| Date | November 2017 | Marks available | 2 | Reference code | 17N.3.SL.TZ0.13 |
| Level | Standard level | Paper | Paper 3 | Time zone | TZ0 |
| Command term | Compare and contrast | Question number | 13 | Adapted from | N/A |
Question
The sea snail Nucella ostrina and the sea star Pisaster ochraceus are predators of the mussel Mytilus trossulus. The mussels live on rocks at the edge of the sea and feed on phytoplankton and zooplankton. The zooplankton feed on the phytoplankton.
Groups of 50 mussels were transplanted to an experimental area and protected from predation until the start of the experiment. Researchers then investigated the effect of the predators on the population of the mussels over a period of 60 days.
[Source: Republished with permission of John Wiley and Sons, from Navarrete, S. A. and Menge, B. A. (1996),
Keystone Predation and Interaction Strength: Interactive Effects of Predators on Their Main Prey. Ecological Monographs, 66: 409–429. doi:10.2307/2963488; permission conveyed through Copyright Clearance Center, Inc.]
Compare and contrast the effects of the predators on the population of the mussels.
The sea star also eats the sea snails. Construct a food web to show the feeding relationships between these five organisms in the ecosystem.
Markscheme
a. both cause the frequency of the mussel to decrease
b. sea star affects the mussel population more than the sea snail
c. when both are together the effect of the sea snail is low
Accept binomial names
Allow numerical answers if expressed as comparisons and the candidates are not simply stating numbers
[Max 2 Marks]
Award [2] for a correct food web
Award [1] for phytoplankton, zooplankton and mussel with correct arrows
Award [1] for mussel, sea snail and sea star with correct arrows
Award [0] if arrows are in wrong direction
Accept binomial or scientific names
