Category Archives: population

The Hardy-Weinberg Principle of Allele Frequencies

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The Hardy-Weinberg Principle is a mathematical law that predicts allelic frequencies, making several assumptions:

  • Large population
  • Random mating
  • No immigration
  • No emigration
  • No natural selection

In nature, these assumptions are extremely unlikely to occur, but it is the deviation from the expected distribution of alleles (according to the HW Principle) that informs us about the action of these natural conditions.

Please complete Activity 13.2 (page 141) Looking at Allele Frequencies – Parts A and B.

Population Ecology

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Population Ecology is the study of the numbers and distribution of groups of organisms. The Biology Q and A site has some good revision for exams at Population Ecology in question and answer format. Breathing Earth is the site I showed you in class with informative, animated graphics that show a comparison of births, deaths and CO2 emissions across the globe. It clearly shows how the rate of births and deaths in China, India and Africa differs to those in Australia and North and South America.

This Predator Prey simulation is based on a cardboard grid and game pieces representing rabbits and lynx. It demonstrates how the population of predator and prey impact on each other. At the Gould League site there is another activity that clearly shows how predator/prey populations interact with each other, this time with kangaroos and dingoes in an Australian ecosystem. Draw a graph of the data, showing the predator and prey in different colours. Then answer the following questions:

  1. What patterns can be seen in the graph? Suggest reasons for any patterns.
  2. Is there any relationship between the kangaroo and the dingo numbers? If so, account for this.
  3. The park management have decided to stop a rabbit prevention program because it is too expensive. What effect could this have on the dingo population and why?
  4. Identify the reasons for the absence of large migratory populations in Australia.

Population Dynamics

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A line transect on a rocky intertidal habitat at Heron Island Research Station.

The study of any species requires an estimate of their density in different habitats. Scientists use many different methods of measuring density, depending on the size, abundance, behaviour and habitat of the species. Our class used a line transect and quadrats to measure the abundance and cover of different floral species in a grassland reserve. Other methods to measure relative density include:

  1. Aerial photographs (eg. herds of moose or other hoofed mammals during migration, flamingos)
  2. Capture-recapture (or mark and release)
  3. Traps (eg. Elliot traps for small mammals, pit-fall traps for reptiles, light traps for insects, harp traps for bats)
  4. Number of fecal pellets
  5. Vocalisation frequency
  6. Pelt records
  7. Catch per unit fishing effort
  8. Number of artifacts (nests, pupal cases, burrows etc.)
  9. Questionnaires (of hunters and trappers for example)
  10. Cover (of plants)
  11. Feeding capacity (amount of bait taken)
  12. Roadside counts

Each method has it’s advantages and disadvantages and is most valubale when used as an adjunct to more direct methods.

Reference: Charles J. Krebs (1985) “Ecology – The Experimental Analysis of Distribution and Abundance” Harper International, New York.

Population Growth

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Today in Biology we looked at population growth, graphing populations of bacteria (hypothetical), bees in a hive, deer and the human population. The rate of population growth is dependent on four factors – births and immigration (increase) and deaths and emigration (decrease). In the hypothetical situation of bacterial growth with no limits, we saw that the population increased exponentially. Bees showed a gradual increase and then reached their equilibrium around the ‘carrying capacity’ of the hive. The ‘carrying capacity’ is dependent on environmental pressures such as space and nutrient availablity. Other environmental factors that will affect population growth include predation, disease (parasites and pathogens), catastrophic weather events (drought, flood, fire, storms) and habitat destruction. The deer population showed a more gradual increase, a peak and then a sharp decline. This may have been as a result of hunting, drought, disease, introduced predators or parasites.  

The human population growth showed an exponential increase, with our global population now at nearly 7 billion people. Check out the Worldometer for current (real-time) population statisitcs. Worldmapper has some interesting thematic maps showing the size of each country relative to different statistics – for example population, infant mortality, life span or income. One good example of how a human population exceeded it’s carrying capacity is Easter Island. “The Mystery of Easter Island” describes how a once flourishing civilisation was reduced to starvation, disease, war and cannabilism. This was also the title of an excellent documentary. We also discussed the article “More than 100 million women are missing” and “Where did 100 million missing women go?”