The nature of ecosystems and samplingEdexcel A-Level Biology B: Subtopic test

1. 1
   In a woodland, one oak tree supports about 2000 caterpillars. The caterpillars are eaten by about 20 blue tits, and the blue tits are eaten by one sparrowhawk. A student draws a pyramid of numbers for this food chain.

   (a)

   What is the main limitation of the pyramid of numbers for this food chain?

   [1 mark]

   • AIt does not take account of the size of organisms, so the pyramid is not a true pyramid shape
   • BIt cannot show which organisms are producers
   • CIt does not show the number of trophic levels
   • DIt can only be drawn by killing the organisms

   (b)

   Which type of pyramid is the most reliable for comparing the transfer of energy between trophic levels?

   [1 mark]

   • APyramid of numbers, because counting is easiest
   • BPyramid of dry biomass, because it is never inverted
   • CPyramid of energy, because it allows for the rate of production and is never inverted
   • DPyramid of fresh biomass, because it shows all the water

   (c)

   A pyramid of dry biomass is often used instead of a pyramid of fresh mass. Explain one advantage and one disadvantage of using dry biomass.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student investigates how the abundance of creeping buttercup changes across a school field, moving from a damp hedge bottom into the open, drier field. She also needs a method for grass in the same field, where individual plants cannot easily be told apart.

   (a)

   Which technique is most suitable for investigating how the abundance of buttercup changes from the hedge into the open field?

   [1 mark]

   • ARandomly placed quadrats over the whole field
   • BMark-release-recapture
   • CQuadrat samples at one point near the centre of the field
   • DA line transect with quadrats at regular intervals

   (b)

   To avoid bias when estimating the mean density of buttercup in a uniform area of the field, how should the positions of the quadrats be chosen?

   [1 mark]

   • AWhere the plant looks most abundant
   • BUsing random coordinates from a random number table
   • CAlong the edge of the field
   • DAt equal intervals along one fence

   (c)

   Explain why percentage cover is more suitable than counting individuals for the grass.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student estimates the population of snails in a pond margin by mark-release-recapture. She collects 48 snails and marks the shell of each with a small spot of non-toxic paint. She releases them at the place of capture. After 24 hours she collects 40 snails, of which 12 are marked.

   (a)

   Calculate the estimated population size of the snails.

   [3 marks]

   (b)

   State two assumptions made in this method and describe two precautions the student should take to use the snails safely and ethically.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A researcher studies a rocky shore. On one transect she records the percentage cover of bladderwrack seaweed in 10 quadrats at increasing distances up the shore, and calculates a Spearman rank correlation coefficient of rs = −0.89 (critical value for n = 10 at p = 0.05 is 0.648). She also compares limpets on an exposed and a sheltered shore, using 10 randomly placed quadrats on each. The mean is 9.8 limpets per quadrat on the exposed shore and 14.2 on the sheltered shore. A t-test gives t = 2.90, and the critical value for 18 degrees of freedom at p = 0.05 is 2.10.

   (a)

   Explain why the researcher used a Spearman rank correlation test, and interpret the result of rs = −0.89.

   [6 marks]

   (b)

   Evaluate the conclusion that sheltered shores support more limpets than exposed shores.

   [6 marks]

   Total for question 4: 12 marks

End of questions