Ecosystems and processes — AQA A-Level Geography
Test yourself on Ecosystems and processes with AQA A-Level practice questions.
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Ecosystems and processes explained
An ecosystem's structure comprises interacting biotic communities and abiotic factors.
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Energy flows through this system unidirectionally, entering primarily via solar radiation captured by autotrophs (producers) during photosynthesis. This energy is transferred through distinct trophic levels, from primary consumers (herbivores) to apex predators. A food chain illustrates a single, linear pathway of this energy transfer, whereas a food web provides a more realistic, complex network of interconnected food chains within the ecosystem. Because energy is lost at each trophic level through respiration and heat (following the 10% rule), food chains rarely exceed four or five levels, fundamentally shaping the ecosystem's carrying capacity and overall biomass distribution.
Application of systems concepts to ecosystems – inputs, outputs, stores and transfers of energy and materials. Concepts of biomass and net primary production.
Ecosystems function as open systems with distinct inputs (solar radiation, precipitation), outputs (runoff, heat loss), stores (biomass, soil, litter), and transfers (decomposition, food web consumption) of both energy and materials. Energy flows linearly and is ultimately lost as heat, whereas materials like carbon and nitrogen cycle continuously. Biomass represents the total dry mass of living organisms per unit area, typically measured in g m⁻². Net primary production (NPP) is the rate at which plants accumulate energy or biomass, calculated as gross primary production minus respiratory losses (NPP = GPP - R). It is usually expressed in g m⁻² yr⁻¹ or kJ m⁻² yr⁻¹, indicating the energy available to higher trophic levels.
Concepts of succession: seral stages, climatic climax, sub-climax and plagioclimax.
Succession is the directional change in a plant community over time. It progresses through sequential seral stages (e.g., pioneer species, shrubs) as organisms modify the environment, making it more suitable for subsequent species. This naturally culminates in a climatic climax, the stable end-point determined by the regional climate, such as temperate deciduous woodland in the UK. A sub-climax occurs when development is temporarily arrested by local factors like waterlogging or poor soil before reaching the climatic climax. Conversely, a plagioclimax is a stable plant community maintained by continuous human intervention, such as controlled burning on heather moorlands or sheep grazing, which prevents the natural climax from establishing.
Mineral nutrient cycling.
Mineral nutrient cycling describes the continuous movement of essential chemical elements, such as nitrogen and phosphorus, through an ecosystem. Using Gersmehl's model, this cycle is visualised through three main stores: biomass (living vegetation and animals), litter (dead organic matter), and soil. Nutrients transfer between these stores via flows: littering moves nutrients from biomass to litter, decomposition transfers them from litter to soil, and root uptake returns them to biomass. The system also experiences inputs, such as dissolved nutrients in precipitation or chemical weathering of parent rock, and outputs, including surface runoff and leaching. For example, in a tropical rainforest, the biomass store is exceptionally large, and rapid decomposition ensures swift nutrient cycling despite heavily leached soils.
Nature of terrestrial ecosystems and the inter-connections between climate, vegetation, soil and topography which produce them. Ecosystem responses to changes in one or more of their components or environmental controls.
Terrestrial ecosystems are complex, open systems shaped by dynamic inter-connections between climate, vegetation, soil, and topography. Climate dictates temperature and precipitation, directly controlling vegetation growth rates and soil weathering. Topography influences local microclimates and drainage, affecting soil depth and plant distribution. Vegetation, in turn, protects soils from erosion and contributes organic matter. Ecosystems respond continuously to changes in these components or environmental controls. For instance, a reduction in precipitation (climatic control) can trigger drought stress, leading to vegetation dieback. This reduces leaf litter, slowing soil nutrient replenishment and increasing vulnerability to erosion. Such feedback loops demonstrate how altering a single variable cascades through the entire ecosystem, forcing it to adapt or degrade.
Factors influencing the changing of ecosystems, including climate change and human exploitation of the global environment.
Ecosystems are dynamic and constantly evolve due to natural and anthropogenic factors. Climate change alters temperature and precipitation regimes, shifting ecosystem boundaries and disrupting phenology, such as the earlier blooming of flora affecting pollinator life cycles. Human exploitation of the global environment drives rapid degradation through deforestation, overfishing, and pollution. For example, the conversion of the Amazon rainforest for cattle ranching fragments habitats, reduces biodiversity, and disrupts local hydrological cycles. Understanding these drivers requires analysing how global-scale exploitation and climatic shifts interact to degrade ecosystem resilience and trigger irreversible tipping points.
Your focus
- Describe the biotic and abiotic components that form the structure of an ecosystem.
- Illustrate the unidirectional flow of energy through various trophic levels.
- Compare and contrast the concepts of food chains and food webs using specific ecological examples.
Show all 18 objectives
- Categorise ecosystem components into inputs, outputs, stores, and transfers of energy and materials.
- Define biomass and explain its significance as a store of energy and matter.
- Calculate and interpret net primary production using the relationship between gross primary production and respiration.
- Describe the progression of vegetation communities through distinct seral stages over time.
- Differentiate between a climatic climax and a sub-climax community using natural environmental factors.
- Explain how human activities deflect natural succession to create and maintain a plagioclimax community.
- Diagram the stores and flows of a mineral nutrient cycle using Gersmehl's model.
- Compare the relative sizes of nutrient stores and flows across different terrestrial biomes.
- Explain how environmental factors influence the rates of nutrient transfer, such as decomposition and weathering.
- Map the inter-connections between climate, vegetation, soil, and topography within a specific terrestrial ecosystem.
- Predict how a specific terrestrial ecosystem will respond to a defined change in an environmental control, such as a prolonged drought.
- Explain the role of feedback loops in maintaining or disrupting the dynamic equilibrium of an ecosystem.
- Identify and describe the primary factors driving changes in global ecosystems.
- Explain the specific mechanisms by which climate change and human exploitation degrade ecosystem functions.
- Evaluate the interconnected nature of anthropogenic climate change and direct environmental exploitation.
Ecosystems and processes exam tips
Marking Points
- Define ecosystem structure as the dynamic interaction between biotic (living) and abiotic (non-living) components.
- Explain that energy flows unidirectionally through an ecosystem, unlike nutrients which are continuously cycled.
- Describe the function of trophic levels, identifying producers, primary, secondary, and tertiary consumers.
- Distinguish between a food chain (linear energy transfer) and a food web (complex, interconnected feeding relationships).
- Apply the concept of energy loss between trophic levels (approximately 90% lost as heat and respiration) to explain limits on food chain length.
- Define ecosystems as open systems exchanging both energy and matter with their surrounding environment.
- Identify specific stores (e.g., biomass, dead organic matter, soil) and transfers (e.g., nutrient uptake, leaf fall, consumption).
- Distinguish between the one-way flow of energy (ultimately lost as heat) and the cyclical transfer of materials (nutrients).
- Define biomass as the total dry mass of living matter within a specific area, noting its role as a major energy store.
- Explain Net Primary Production (NPP) as the energy remaining after autotrophic respiration (R) is subtracted from Gross Primary Production (GPP).
- Describe succession as a series of predictable changes in vegetation communities over time, driven by environmental modification.
- Identify seral stages as the distinct intermediate communities that replace one another during succession.
- Define climatic climax as the final, stable community in equilibrium with the prevailing regional climate.
- Explain sub-climax as a stage where succession is halted by natural, non-climatic factors such as edaphic (soil) or topographic conditions.
- Define plagioclimax as an ecosystem whose development is permanently deflected or maintained by human activities like grazing or deforestation.
- Identification of the three main nutrient stores: biomass, litter, and soil.
- Explanation of the flows between stores, including littering, decomposition, and root uptake.
- Description of system inputs (precipitation, weathering) and outputs (leaching, surface runoff).
- Application of Gersmehl's nutrient cycle model to specific biomes, such as the rapid cycling in tropical rainforests versus slow cycling in tundra.
- Explanation of how climate (temperature and precipitation) acts as the primary macro-control on biome distribution and productivity.
- Analysis of the reciprocal relationship between vegetation and soil, such as plants providing organic matter while soils supply anchorage and nutrients.
- Description of topography's role in modifying local ecosystems through altitude, slope angle, and aspect affecting drainage and sunlight.
- Evaluation of ecosystem responses to environmental changes, illustrating positive and negative feedback loops triggered by events like climate change or deforestation.
- Application of systems theory to show how altering one component (e.g., removing vegetation) impacts interconnected stores and flows (e.g., increased soil erosion).
- Explain how climate change alters abiotic conditions, forcing species to migrate to higher latitudes or altitudes to survive.
- Detail specific mechanisms of human exploitation, such as agricultural expansion, resource extraction, or overfishing, and their direct impact on biodiversity.
- Discuss the synergistic effects where climate change exacerbates the vulnerability of ecosystems already weakened by human exploitation.
- Evaluate the scale of these changes, contrasting localised human impacts with the pervasive, global nature of climate change.
Examiner Tips
- 💡Always draw arrows in food webs pointing in the direction of energy transfer, representing the flow of biomass.
- 💡Use specific examples of organisms for each trophic level when describing a local or global ecosystem.
- 💡Reference the second law of thermodynamics when explaining why biomass and energy decrease at higher trophic levels.
- 💡Use specific examples of inputs (e.g., solar energy) and outputs (e.g., leaching) when describing an ecosystem as an open system.
- 💡Always include appropriate units when discussing NPP, such as g m⁻² yr⁻¹ or kJ m⁻² yr⁻¹, to demonstrate mathematical understanding.
- 💡When evaluating ecosystem productivity, explicitly state the formula NPP = GPP - R to structure your explanation.
- 💡Provide a named example of a plagioclimax, such as UK heather moorlands managed by grouse shooting, to support your definitions.
- 💡Use the term 'deflected succession' when explaining how human activities create a plagioclimax community.
- 💡Clearly distinguish between primary succession (starting on bare rock) and secondary succession (starting on existing soil) when discussing seral stages.
- 💡Draw and label a Gersmehl diagram to illustrate nutrient cycling in a specific biome, ensuring the size of circles and arrows reflects the relative size of stores and flows.
- 💡Use specific examples of mineral nutrients, such as nitrogen, phosphorus, or potassium, rather than just referring to 'nutrients' generically.
- 💡Link the speed of nutrient cycling to climatic factors, explaining how temperature and moisture control the rate of decomposition.
- 💡Use a specific case study, such as a tropical rainforest or savanna, to exemplify the inter-connections between climate, soil, and vegetation.
- 💡When discussing ecosystem responses, explicitly use systems terminology like 'thresholds', 'dynamic equilibrium', and 'feedback mechanisms'.
- 💡Structure essay answers by dedicating paragraphs to specific inter-connections (e.g., climate-soil, vegetation-topography) before synthesising how they function as a whole.
- 💡Use specific, named examples of ecosystems, such as the Great Barrier Reef or the Amazon, when discussing the impacts of climate change and exploitation.
- 💡Assess the relative importance of different factors; for instance, argue whether direct human exploitation or indirect climate change is the primary threat to a specific ecosystem.
- 💡Link changes in ecosystems to broader geographical concepts like the carbon and water cycles to demonstrate synoptic understanding.
Common Mistakes
- Stating that energy cycles through an ecosystem; correction: specify that energy flows unidirectionally and is eventually lost as heat, whereas nutrients cycle.
- Drawing food chain arrows pointing towards the organism being eaten; correction: ensure arrows point in the direction of energy flow, from the prey to the predator.
- Confusing food chains with food webs; correction: define a food chain as a single linear pathway and a food web as a complex network of multiple interconnected chains.
- Confusing energy flow with material cycling; energy flows linearly and is lost as heat, whereas materials cycle continuously through the system.
- Using wet mass instead of dry mass to define biomass; biomass must be measured as dry mass to eliminate variations caused by fluctuating water content.
- Equating Gross Primary Production (GPP) with Net Primary Production (NPP); NPP is GPP minus the energy used by plants for respiration.
- Using sub-climax and plagioclimax interchangeably; sub-climax is halted by natural factors, whereas plagioclimax is maintained by human activity.
- Assuming the climatic climax is identical globally; the climax community depends entirely on the specific regional climate (e.g., tropical rainforest versus tundra).
- Believing pioneer species remain dominant throughout succession; pioneer species are outcompeted and replaced in later seral stages as the environment changes.
- Confusing leaching with surface runoff; leaching is the downward movement of soluble minerals through the soil profile, whereas runoff occurs over the surface.
- Assuming the soil is always the largest nutrient store; in tropical rainforests, the biomass is the largest store due to rapid uptake and heavy leaching.
- Treating the nutrient cycle as a closed system; ecosystems have inputs like precipitation and weathering, and outputs like leaching, making them open systems.
- Treating ecosystem components in isolation; students must explicitly describe the two-way interactions, such as how soil affects vegetation and how vegetation simultaneously affects soil.
- Ignoring topography as a control; students often focus solely on climate, forgetting that altitude and slope aspect create distinct microclimates and soil conditions.
- Confusing environmental controls with human impacts; while humans cause change, environmental controls refer to the fundamental physical drivers like climate, geology, and topography.
- Error: Confusing natural climate variability with anthropogenic climate change. Correction: Explicitly distinguish between long-term anthropogenic climate change and short-term natural cycles like the El Niño-Southern Oscillation.
- Error: Treating human exploitation and climate change as entirely separate issues. Correction: Acknowledge the feedback loops, such as deforestation releasing carbon dioxide, which in turn accelerates climate change.
- Error: Using vague terms like 'pollution' without context. Correction: Specify the type of exploitation or pollution, such as agricultural runoff causing eutrophication in aquatic ecosystems.