WJEC GCSE Biology 1.6 Ecosystems, Nutrient Cycles and Human Impact on the Environment Exam Questions

About WJEC GCSE Biology 1.6 Ecosystems, Nutrient Cycles and Human Impact on the Environment

This final Unit 1 topic looks at how energy and nutrients pass through ecosystems and how human activity affects them. You need to interpret food chains and webs, draw pyramids of numbers and biomass, and explain why energy is lost between trophic levels. You must describe the carbon and nitrogen cycles, including decay by micro-organisms, nitrogen fixation and denitrification. The topic then covers intensive farming, using indicator species such as lichens to monitor pollution, bioaccumulation of heavy metals and how sewage and fertilisers cause oxygen loss in rivers. Examiners often set efficiency calculations and extended answers on eutrophication.

What you need to know

• Feeding relationships Food chains start with a producer, then primary, secondary and tertiary consumers. Know herbivores, carnivores and decomposers, and how a change in one population affects others in a food web.
• Pyramids and energy transfer Pyramids of numbers and biomass show each trophic level. Energy is lost at each stage in waste (faeces, urine) and as heat from respiration, and some is used for movement, so only a small fraction passes on. Efficiency = energy passed on / energy received x 100.
• Decay and the carbon cycle Micro-organisms decay dead material and waste, releasing carbon dioxide by respiration. Photosynthesis removes carbon dioxide from the air; respiration and combustion of fossil fuels return it. In a stable community these processes are balanced.
• Nitrogen cycle Decomposers turn proteins in dead material into ammonia, then nitrifying bacteria convert it to nitrates. Nitrogen-fixing bacteria (in soil and root nodules) convert nitrogen gas into nitrogen compounds; denitrifying bacteria return nitrates to nitrogen gas in waterlogged soil.
• Human impact Intensive farming increases yield but reduces biodiversity and can cause pollution. Lichens indicate air pollution (sulfur dioxide); invertebrate indicator species, pH and oxygen levels indicate water pollution. Heavy metals and pesticides can bioaccumulate, reaching toxic levels in top consumers.
• Eutrophication Sewage or fertiliser run-off adds nutrients, causing rapid algal growth. Algae block light and die; decomposing bacteria respire and use up the oxygen, so fish and other animals die.

Common mistakes in 1.6 Ecosystems, Nutrient Cycles and Human Impact on the Environment

• Saying energy is lost as heat from movement only. Most energy is lost as heat released by respiration, plus energy in waste materials and uneaten parts.
• Writing that algae use up the oxygen in eutrophication. It is the bacteria decomposing the dead algae that respire and use up the dissolved oxygen.
• Confusing nitrifying and nitrogen-fixing bacteria. Nitrifying bacteria convert ammonia into nitrates; nitrogen-fixing bacteria convert nitrogen gas into nitrogen compounds.
• Drawing a pyramid of biomass using numbers of organisms. Biomass is the dry mass of living material at each level, so one large tree gives a wide base even though it is a single organism.

Try three 1.6 Ecosystems, Nutrient Cycles and Human Impact on the Environment exam-style questions