Changes 1Populations
Topic 2.6
Death
Immigration
Population
Emigration
Birth
Factors that increase population size:
•Natality (Birth rate) is recruitment to a population through reproduction
•Immigration from external populations e.g. Bird migration
Factor reducing population size:
•Mortality (death rate) which is the death rate from any source e.g.
predation
•Emigration, where individuals leave the population for another habitat
For a stable population:
Immigration
+ Natality
=
Emigration
+ Mortality
Decline phase
Plateau phase
Transitional Phase
Exponential Phase
Lag phase
Population size
A Sigmoid (s - shaped) Population Curve
Time
•
•
•
•
•
Lag phase: Population growth begins slowly from a few individuals.
Exponential Phase: Exponential growth occurs, the conditions are ideal and
maximum growth rate is reached.
Transitional Phase: Growth rate begins to slow down as factors such as food,
water and space become limiting.
Plateau phase: Carrying capacity for the population has been reached and the
population number becomes stable. The carrying capacity is the population size
that can be supported by a particular environment.
Decline phase: If there is a sudden change in the environment meaning that the
environment can no longer support the population, such as a drought causing
food shortage, the population will crash and the whole process begins again.
Carrying Capacity.
• An ecosystem can only support a certain
size of a population – its carrying capacity.
• Carrying capacity is affected by the
amount of energy available to the
organism – more energy available means
a higher population!
Population size can be affected by the
following:
• Density dependent factors: These are any
factors, dependent on the density of the
population in question. Generally their effects
increase as population increases. Some
examples of these are predation, disease and
competition. They tend to form a negative
feedback mechanism that controls population.
• Density independent factors: These are any
factors, not dependent upon the density of the
population in question. Some examples of these
are climate or weather and catastrophe.
Internal and external factors
affecting populations
• Internal – factors that act within a species.
Eg. limited food supply, availability of
territory, density dependent fertility. (Take
a look at this interesting article about
overcrowding by Aidan Sammons – link )
• External – factors that act between
different species. Eg. Predation and
disease.
Case studies
• Rabbits
• The Lynx and the
Snowshoe Hare.
Rabbits in Australia
• In 1859, Thomas Austin brought out 24
rabbits, 5 hares and 72 partridges and
released them on his property, in Victoria.
• In 1887, Rabbits were first sighted in
Northern Territory.
The gestation period
for a rabbit averages
31 days.
A rabbit can live as
long as 10 years
Rabbits can start
breeding as early as
3-4 months of age
European Rabbits
have an average litter
size of 7 – 9 but may
be as high as 15!
8 Rabbits eat as
much grass as 1
sheep!
• By 1907 Rabbit numbers had reached
plague proportions (up to an estimated
600million) causing untold environmental
damage:
So from 1901 -1908 they tried to
build a….
It didn’t work…..
So in 1950, after research carried out by Frank Fenner,
Myxomatosis was deliberately released into the rabbit
population, causing it to drop from an estimated 600 million to
around 100 million.
Genetic resistance in the remaining rabbits allowed the
population to recover to 200-300 million by 1991.
• To combat this trend, Commonwealth Scientific
and Industrial Research Organisation (CSIRO)
scientists developed and accidentally released
calicivirus (also known as Rabbit Haemorrhagic
Disease or RHD) in 1996.
The Australian Government
refuses to legalise a
vaccine to protect pet
rabbits against
Myxomatosis, for fear the
rabbit may escape and
transmit resistance to the
disease. A legal vaccine
exists in Australia for RHD.
There is no cure for either
Myxomatosis or RHD, and
many affected pets are
euthanised.
Activity:
Using the information in this power-point:
• Draw a sketch of the population curve for
Rabbits in Australia from 150 to present
day.
• Label relevant parts of the curve and
include approximate dates and population
sizes.
• Discuss with a friend whether the methods
of control were ethically or environmentally
justifiable.
Predator Prey
Relationships
• The lynx and the Snow
Hare
A lynx
-Eats
Snowshoe
hares
A
Snowshoe
Hare.
- Eaten by
Lynxs.
Activity:
• How will increasing numbers of Snowshoe
hare affect the numbers of lynx?
• How will increasing numbers of Lynx affect
the numbers of Snowshoe hare?
• How will decreasing numbers of
Snowshoe hare affect the numbers of
lynx?
• Does this explain the population graph?
Population
J – population curves
Show a ‘Boom and Bust’.
Population
Time
Time
• Exponential growth followed by
‘Dieback’.
• Growth is not limited by density
dependent factors.
• Often ‘overshoots’ the carrying
capacity of the ecosystem.
• Examples: Bacteria, algae,
invertebrates, many fish species
and some small mammals.
There are 2 groups of species with differing reproductive
strategies. Those that go for rapid growth (r-strategists) and
those that go for Karrying Kapacity (K-strategists). These are
extremes – most species fall somewhere in the middle.
r-strategists
k-strategists
Life span
Short
Long
Growth rate
Rapid
Slow
Age of maturity
Early
Late
How many offspring?
Loads
Few
How large offspring?
Small
Large
Little or none
Lots
Unstable
Stable
External
Internal
Lower
Higher
Weeds, plankton
Oak trees, elephants
Parental care
Stability of environment it
lives in
Internal or external
population regulation?
Trophic levels
Example
Survivorship curves.
• This is the fate of groups of individuals
through their life span.
% Population of survivors
K- strategists – Few young, lots of
parental care, so mortality at young
ages is low. Most organisms live to
average lifespan.
r- strategists – many young, limited
parental care, so high infant mortality.
Few organisms live to full lifespan.
Most organisms come somewhere
in between.
Age
Activity:
•
•
A survival model for a
species of bird
Population pool- birds
of breeding age.
200
eggs
laid
Deaths: •
100
young
and
•
eggs
•
•
Deaths:
100
•
birds
100
0
1
50
25
12
2
3
Age in years
4
•
What is the lifespan
of these birds?
What is the potential
population size?
How many survivors
are there after the 1st
year?
What is the
percentage mortality
after the1st year?
What is the
percentage mortality
after the 2nd year?
What is the
percentage mortality
after the 3rd year?
What would the
survivorship curve for
this population look
like?
Is this a K or an r
strategist?
Question form Page 164 – Environmental Systems and Societies Course companion – Jill Rutherford – Oxford University press 2009
Picture sources:
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Slide 4 http://www.s-cool.co.uk/alevel/biology/ecological-concepts/populations.html
Slide 7 http://www.creationsbydawn.net/pi/tutorials/rabbit.jpg
Slide 8 http://wwp.greenwichmeantime.com/timezone/australia/_derived/map.htm_txt_australia-map.gif
http://library.thinkquest.org/03oct/00128/en/rabbits/history.htm
Slide 9
http://highlandviewrabbitry.tripod.com/sitebuildercontent/sitebuilderpictures/breeding.jpg
http://www.threelittleladiesrabbitry.com/rabbitfacts.php
Slide 10 http://upload.wikimedia.org/wikipedia/en/3/3e/Rabbit-erosion.jpg
Slide 11 http://www.onesteelwaratah.com.au/media/81204/rabbit%20fence%20sign.jpg
Slide 12 http://en.wikipedia.org/wiki/Rabbits_in_Australia#Biological_measures
http://images.encarta.msn.com/xrefmedia/sharemed/targets/images/pho/t978/T978475A.jpg
Slide 13 http://members.iinet.net.au/~rabbit/rcd2.jpg
Slide 16
http://4.bp.blogspot.com/_W9mOaqcNMB4/SVVbtbqziJI/AAAAAAAABgc/WXsJp9Ysgpo/s40
0/lynx+young+walking+on+snow.jpg
Slide 17 http://www.ngsprints.co.uk/images/M/100148.jpg
Slide 18 https://fp.auburn.edu/sfws/ditchkoff/images/Lecture%20Images/Carnivores/lynxhare_cycle.gif