5.3 Populations

15/05/2013 § Leave a comment

So we’re back to our regularly scheduled program, with only three blogs left, including this one. Jeez. This one will be a short blog about animal populations, how they change, and how they are graphically represented.

So our trusty study guide defines a population as a group of organisms of the same species, who live in the same area at the same time. Populations can change in four ways, which can be represented by the following math equation:

Population change = (natality + immigration) – (mortality + emigration)

Natality is defined as the [birth] production and addition of offspring into a population. Mortality is the exact opposite, wherein individuals die are lost from the population. Immigration, like with humans, is when individuals move in from somewhere else and join the population. Emigration is quite the opposite, where the individuals move away from the area to join a different population.

Population is illustrated by a sigmoid growth curve, which is S-shaped. There are three important sections of the sigmoid growth curve that we need to know, and these are the exponential phase, transition phase, and plateau phase.

The exponential phase is the stage at which natality (or birth rate) is higher than mortality (death rate) and the population feels an increase. This is why the curve increases “exponentially.” The following phase is the transition phase, wherein natality and mortality start to switch; natality starts to decrease while mortality starts to increase. At this point, natality is still higher than mortality so the population still feels an increase, albeit much slower than in the exponential phase. The final phase is the plateau phase, which is when the environment reaches its limits (it can only support so many individuals). This is called its carrying capacity, which is what the population reaches when it feels a shortage of resources to allocate amongst the individuals in the population. At this point, natality and mortality are equal so the population size remains constant and no more individuals can be added, which is why the graph reaches a plateau.

The factors that limit an environment from always receiving more individuals and a population from constantly growing include competition for resources, a shortage of resources, more predators, diseases, or parasites, and an increase in the toxic byproducts of metabolism.


Essay Questions

  1. Outline what is meant by the trophic level of an organism with three examples from one named habitat. (4 max)
  2. Compare the ways in which autotrophic, heterotrophic and saprotrophic organisms obtain energy. (6 max)
  3. Draw a labelled sigmoid population growth curve. 4 marks
  4. Explain the factors that cause a population to follow the sigmoid ( S-shaped) growth curve. (8 max)
  5. Apply the concept of carrying capacity to the struggle for survival resulting from overproduction of offspring. (5 max)
  6. Outline the international system used for naming species of living organisms. (4 max)
  7. Discuss the definition of the term species. (8 max)
  8. Name the levels and the specific taxa in the hierachy of classification using humans as an example. (2 max)
  9. Describe the relationship between the rise in the concentration of atmospheric carbon dioxide and the enhanced greenhouse effect. 5 marks
  10. Outline the consequences of a global temperature rise on arctic ecosystems. 6 marks
  11. Outline the precautionary principle. 5 marks
  12. Outline the structural differences which characterize bryophytes, filicinophytes, coniferophytes and angiospermophytes. 9 marks
  13. List the structural differences between bryophytes and angiospermophytes. 5 marks
  14. Briefly explain Darwin`s theory of evolution. 4 marks
  15. Outline five types of evidence which support the theory of evolution by natural selection. 6 marks
  16. Outline one modern example of observed evolution by natural selection. 2 marks
  17. Explain the evidence from homologous anatomical structures that supports the theory of evolution. 6 marks
  18. Outline how antibiotic resistance in bacteria can arise in response to environmental change. 5 marks
  19. Antibiotic resistance in bacteria is an example of evolution in response to environmental change. Using another example, explain how an environmental change can lead to evolution. 8 marks

Oh, man, we’ve done so much.


Chapter 15 Questions « Currently in works! Give it some time. »

1. Distinguish between the words within each of the following pairs of terms:

a) natality and mortality

b) ecosystem and community

c) heterotroph and autotroph


a) Calculate the energy lost by plant respiration.

b) Construct a pyramid of energy for this grassland.


a) The chart shows that 99.17% of the sunlight energy in the temperate forest is lost. Predict with a reason whether a greater or lesser percentage of sunlight energy would be lost in desert.

b) Only a small part of the net production of plants in the temperate forest passes to herbivores. Explain the reasons for this.


a) State the year in which there was the greatest increase in biomass of trout caught by fishermen, compared with the previous year. —> 1968

b) Using the data from the bar chart, discuss whether cormorants caused the decrease in the number of trout caught by the fishermen. —> Though the cormorants are part of the reason that the fishermen caught less trout, they are not the sole cause. Since we know that cormorants started to increase starting from 1970, the population of trout would start decreasing from there on if the cormorants were the cause of the fishermen’s smaller catches. This is not what occurred and the amount of trout the fishermen caught between 1970 and 1975/’76 continued to grow. The cormorants only started catching trout from 1983 onwards, so though they take some of trout from the fishermen, they aren’t the sole cause.


a) Use the data to construct two separate pyramids of energy. They should both be drawn to the same scale.

b) Compare the two pyramids.

c) Explain the low biomass and low numbers of organisms in higher trophic levels.


a) Explain the changes in population size:

i. in the first four years

ii. from year 4 to year 6

b) Predict with reasons what would have happened to the population after year 6.

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