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Populations - PowerPoint PPT Presentation

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Populations. Topic 5.3. Death. Population. Immigration. 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 :

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Topic 5.3






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 Emigration

+ Natality + Mortality


A Sigmoid (s - shaped) Population Curve

  • Lag phase: Population growth begins slowly from a few individuals.

  • Log phase or Exponential Phase: Exponential growth occurs, the conditions are ideal and maximum growth rate is reached.

  • S-phase or Transitional Phase: Growth rate begins to slow down as factors such as food, water and space become limiting.

  • Stable (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
Case studies

  • Rabbits

  • The Lynx and the Snowshoe Hare.

The gestation period for a rabbit averages 31 days. 72 partridges and released them on his property, in Victoria.

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!

So from 1901 1908 they tried to build a
So from 1901 -1908 they tried to build a…. causing untold environmental damage:

It didn t work
It didn’t work…. causing untold environmental damage:.

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, causing untold environmental damage: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: causing untold environmental damage:

  • 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
Predator Prey Relationships causing untold environmental damage:

  • The lynx and the Snow Hare

A lynx causing untold environmental damage:

-Eats Snowshoe hares

A Snowshoe Hare. causing untold environmental damage:

- Eaten by Lynxs.

Activity: causing untold environmental damage:

  • 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
Picture sources: causing untold environmental damage:

  • 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/time-zone/australia/_derived/map.htm_txt_australia-map.gif


  • Slide 9 http://highlandviewrabbitry.tripod.com/sitebuildercontent/sitebuilderpictures/breeding.jpg


  • 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


  • Slide 13 http://members.iinet.net.au/~rabbit/rcd2.jpg

  • Slide 16 http://4.bp.blogspot.com/_W9mOaqcNMB4/SVVbtbqziJI/AAAAAAAABgc/WXsJp9Ysgpo/s400/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/lynx-hare_cycle.gif