OCR GCSE Biology B B3.3 How are organisms in an ecosystem interdependent? Exam Questions

About OCR GCSE Biology B B3.3 How are organisms in an ecosystem interdependent?

B3.3 looks at how organisms depend on each other and on the cycling of materials. You need to interpret food chains and webs, explain how changes in one population affect others, and describe competition for resources. Biomass and energy are lost at each trophic level, so you should be able to draw pyramids of biomass and calculate the efficiency of transfer. Decomposers recycle materials through the carbon, nitrogen and water cycles. This builds on photosynthesis in B3.1 and leads into B3.4 and biodiversity in B6.4. Examiners often set food web change questions and efficiency calculations.

What you need to know

• Food chains and webs Food chains start with a producer. Arrows show the direction of energy and biomass transfer. A food web shows interlinked chains, with consumers at different trophic levels.
• Interdependence Organisms depend on each other for food, shelter, pollination and seed dispersal. A change in one population affects others in the web.
• Competition Plants compete for light, space, water and mineral ions; animals compete for food, mates and territory.
• Biomass transfer Only a small proportion of biomass passes to the next trophic level, as some is not eaten, some is lost in faeces and urine, and much is used in respiration. Efficiency = biomass transferred / biomass available x 100.
• Pyramids of biomass Pyramids of biomass are drawn to scale and are almost always pyramid shaped because biomass decreases at each level.
• Decomposers and cycles Bacteria and fungi break down dead material, releasing carbon dioxide by respiration and mineral ions to the soil. Carbon cycles via photosynthesis, respiration and combustion; nitrogen-fixing, nitrifying and denitrifying bacteria cycle nitrogen.

Common mistakes in B3.3 How are organisms in an ecosystem interdependent?

• Drawing food chain arrows from predator to prey. The arrow points to the organism doing the eating.
• Saying energy is used up or destroyed. Energy is transferred to the surroundings, mainly as heat from respiration.
• Predicting only one effect of a change in a food web. Consider all the organisms that eat or are eaten by the affected species.
• Writing that plants get their food from soil. Plants make food by photosynthesis and only take mineral ions from soil.
• Dividing the wrong way in efficiency questions. Divide the biomass at the higher level by the biomass at the lower level.

Try three B3.3 How are organisms in an ecosystem interdependent? exam-style questions