OCR GCSE Biology A B4.1 Ecosystems Exam Questions

About OCR GCSE Biology A B4.1 Ecosystems

B4.1 opens Topic B4 Community level systems on the OCR Gateway Biology A course, assessed in Paper 2 (Foundation) or Paper 4 (Higher), where knowledge of B1 to B3 is also assumed. It looks at how organisms interact with each other and their environment, how materials are recycled through ecosystems and how biomass is transferred along food chains. It links to photosynthesis and respiration in B1 and to human impacts on the environment in B6.1. Examiners frequently test the carbon and nitrogen cycles, factors affecting the rate of decomposition, predator-prey graphs and calculating the efficiency of biomass transfer between trophic levels.

What you need to know

• Levels of organisation Individual organisms form populations of one species; populations of different species form a community; a community and the non-living parts of its environment form an ecosystem.
• Biotic and abiotic factors Abiotic factors are non-living, such as light intensity, temperature, moisture, pH and soil minerals. Biotic factors are living, such as food availability, predators, pathogens and competition.
• Interdependence and competition Species depend on each other for food, shelter, pollination and seed dispersal, so a change in one population affects others. Plants compete for light, water and mineral ions; animals compete for food, mates and territory.
• Cycling of materials In the carbon cycle, carbon dioxide is removed by photosynthesis and returned by respiration, decomposition and combustion. In the nitrogen cycle, nitrogen-fixing bacteria, decomposers and nitrifying bacteria make nitrates available to plants, while denitrifying bacteria return nitrogen to the air.
• Decomposition Decomposers such as bacteria and fungi break down dead material, releasing carbon dioxide and mineral ions. Decay is faster when it is warm, moist and has plenty of oxygen.
• Biomass transfer Pyramids of biomass show the dry mass at each trophic level. Only a small proportion of biomass is transferred to the next level because of respiration, egestion, excretion and parts not eaten; efficiency = biomass transferred to next level / biomass available x 100.

Common mistakes in B4.1 Ecosystems

• Saying energy or biomass is 'lost' without explaining how. Give specific reasons such as respiration, faeces, urine or parts of organisms not eaten.
• Writing that plants take in nitrogen gas from the air. Most plants absorb nitrate ions from the soil through their roots.
• Confusing nitrifying and denitrifying bacteria. Nitrifying bacteria convert ammonium compounds to nitrates; denitrifying bacteria convert nitrates to nitrogen gas.
• Describing a predator-prey graph without noting the time lag. Predator numbers peak after prey numbers because predators need time to reproduce.
• Treating temperature as always increasing decay. Very high temperatures denature the enzymes of decomposers and slow decay.

Try three B4.1 Ecosystems exam-style questions