CCEA A Level Biology 4.4 Ecosystems Exam Questions

About CCEA A Level Biology 4.4 Ecosystems

Section 4.4 of Unit A2 1 brings together populations, communities, ecological energetics and nutrient cycling. You need the phases of population growth, carrying capacity and environmental resistance, r- and K-selected species, interactions such as competition, mutualism, predation and parasitism, and biological control. Practicals include counting yeast with a haemocytometer and estimating population size by capture-mark-recapture using the Lincoln index. You also need succession to a climax community, trophic levels, GPP and NPP, ecological pyramids, efficiency of energy transfer, applications to agriculture, and the carbon and nitrogen cycles. Calculations and graph interpretation appear in most papers.

What you need to know

• Population growth Phases are lag, exponential (log), stationary and decline. Carrying capacity is the maximum population the environment can support; environmental resistance is the factors that limit growth. r-selected species reproduce rapidly with little parental care; K-selected species have fewer young and stay near carrying capacity.
• Interactions and control Competition harms both (-/-) and may eliminate one species; mutualism benefits both (+/+); predation, parasitism and grazing are +/-. Biological control uses a predator or competitor to reduce a pest below economic damage levels.
• Practicals Count yeast cells in a haemocytometer grid of known volume to calculate density. Estimate population size by capture-mark-recapture: N = (M x C) / R, assuming marks are not lost, marking causes no harm, and no migration, births or deaths occur between samples.
• Succession Pioneer species colonise, change the environment and are replaced in stages until a climax community forms. Primary succession starts on bare ground; secondary succession is faster, after fire, flooding or clearing.
• Energy flow NPP = GPP - respiration. Energy is lost between trophic levels through respiration (heat), egestion, excretion and parts not eaten. Efficiency = energy transferred to next level / energy in previous level x 100. Pyramids of energy are never inverted.
• Agriculture Eating plant products is more energy-efficient than animal products. Fertilisers raise primary productivity; housing animals reduces respiratory losses; silage and high-protein feed improve production.
• Nutrient cycles Carbon: respiration, combustion and decomposition release CO2; photosynthesis removes it. Nitrogen: decomposers release ammonium; nitrifying bacteria oxidise it to nitrite then nitrate; nitrogen-fixing bacteria make organic nitrogen compounds; denitrifying bacteria return N2 to the air.

Common mistakes in 4.4 Ecosystems

• Writing the Lincoln index the wrong way round. N = number marked first time x number caught second time / number marked in second sample.
• Saying energy is lost as heat only from respiration of plants. Losses occur at every trophic level, especially as heat in endotherms.
• Confusing nitrification and nitrogen fixation. Nitrification converts ammonium to nitrate; fixation converts N2 to nitrogen compounds.
• Saying a pyramid of biomass can never be inverted. It can be (for example phytoplankton); pyramids of energy cannot.
• Describing exponential growth as steady growth. In the log phase numbers double at a constant rate because resources are not limiting.

Try three 4.4 Ecosystems exam-style questions