Download Population changes

2.6.1-2.6.4
Population Changes
LO: to understand ‘curves’!
The basics
2.6.1 Explain the concept of limiting factors and carrying capacity in the
context of population growth
 Populations change i.e. Bacteria reproduce asexually by splitting in
two (binary fission): 1, 2, 4, 8...
 Population growth without things affecting it down is called
exponential or geometric growth
Limiting factors
2.6.3 Describe the role of density-dependent and density-independent
factors, and internal and external factors, in the regulation of populations
 Things that affect growth are called ‘limiting factors’
Brainstorm possible
‘limiting factors’ for the
human population
.
Two types of limiting factors:
1.
Density dependent: usually biotic; effects increase with
population size; negative feedback mechanisms; lead to
stability/ regulation; can be divided into internal (within a
species) and external (between different species) factors
2.
Density Independent: usually abiotic; not related to
population size, no feedback
The red words are examples of each of the three types:
1a. Density dependent internal
1b. Density dependent external
Weather
Limited food supply
war
2. Density independent
predation
limited territory
disease
Can you match them to their correct area????
1a. Density dependent internal: limited food supply and limited
territory
1b. Density dependent external: predation and disease
2. Density independent: weather and war

Positive feedback: leads to increasing change in a system—it accelerates
deviation

Negative feedback: a self-regulating method of control leading to the
maintenance of a steady-state equilibrium—it counteracts deviation
Rabbit population
increase, more grass is
eaten, less food is
available, rabbits die, less
grass is eaten, more
grass grows, more grass
is available, rabbit
population increases...
Global warming causes
an increase in
temperatures, this melts
ice, there is less ice, less
radiation is reflected so
more is absorbed, global
temperatures increase,
more ice melts...
What’s all the ‘curves’ about?
2.6.2 Describe and explain S and J population curves
 Limiting factors lead to two types of curve:
 ‘s’
‘j’
Which do you think is density
dependent?Which is density
independent?
Carrying capacity
 ‘the maximum population size of an ecosystem’
 Both ‘s’ curve and ‘j’ curve population have a response around the carrying
capacity...
 Overall, no gradual slow down is
shown with ‘j’ curves (as with ‘s’
curves) so probably a densityindependent factor causes the
dieback
 Both types of curve are ‘models’ of
reality. In practice, a combination of
both is probably nearer to the truth
And now for something completely
different...questions!
Answer these questions in the space provided on the sheet using pages 162-163
 1. Can carrying capacity be exceeded in the short term?
 2. What term describes this?
 3. What term describes the return to below carrying capacity?
 4. Do you agree that humans are ‘overshooting’?
 5. What environmental resistance has the human population faced?
 6. How have we overcome this resistance?
 7. What to YOU think may cause ‘dieback’ in terms of human population
growth?
Review
 What term defines how large a population can be sustained in




an area?
What two types of curve are there to show population
growth?
What term describes a population exceeding its CC?
What term defines the things that hinder population growth?
The answer to the previous question can be divided into
three categories- What? And give and example of each
Reproductive strategies
Species
are competing for their niche.
The main aim for all
K- strategists
r- strategists
species
is to reproduce and for their
to grow.
 i.e.population
Fish
 i.e. Humans
Fast reproduction
Few off-spring
In order for this to happen there are
twoofcommon
strategies
Loads
babies
High care effort
1. Have one offspring at a time, investing
time
to look after
Little
care
effort
Population Klose to carrying
it very carefully
(K)
Rapid
colonisers,
capacity
2. Invest time into reproducing lots opportunistic
of offspring all the time,
Good competitors
but not into lookingOften
aftercrash
them (r)
Outnumber r in stable
Outnumber K in unstable
ecosystems
ecosystems
Can you think of an example
of each type?
Survivorship curves
2.6.4 Describe the principles associated with survivorship curves
including k- and r- strategists
 These are graphs that show the fate of a group of individuals
of a species....There are 3 ‘model’ curves based on what
happens to k strategists, r strategist and a very rare third
possibility where a species has an equal chance of dying at any
age, for example in the hydrozoan hydra
Copy the Survivorship curves
on p164 and annotate each
curve to explain what it shows
NB: Logarithmic scales
 Used to show large changes in a variable within a manageable
scale. The Richter scale of earthquake strength is a common
example:
HW
Complete questions 4-10 on page 164 in FULL SENTENCES
OR
Exam question
(MS)