow down?

Predators, less food, less space,
less resources, loss of habitat, disease,
etc.
Logistic Growth

Logistic growth occurs when a
population’s 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
Number
Logistic Growth
Time (hours)
Logistic Growth
 Occurs
after a period of exponential growth
 When a population
reaches carrying capacity
Logistic Growth
capacity – the largest number of
individuals an environment can support
 Carrying
Most plant and animal populations follow this
type of
Carrying capacity
growth
Number of Yeast Cells

Time (hours)
Comparing Exponential and
Logistic Growth
Limits to Growth
 Limiting
Factor: any factor that
causes population growth to stop
 Limits the size of a population
 Almost anything can be a limiting factor…
 Examples: competing for sunlight, space,
water, food, etc.
 Predation, parasitism,
disease, natural disasters,
etc.
Limits to Growth
 As
a population increases,
so does competition
 Not all individuals will get what they need
to survive and will die
 The number of births decreases and the
number of deaths
increases
 The population
gets smaller
Limits to Growth
 Plant
roots compete for space, nutrients,
and water with the roots of other plants
 Taller plants block out sun for shorter
plants