1. 1
   A microbiologist introduces a small number of bacteria to a flask of fresh nutrient broth and monitors the population size over several hours, producing a sigmoid (S-shaped) population growth curve with four distinct phases.

   (a)

   A small number of bacteria are introduced to a flask of nutrient broth. For the first hour, the number of bacteria barely increases, as the bacteria adjust to their new environment. What is this initial phase of the population growth curve called?

   [1 mark]

   • AExponential (log) phase
   • BLag phase
   • CStationary phase
   • DDeath phase

   (b)

   After the initial slow period, the bacterial population begins to increase very rapidly, roughly doubling in number at regular short intervals, as nutrients are still abundant. What is this rapid growth phase called?

   [1 mark]

   • ALag phase
   • BDeath phase
   • CStationary phase
   • DExponential (log) phase

   (c)

   Explain what causes the bacterial population eventually to stop increasing and enter the stationary phase, and what typically happens after this to bring about the death phase.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A population of deer is introduced to an island with abundant food and no natural predators. The population grows rapidly at first, but ecologists predict that as the population continues to grow, its rate of increase will eventually slow due to several factors, including the later arrival of a predator species to the island.

   (a)

   A population of deer on an island increases rapidly after being introduced, since food is initially abundant and there are no natural predators. Which factor is most likely to eventually limit further growth of this deer population?

   [1 mark]

   • AFood supply
   • BAn increase in the number of available mates
   • CA decrease in the deer's need for oxygen
   • DAn increase in island size

   (b)

   Several years later, a population of wolves is introduced to the same island. What effect would the presence of this new predator most likely have on the deer population's growth curve?

   [1 mark]

   • AIt would have no effect, since predation does not influence population growth
   • BIt would cause the deer population to grow faster than before
   • CIt would help slow the rate of population growth by increasing the deer death rate
   • DIt would only affect the deer's food supply, not their population size

   (c)

   State two factors, besides predation, that could affect the rate of growth of the deer population on the island, and explain how one of these factors could limit population growth.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student investigates population growth using two different organisms: a yeast culture grown in a flask of sugar solution, monitored hourly for 24 hours, and a population of rabbits introduced to a large enclosed field with plentiful grass and no predators, monitored monthly for two years. Both populations show a similar sigmoid (S-shaped) growth pattern.

   (a)

   A yeast culture is grown in a flask of sugar solution, and its population size is recorded every hour for 24 hours, producing a sigmoid population growth curve. Describe the four phases of this sigmoid curve in the order in which they occur, naming each phase.

   [3 marks]

   (b)

   The rabbit population in the enclosed field shows a similar sigmoid growth pattern to the yeast population, but over a much longer timescale (months rather than hours). Explain the factors that would cause the rabbit population to enter its stationary phase, and explain one similarity and one difference between the factors limiting growth in the rabbit population compared with the yeast population.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A conservation project is monitoring two newly introduced populations to understand population dynamics: a population of fish introduced to a large, previously fish-free lake with abundant food and no predators, and a population of insects introduced to a greenhouse containing a limited number of host plants. Both populations are expected to follow a sigmoid growth pattern, but researchers want to understand how graphs of these curves can be interpreted.

   (a)

   A population of fish is introduced to a large, previously fish-free lake that has abundant food and no natural predators. Explain fully, referring to all four phases of the sigmoid population growth curve, how the fish population would be expected to change over several years, and explain the biological reasons for the transition between each phase.

   [6 marks]

   (b)

   A population of insects is introduced to a greenhouse containing a limited number of host plants for them to feed on. Researchers plot a graph of insect population size against time over several months, producing a sigmoid curve. Explain how a researcher could use such a graph to identify the lag, exponential, stationary and death phases, describing what features of the graph (such as its shape and gradient) would indicate each phase, and explain why interpreting such graphs is a useful skill for predicting the effects of introducing a population to a new, resource-limited environment.

   [6 marks]

   Total for question 4: 12 marks

End of questions