Populations
Topic 5.3
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
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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.
Population size can be affected by the
following:
• Density dependent factors: These are any
factors, dependent on the density of the
population in question. Some examples of these
are predation, disease and competition.
• Density independent factors: These are any
factors, not dependent upon the density of the
population in question. Some examples of these
are climate and catastrophe.
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 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:
• Draw a 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 your neighbour 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?
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