Agroecosystems — AQA A-Level Environmental Science

Agroecosystems represent controlled terrestrial habitats where natural ecological succession is arrested and species diversity is deliberately constrained to optimise the yield of target organisms. This topic explores the fundamental differences between natural ecosystems and agricultural systems, focusing on nutrient cycling, genetic diversity, energetics, and the environmental footprint of food production.

Mastering agroecosystems bridges biological principles with environmental management, illustrating how anthropogenic inputs such as fertilisers, pesticides, irrigation, and selective breeding alter ecosystem stability. It forms a cornerstone of AQA Environmental Science by providing the critical context needed to evaluate global food security against land degradation and biodiversity conservation.

• →Agroecosystems vs Natural Ecosystems: Contrast in species richness, trophic web complexity, genetic variation, stability, and reliance on auxiliary energy inputs.
• →Energy Ratios and Food Trophics: Energy subsidies (embodied fossil energy in machinery, fertilisers, transport) versus usable caloric yield, and the thermodynamic advantage of plant-based trophic levels.
• →Biogeochemical Disruptions: Accelerated cycling and open-ended nutrient loss, including nitrogen leaching, phosphate runoff, and depleted soil organic matter.
• →Pest Dynamics in Monocultures: Reduced natural predator reservoirs and uniform host genotype distributions promoting density-dependent pest and disease proliferation.
• →Sustainable Intensification: Agroecological interventions including agroforestry, integrated pest management (IPM), intercropping, and conservation tillage.

• Compares at least two named insecticide groups on shared criteria such as persistence, spectrum, toxicity and bioaccumulation
• Explains the mode of action of each group compared and links it to the resulting advantage or disadvantage
• Weighs persistence against environmental harm, noting how long residues remain and how they enter food chains
• Assesses effects on non-target organisms, including beneficial insects and human health, for each group
• Explains how repeated use of a group can select for resistance and what that means for its long-term value
• Reaches a reasoned judgement about which group is preferable for a stated management aim and justifies the choice

• 💡Set out the comparison criterion by criterion rather than group by group, so each paragraph makes a direct contrast
• 💡Name the groups explicitly and give a concrete example of a crop or pest context for each judgement
• 💡Finish with a short evaluative conclusion that states which group you would choose for a named aim and why
• 💡Always state whether you are discussing intensive or extensive systems when describing agricultural practices, as energy ratios and biodiversity impacts differ radically between them.
• 💡Reference specific named techniques (e.g., 'no-till farming', 'leguminous green manures', 'strip cropping') rather than vague generalities like 'environmentally friendly methods'.
• 💡Remember that the AQA syllabus demands evaluation of both ecological impacts and economic viability; do not focus entirely on environmental detriments while ignoring crop yield requirements.

• Lists advantages of one group and disadvantages of another without comparing them on the same criteria; compare each group against identical criteria
• Describes a single insecticide rather than an insecticide group; keep the comparison at the level of chemical groups
• States that a persistent insecticide is simply better because it lasts longer; weigh persistence against bioaccumulation and non-target harm
• Asserts that one group is always best without stating the management aim; tie the judgement to a specific aim such as protecting pollinators
• Believing intensive livestock farming is more energy-efficient than arable farming because animals grow quickly; in reality, 90% of energy is lost between trophic levels, making livestock systems net consumers of energy.
• Assuming organic farming produces zero environmental impacts; organic practices require larger land areas to achieve equivalent yields, which can result in greater land conversion pressure and habitat fragmentation.
• Equating agroecosystem productivity solely with solar radiation; modern intensive agroecosystems depend fundamentally on fossil-fuel subsidies for synthetic nitrogen synthesis via the Haber process.

1. 1Day 1-3: Review comparative tables of natural vs agroecosystems; master the definitions of intensive, extensive, monoculture, and polyculture.
2. 2Day 4-6: Practice energy balance calculations, calculating output:input ratios for livestock versus arable farming scenarios.
3. 3Day 7-9: Study soil degradation mechanisms and evaluate remediation methods (e.g., contour ploughing, cover crops, conservation tillage).
4. 4Day 10-12: Complete 6-mark and 9-mark past exam questions evaluating Integrated Pest Management (IPM) and agroforestry systems against continuous monoculture.

• 📋Calculation questions: Determining agricultural energy ratios, percentage change in crop yield, or nutrient application rates per hectare.
• 📋Comparative data analysis: Interpreting tables comparing soil organic carbon, bulk density, or runoff volumes under conventional tillage versus conservation agriculture.
• 📋Extended response (Evaluate/Discuss): 9-mark essay questions assessing the environmental sustainability and food security trade-offs of modern agricultural practices.