Today we are covering from the specification:
Population Growth
There is a “standard” pattern of growth
changes seen when an organism is
introduced into a new environment.
TASK: Using the sheet, calculate the mean
population data for the yeast and plot the graph.
EXTENSION: Try the task at the bottom of the sheet.
Sigmoidal Growth Curve
There are 4 key phases:
• Lag phase
• Log/exponential phase
• Stationary phase
• Decline or death phase
(in closed population).
What is happening at each
stage? Why?
Bacterial Growth
• Eventually the
bacteria will grow
until a factor limits
growth.
• Amazingly these are
called limiting
factors…
Limiting Factors
Limiting factors in growth culture?
• Nutrient availability
• Oxygen concentration
• Levels of excretory factors
In the wild a variety of factors may be limiting.
Population Changes
Although most wild populations stay
relatively constant, there are highly
significant fluctuations in numbers due to
several key factors – what are they?
Key Factors Affecting Populations
Birth Rate
 Death Rate
 Competition
 Availability of food (or prey)
 Predators
 Disease
 Parasites
 Space/territory
 Migration (emigration and immigration)

Competition

Intraspecific
 Interspecific
 Between members of  Between members
the same species,
of different species
who will have very
that “overlap” for
similar needs.
resources.
Predator/Prey Cycles
One of the most significant factors
affecting populations.
 In some environments the numbers of a
particular animal is critically dependent
on the availability of its prey.
 Equally, the presence or absence of
significant numbers of predators has a
dramatic effect on prey populations.

Questions
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What is a predator?
What is a prey organism?
How are predators adapted to catch
their prey?
How can prey animals adapt to avoid
predators?
Predation
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One animal consumes another.
In the lab this results in extermination
of the prey….why does this not happen
in nature?
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What happens when there are too few
predators?
http://www.bbc.co.uk/learningzone/clips/
population-size-and-control-predatorsand-prey/5499.html
Snowshoe Hare and Lynx
Charles Elton
(on the right)
Studied living
organisms in
relation to
their natural
environment,
or ECOLOGY
He began his
studies in
Oxford in the
1920s and
died in 1991
Lynx
Young
Lynx
Snowshoe
Hare
 In Canada in the early
1900s, lynx and hares were
both killed for their pelts.
 Elton saw graphs of the
annual fur returns of the
Hudson’s Bay Fur Trading
Company.
 He noticed regular 10 year
cycles of lynx and hare skins
being collected.
Elton was made Biological Consultant
to the Hudson’s Bay Fur Company.
There he examined records of
Canada Lynx (numbers caught) back
to 1736 (300 years).
His results were published in 1942.
The Typical Cyclical Oscillation of
the Predator/Prey Relationship
Lynx cycle lags behind hare by 1-2 yrs.
Why?
What is the cause of these
population changes?
When hare numbers increase, 2
things happen:
1. Shortage of food for hares
2. Increase in Lynx number
(predation)
Lynx numbers increase when hare
numbers are high, but if food for
hares is low and predation is
high……………………..
…………soon the hare population
will crash (drop).
This drop in hare numbers leads to
3 things …
(1) Some lynxes leave (emigrate) to where
food is more plentiful.
(2) Lynxes eat other prey when hare
numbers low.
(3) More competition between lynxes.
A CRASH in the lynx population.
MEANWHILE…..
Lower hare numbers – vegetation starts to grow
again
+ Hares have fewer lynx to watch out for
+ There is more vegetation to eat
SO, the number of hares begins to increase
The PREDATOR PREY cycle begins
again.
Highest peaks always belong to PREY
Why?
General Predator/Prey
relationship
Effect on Populations
Sketch a predator-prey graph and
label the following points:
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Predators eat prey – population is reduced.
Predators are now in greater competition
with each other for the remaining prey.
Predator population falls.
Fewer predators, so fewer prey are eaten.
More food available for prey.
Prey population increases.
Predator population has more food,
population increases.
The Reason for the Cycle
Limiting Factors in the wild
• More complex than bacteria.
• Will also need nesting sites
• Effect of predators and
parasites.
• All of the above will put an
upper limit on numbers –
carrying capacity.
Carrying Capacity
• Unlike the bacteria in the flask, food never
totally runs out.
• Environmental factors affect numbers
• Such as the weather.
• Difficult to control all facts when we study
population sizes.
Evaluation
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This is a cyclic relationship.
BUT not the only reason for cyclic
fluctuations in populations – disease and
climactic factors also play a part.
These crashes create ‘selection pressures’
which are important in evolution.
The population evolves and becomes better
adapted to the prevailing conditions
more stable population
Examples - answers
a) Lag – few cells take time to divide
enough for numbers to rise steeply.
Log/exponential – increasing rate of
growth as there are sufficient resources to
support them/little competition
Stationary – increasing competition for
resources means reduced rate of population
growth.
1.
1. b) Yeast grows more quickly/reaches carrying
capacity more quickly
Daphnia shows a decline/death phase
Daphnia population increases past carrying
capacity before declining
Use of figures
1. c) Relies on sexual reproduction which takes
longer (than asexual in yeast)
May be ‘in the wild’ – other factors affect
growth rate (e.g. predation) therefore carrying
capacity may be changing.
2. a) P.aurelia population grows more slowly at first
P.aurelia population reaches a higher population
density
Use of figures.
2. b) P.caudatum population grows more slowly
P.caudatum population declines after 6 days but
P.aurelia continues to increase
Neither population reaches the same size as
when grown separately.
2. c) Reproduction in P.aurelia usually slower
P.aurelia is a better competitor
P.caudatum outcompeted for
resources/nutrients/oxygen
Results in death of P.aurelia
Presence of both organisms results in
lower populations for both (as they are
competing interspecifically).