Download Population Biology

A population is a group of organisms
belonging to the same species and living
within a certain area.
 The number of organisms within the
group is the population size.
 Under ideal conditions, unlimited food,
absence of disease etc, the size of a
population would increase indefinitely.

Increase in the size of a population over
time is termed population growth.
 Here is an example of a J-shaped growth
curve.

Under ideal conditions, many species of
bacteria can double their population
size every 20 minutes.
 In 36 hours, one bacterium could give
rise to enough bacteria to cover the
Earth with a layer 30 cm deep.
 The highest rate of reproduction under
ideal conditions is called a population’s
biotic potential.

Most organisms never reach their biotic
potential.
 Limiting factors are circumstances that
keep organisms from reaching their
biotic potential.


Temperature, disease, predators,
habitat, and lack of food can all be
considered limiting factors.
How do we know how populations grow
and respond to the environment?
 In a lab, growth can be studied easily
because conditions can be controlled.
 To study a population of yeast cells, a
culture medium can be inoculated with
a few yeast cells and growth can be
measured.

Population Age (Hours) Number of Cells
Increase in Number
0
10
2
29
19
4
71
42
6
175
104
8
351
176
10
513
162
12
595
82
14
641
46`
16
656
15
18
662
6
10
700
600
500
400
10
300
200
100
0
0
5
10
15
20
When a population arrives at a point
when its size is no longer increasing, it has
reached its carrying capacity of the
environment at that particular time.
 This number is not definite or permanent.
 What do you think this means?
 At carrying capacity, the birth rate for a
population will roughly equal the death
rate, (everything else being equal.)

Limiting factors for organisms that follow
an S-shaped growth curve are almost
always density-dependent.
 This means the influence of any limiting
factor varies with the size of the
population density.
 Example: Frogs in a pond.


Insects usually show a different kind of population
growth. Resembles a J curve at first, then crashes. Ex:

DI factors, like temp, fire, oxygen supply, hurricanes, etc.
are generally physical aspects of the environment.

Questions 1-5 page 766
DD limiting factors are more often
biological rather than physical.
 Predation and food: Often hard to
determine what effect predation has on
a population.
 Predation as a DD factor: we will work
on an activity with this.
 Is predation beneficial?

Parasites can be limiting factors for a
population.
 They usually harm their host, but do not
kill it.
 If they killed the host, they would be
without a home.
 Why is parasitism a DD factor?
 What about disease and populations?
Are they DD as well?

When populations of different species
compete for same food source.
 Its DD.
 When it occurs, three things may happen:

› Extinction (local)
› Movement – warblers and spruce trees.
› Adaptation – Darwin’s finches – seeds vs. insects.
Virtual Lab involving Paramecium caudatum and
Paramecium aurelia.
Competition between members of the
same species.
 More severe, because all members of the
same species have same requirements to
survive.
 Also DD: how organisms adapt

› Life cycles and life spans – tadpoles and frogs;
insects, dispersal of plant seeds by wind.
› Dominance and social hierarchy – wolves.
Increases order and decreases aggression
throughout the species. If not all can mate due
to poor conditions, dominant ones will and lesser
ones will not.
Role separation – ants and bees
 Behavioral and physiological changes –
birth and death rates can be affected
by stress in populations.
 Emigration – move out to a new area.
 Territoriality – song birds, coyotes, fox,
howler monkeys. Reduces conflict within
species and can help keep populations
in check.

1.2
1
0.8
0.6
0.4
0.2
0
0
0.2
0.4
0.6
0.8
1
1.2
Right now, it appears to be a J curve
growth.
 Growth rates are declining in developed
countries.
 Not the case in undeveloped countries.

› More sanitary populations
› Antibiotics
› Technology has expanded the limits
Thomas Malthus wrote an essay on
population growth.
 Dealt with people’s tendency to take
care of what they owned, and not take
care of what they did not.
