| Date | May 2017 | Marks available | 2 | Reference code | 17M.2.SL.TZ0.1 |
| Level | Standard Level | Paper | Paper 2 | Time zone | Time zone 0 |
| Command term | Outline | Question number | 1 | Adapted from | N/A |
Question
The concentration of DDT at different trophic levels of the food chain.
Figure 1: Levels of concentration of DDT in food chain
[Source: © International Baccalaureate Organization, 2017]
State the main source of energy for the food chain in Figure 1.
State the trophic level labelled X in Figure 1.
Identify one use of DDT that has led to its presence in the environment.
With reference to the concepts of bioaccumulation and biomagnification, outline how the concentration of DDT has changed along the food chain.
State the relationship between large and small fish in Figure 1.
Outline how this relationship may be of benefit to the populations of both species.
Markscheme
Sun/sunlight/insolation/solar energy
(primary) producer / autotrophs / phototrophs / phytoplankton / green plants
The command term “State” requires a “specific name” so do not credit “trophic level 1”.
insecticide/pesticide / kill pests/insects / control malarial mosquitoes / plague / OWTTE
bioaccumulation: DDT absorbed by a given trophic level is not broken down/excreted / is non-biodegradable so accumulates in tissues [1 mark]
biomagnification: the concentration of DDT will increase as it’s passed along/up food chain (because other biomass is lost) [1 mark]
Do not allow simply “organisms at higher levels eat/consume more”. Higher trophic levels do not consume more biomass in a given time than lower trophic levels.
predator-prey / carnivory / predation / OWTTE
Do not credit “small fish are eaten/consumed by large fish”.
the predator benefits by gaining food from prey / its population is limited/stabilized by amount of prey available [1 mark]
they prey benefits by predators limiting/stabilizing its population / selecting out weaker/diseased individuals / maintaining healthy gene pool in prey / reducing competition in prey for available food/resources [1 mark]
Award full marks is response describes negative feedback between both populations, or indicates that each group regulates population of the other.
Examiners report
Only around half the candidates were able to identify solar energy as the energy source for food chains.
Great majority were able to identify plants/primary producers as base of food chain.
Most were able to identify DDT as a pesticide … students should be advised that where question specifies a single response it is not a wise strategy to offer a selection of responses.
Very few candidates were able to apply these terms of “bioaccumulation” & “biomagnification” appropriately …some confused the two; some made no reference to them as required by the question. More were able to identify biomagnification correctly than bioaccumulation.
Most were able to identify predation but a surprisingly large number failed to do so.
The great majority were able to identify benefits to both populations.
