Welcome to Small Scale Ecosystems
Welcome to your study notes for Theme 2B: Small Scale Ecosystems, part of AS 1: Physical Geography (CCEA Specification 3910). Ecosystems can seem complicated at first because they involve living organisms, chemical nutrients, and physical landscapes all interacting at the same time. Don't worry if this feels like a lot to take in right now—we will break down every concept step-by-step with clear definitions, visual models, and case study examples to help you secure top marks in your 1 hour 15 minute AS 1 exam.
Quick Exam Fact: Unit AS 1 is worth \(40\%\) of your AS grade (or \(16\%\) of your full A Level). In Section A and Section B, examiners look for clear definitions, accurate diagrams (like the Gersmehl model), and specific real-world examples.
---1. Ecosystem Components and Structure
What is an Ecosystem?
An ecosystem is a dynamic, open system made up of a community of living organisms (biotic components) interacting with each other and with their non-living physical environment (abiotic components) within a defined area.
Because it is an open system, both energy and matter can enter and leave the system across its boundaries.
• Biotic components: All living flora (plants) and fauna (animals), as well as microscopic organisms like bacteria and fungi.
• Abiotic components: The non-living physical and chemical elements, including solar radiation, temperature, rainfall, soil type, rock geology, water chemistry, and air.
Classification of Organisms (Trophic Roles)
Every living organism in an ecosystem has a specific feeding role. Organisms are classified into four main categories:
1. Producers (Autotrophs):
These are green plants and algae. They capture solar energy from the sun and convert it into chemical energy (carbohydrates) via photosynthesis. Producers form the foundational base of all ecological food chains.
2. Primary Consumers (Herbivores):
Organisms that feed directly on producers to obtain their energy (for example, caterpillars eating oak leaves, or rabbits grazing on grasses).
3. Secondary and Tertiary Consumers (Carnivores and Omnivores):
• Secondary consumers: Predators that eat primary consumers (e.g., blue tits feeding on caterpillars).
• Tertiary consumers: Apex predators that feed on secondary consumers (e.g., sparrowhawks feeding on smaller birds).
4. Decomposers (Saprotrophs and Detritivores):
Organisms such as fungi, bacteria, earthworms, and woodlice that break down dead organic matter and waste. They release essential mineral nutrients back into the soil so plants can absorb them again.
Energy Flow and Trophic Levels
Energy enters an ecosystem as solar radiation and moves through the ecosystem along a feeding pathway known as a food chain. Each step or feeding stage in a food chain is called a trophic level.
The Unidirectional Rule: Unlike nutrients, energy does not cycle. Energy flows unidirectionally (one-way) through trophic levels and is eventually lost to the atmosphere as heat.
The \(10\%\) Energy Transfer Rule:
Energy transfer between trophic levels is inefficient:
• Approximately \(90\%\) of energy is lost at each trophic stage.
• Energy is lost via metabolic respiration, movement, excretion/unassimilated waste, and heat.
• Only approximately \(10\%\) of energy is converted into new tissue (biomass) and made available to the next feeding level.
Because energy decreases dramatically at each level, ecosystems rarely support more than 4 or 5 trophic levels. This can be illustrated using Ecological Pyramids:
• Pyramids of Numbers: Show the total count of individual organisms at each trophic level.
• Pyramids of Biomass: Show the total dry mass of living biological matter at each trophic level.
• Pyramids of Energy: Show the total amount of energy entering each level over time (always pyramidal in shape).
Key Takeaway: Energy flows one way through trophic levels with \(\sim 90\%\) lost at each stage, while decomposers recycle dead organic material back into the physical environment.
---2. Nutrient Cycling: The Gersmehl Model
Understanding the Gersmehl Model
While energy flows straight through an ecosystem, mineral nutrients (such as nitrogen, phosphorus, and potassium) are constantly recycled and conserved. In 1976, P. F. Gersmehl developed a classic model representing how nutrients move through three main storage compartments.
The Three Nutrient Stores
In a Gersmehl diagram, stores are represented by circles:
• Biomass (\(B\)): The total mass of living organisms (plants, animals, microbes) within the ecosystem.
• Litter (\(L\)): The layer of dead, decaying organic matter resting on the surface of the soil (fallen leaves, twigs, dead wood, animal carcasses).
• Soil (\(S\)): The weathered topsoil containing dissolved, mineralised nutrients held on soil colloids and available for plant roots.
Transfers, Inputs, and Outputs
Nutrients move between these three stores along specific pathways (shown as arrows on a diagram):
Internal Transfers:
• Plant Uptake (\(S \to B\)): Plant roots absorb dissolved nutrients from the soil moisture to build living tissue.
• Litterfall / Fallout (\(B \to L\)): Leaves, twigs, branches, and dead organisms drop from the biomass onto the ground surface.
• Decomposition and Humification (\(L \to S\)): Decomposers (fungi and bacteria) break down dead litter, converting organic compounds into inorganic soil minerals (humus).
Inputs into the Ecosystem:
• Precipitation / Atmospheric Deposition (into \(L\)): Rain, snow, and dust carry dissolved nutrients directly into the litter layer.
• Weathering of Parent Rock (into \(S\)): Chemical and physical breakdown of bedrock releases mineral ions directly into the soil.
Outputs leaving the Ecosystem:
• Runoff (out of \(L\)): Surface water washes loose litter and dissolved nutrients away into streams and rivers.
• Leaching (out of \(S\)): Downward percolation of water washes soluble nutrients down through the soil profile beyond the reach of plant roots.
• Gas loss / Denitrification: Nitrogen compounds lost back to the atmosphere as gases.
Exam Diagram Tip: When drawing the Gersmehl model for a specific ecosystem, the size of the circles must reflect the relative size of the stores, and the thickness of the arrows must represent the speed/volume of the transfers.
Key Takeaway: The Gersmehl model consists of three main stores (\(B\), \(L\), \(S\)) connected by internal transfers (uptake, litterfall, decomposition) and balanced by inputs (weathering, precipitation) and outputs (leaching, runoff).
---3. Plant Succession
What is Succession?
Plant succession is the directional, non-seasonal process of change in the species structure of an ecological community over time. As plants grow, die, and alter the environment (microclimate and soil), they make conditions more suitable for newer, more competitive species.
Primary vs Secondary Succession
A frequent exam question asks students to compare these two types:
1. Primary Succession (Prisere):
• Starts on a completely barren, newly exposed substrate with no pre-existing soil, organic matter, or seed bank.
• Examples: Bare rock exposed by a retreating glacier, cooled volcanic lava, sand dunes (psammosere), or open freshwater (hydrosere).
• The process is very slow because soil must first be formed from scratch.
2. Secondary Succession (Subseres):
• Occurs on an area where an existing ecosystem was disturbed, damaged, or cleared, but where pre-existing soil and seed banks remain intact.
• Examples: Abandoned agricultural fields, cut-down woodland, or land recovering after a forest fire.
• Succession proceeds much faster than primary succession because fertile soil and seeds are already present.
The Seral Stages of Succession
As succession progresses, the ecosystem passes through several recognisable stages called seres or seral stages:
Stage 1: The Pioneer Community
• Pioneer species are hardy, stress-tolerant organisms capable of surviving extreme conditions (e.g., lichens on bare rock, or marram grass on sand dunes).
• Role: They break down minerals, trap moisture, and when they die, their organic remains create the very first thin layer of soil (humus).
Stage 2: Intermediate / Seral Stages
• As soil depth, water retention, and nutrient levels increase, pioneer species are outcompeted by larger plants.
• Grasses, flowering herbs, and ferns arrive first, followed by fast-growing shrubs and scrub species (such as gorse, bramble, or hawthorn).
• Biodiversity, biomass, and structural complexity all increase.
Stage 3: The Climatic Climax Community
• The final, fully developed, stable community that exists in equilibrium with the prevailing regional climate.
• In the UK and Ireland, the climatic climax is typically temperate deciduous woodland dominated by mature oak (Quercus robur), ash, and beech trees.
Plagioclimax and Deflected Succession
When human activities prevent an ecosystem from reaching its natural climatic climax, the succession is deflected. The resulting artificial, human-maintained community is known as a plagioclimax.
• Real-World Example: Heather Moorlands in the UK. Without human interference, these upland areas would naturally succeed into birch and oak woodland. However, regular controlled burning (swaling) and continuous sheep grazing prevent tree saplings from establishing, arresting the ecosystem in a permanent heather-dominated plagioclimax.
Memory Trick: Remember P-I-C for succession:
• Pioneer (tough colonisers)
• Intermediate (shrubs and small trees)
• Climax (stable, mature forest)
Key Takeaway: Primary succession begins on bare surfaces without soil, progressing from pioneer species through intermediate seres to a stable climatic climax. Human interference creates a plagioclimax.
---4. Small Scale Ecosystem Case Studies
For Section B of AS 1, you must support your geographical answers with precise facts from a named small-scale ecosystem.
Case Study Option 1: Sand Dune Succession / Psammosere (e.g., Murlough Nature Reserve, Co. Down / Magilligan Foreshore)
A sand dune system provides a classic example of primary succession moving inland from the seashore:
• Embryo and Fore Dunes (Pioneer Stage): Extremely harsh abiotic conditions (high salinity, mobile sand, strong wind, alkaline pH \(> 8.0\), low water retention). Pioneer species like Sea Couch Grass and Marram Grass (Ammophila arenaria) colonise. Marram grass has deep, binding root networks that stabilise the moving sand.
• Yellow / Mobile Dunes: Sand accumulates; marram grass thrives and traps more sand. Organic matter begins to build up.
• Grey / Fixed Dunes: Sand is completely stabilised. A continuous carpet of mosses, lichens, and grasses covers the dunes. Soil organic content rises, reducing soil alkalinity (pH drops towards neutral \(\sim 6.5\)).
• Dune Slacks: Low-lying damp depressions between dune ridges where the water table reaches the surface, supporting moisture-loving species like creeping willow, rushes, and reeds.
• Climatic Climax / Dune Heath: Acidic, nutrient-rich brown earth soils develop, supporting heather, gorse scrub, and eventually deciduous birch or oak woodland.
Case Study Option 2: Temperate Deciduous Woodland (e.g., Epping Forest or Local NI Woodland)
• Trophic Structure:
1. Producers: Canopy trees (Oak, Beech), understorey shrubs (Holly, Hazel), and field layer plants (Bluebells, Ferns).
2. Primary Consumers: Herbivores such as caterpillars, bank voles, and woodland deer grazing on leaves and bark.
3. Secondary / Tertiary Consumers: Blue tits, tawny owls, and foxes.
4. Decomposers: Woodlice, earthworms, bracket fungi, and soil microbes breaking down leaf litter into fertile mull humus.
• Gersmehl Model in Deciduous Woodland:
The Biomass (\(B\)) store is the largest because of the massive volume of standing wood in mature trees. The Soil (\(S\)) store is also large and nutrient-rich due to continuous decomposition. The Litter (\(L\)) store is small-to-moderate because deciduous leaves decompose rapidly during warm, moist autumn months.
• Human Influences: Traditional coppicing/pollarding, recreational trampling along footpaths (leading to soil compaction and reduced infiltration), and management for conservation.
Case Study Option 3: Freshwater Pond / Wetland System
• Structure: Open water zonations with submerged plants (pondweed), floating-leaved plants (water lilies), and emergent reed beds at margins.
• Trophic Links: Microscopic algae (producers) \(\to\) daphnia/water fleas (primary consumers) \(\to\) dragonfly nymphs and small fish (secondary consumers) \(\to\) herons and pike (tertiary consumers).
• Environmental Vulnerability: Susceptible to agricultural runoff causing eutrophication (excess nutrient input leading to algal blooms, oxygen depletion, and aquatic organism mortality).
Key Takeaway: In extended case study questions, always name specific species (e.g., Marram grass, Oak, Blue tit) and quote precise environmental changes along the succession gradient (e.g., shifting pH, soil depth, and humus content).
---5. Common Pitfalls and Examiner Tips
Make sure you avoid these common mistakes highlighted in CCEA examiner reports:
1. Confusing Energy Flow with Nutrient Cycling:
• Energy is an open, one-way flow entering from the Sun and lost as metabolic heat (\(\sim 90\%\) loss per trophic level).
• Nutrients are recycled indefinitely between living biomass, dead litter, and the soil substrate.
2. Confusing Primary and Secondary Succession:
• Primary succession begins on completely bare ground with no soil.
• Secondary succession occurs where soil and seeds are already present following a disturbance.
3. Drawing Generic Gersmehl Diagrams:
• Never draw three identical equal circles! Adjust the size of the circles (\(B\), \(L\), \(S\)) to reflect the ecosystem you are describing (e.g., in a temperate deciduous forest, Biomass is the largest circle; in a desert, Soil is the largest circle).
4. Lack of Case Study Specifics:
• Do not write generic biological descriptions. Always state the name and location of your case study (e.g., Murlough Sand Dunes, County Down or Epping Forest) and include named flora and fauna.
Quick Chapter Review
• Ecosystem: An open community of living (biotic) organisms interacting with non-living (abiotic) components.
• Energy Transfer: \(\sim 10\%\) efficiency between trophic levels; \(\sim 90\%\) lost via respiration, excretion, and heat.
• Gersmehl Model: Three nutrient stores—Biomass (\(B\)), Litter (\(L\)), Soil (\(S\))—linked by uptake, litterfall, decomposition, weathering, precipitation, leaching, and runoff.
• Succession: Seral progression from pioneers to climatic climax. Primary starts without soil; secondary starts with soil.
• Plagioclimax: Deflected succession permanently maintained by human activity (e.g., sheep grazing on heather moorland).