Question 19M.3.HL.TZ2.6a.ii
| Date | May 2019 | Marks available | [Maximum mark: 2] | Reference code | 19M.3.HL.TZ2.6a.ii |
| Level | HL | Paper | 3 | Time zone | TZ2 |
| Command term | Determine | Question number | a.ii | Adapted from | N/A |
The shore crab (Carcinus maenas) forages for food in the intertidal zone on rocky seashores. An experiment was carried out to assess predation at specified levels of food abundance. An area of shore was enclosed in a wire cage. All other food was removed from the encaged area except the mussels provided by the investigators. The number of mussels consumed was measured while the crabs foraged at densities of one, two or four crabs per cage.
[Source: reprinted from B D Griffen and D G Delaney, Ecology, 88 (12), pages 3012–3021,
copyright 2007, with permission, the Ecological Society of America]
Determine whether foraging by crabs is dependent on number of prey (mussels) or number of predators (crabs).
[2]
a. «foraging» depends on number of predators/crabs ✔
b. mean number of mussels/prey consumed per crab decreases as number of crabs increases ✔ Accept vice versa.
c. crabs compete for prey/mussels ✔
d. both «prey and predator» affect foraging success ✔
Most candidates figured out that the plateau occurred when the crab could eat no more but it was more difficult for certain to determine that the foraging depends on the number of predators and to justify it.
