1. 1
   Students estimated the population of woodlice (Oniscus asellus) living in leaf litter under a hedge. On the first night they caught 60 woodlice in pitfall traps, marked each one with a small dot of non-toxic paint on the underside and released them where they were caught. Two nights later they caught 75 woodlice in the same traps, of which 15 were marked.

   (a)

   What is the Lincoln index estimate of the size of this woodlouse population?

   [1 mark]

   • A12
   • B135
   • C300
   • D4500

   (b)

   Which is an assumption of the capture–mark–release–recapture method?

   [1 mark]

   • AMarked individuals mix randomly with the rest of the population before the second sample
   • BAll individuals in the population are caught in the first sample
   • CMarked individuals are more likely to be recaptured than unmarked ones
   • DThe organisms being sampled are sessile

   (c)

   Suggest how the estimate would be affected if the paint made marked woodlice more visible to predators such as birds, and explain why.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Hard corals on a tropical reef contain single-celled algae called zooxanthellae living inside the cells of the coral polyps. During a marine heatwave, many corals expelled their zooxanthellae and turned white (bleaching). Divers also recorded crown-of-thorns starfish (Acanthaster planci) on the reef; these starfish feed on coral polyps, killing them.

   (a)

   How do the zooxanthellae benefit the coral?

   [1 mark]

   • AThey fix nitrogen gas into ammonia for the coral
   • BThey supply the coral with carbon compounds made by photosynthesis
   • CThey sting and deter predators of the coral
   • DThey digest the coral's waste to release energy for the coral

   (b)

   Which category of interspecific relationship is shown by crown-of-thorns starfish and the coral?

   [1 mark]

   • AMutualism
   • BHerbivory
   • CInterspecific competition
   • DPredation

   (c)

   Explain why corals often die if the zooxanthellae do not return after bleaching.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Ecologists recorded the presence or absence of two plant species, P and Q, in 80 randomly placed quadrats in a grassland. Both species were present in 8 quadrats, P only in 32 quadrats, Q only in 28 quadrats, and neither species in 12 quadrats. The critical value of chi-squared at p = 0.05 with one degree of freedom is 3.84.

   (a)

   Calculate the value of chi-squared for these data, showing the expected number of quadrats for each combination.

   [3 marks]

   (b)

   Evaluate the conclusion that species P and Q compete with each other.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student placed 10 fronds of duckweed (Lemna minor) in a tank of pond water with added mineral nutrients, kept in constant light and temperature, and counted the fronds every 3 days. The counts from day 0 to day 30 were: 10, 20, 41, 79, 150, 260, 380, 450, 480, 490, 492.

   (a)

   Analyse the data to explain the pattern of growth of the duckweed population.

   [6 marks]

   (b)

   Discuss the extent to which the sigmoid growth model can be used to predict how populations change in natural ecosystems.

   [6 marks]

   Total for question 4: 12 marks

End of questions