Ecology

Ecology

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1. Ecosystems and Levels of Organisation

1.1 Levels of Organisation in an Ecosystem

\mathrm{Individual \to \mathrm{Population \to \mathrm{Community \to \mathrm{Ecosystem \to \mathrm{Biosphere

• Individual: A single organism.
• Population: All the organisms of one species in a habitat. The size of a population can change over time due to birth rate, death rate, immigration, and emigration.
• Community: All the populations of different species in a habitat. The interactions between species in a community include predation, competition, and symbiosis.
• Ecosystem: A community of organisms interacting with their non-living environment. An ecosystem includes both biotic (living) and abiotic (non-living) components.
• Biosphere: All the ecosystems on Earth. The biosphere extends from the deepest ocean trenches to the highest mountains, and includes the atmosphere, the hydrosphere, and the upper layers of the lithosphere.

1.2 Biotic and Abiotic Factors

Biotic factors (living): Predation, competition, disease, food availability, parasitism.

Abiotic factors (non-living): Temperature, light intensity, water availability, soil pH, wind Speed, mineral availability, oxygen concentration.

How biotic and abiotic factors interact. Abiotic factors set the conditions for life in an Ecosystem, but biotic factors can modify abiotic conditions. For example, trees create shade (reducing light intensity and temperature), and plant roots stabilise soil and influence water Retention. Similarly, the presence of a predator (biotic) can affect the distribution of prey Species, which in turn affects the vegetation (affecting soil stability, an abiotic factor).

Worked Example: How biotic and abiotic factors affect a population.

A population of rabbits lives in a grassland. The population size is affected by:

Abiotic factors:

• Temperature: cold winters reduce food availability and increase energy expenditure for thermoregulation, potentially reducing the population.
• Rainfall: drought reduces grass growth, reducing food availability.
• Soil pH: affects the growth of plants that the rabbits eat.

Biotic factors:

• Predation: foxes eat rabbits, reducing the population.
• Competition: other herbivores (e.g., deer) compete with rabbits for grass.
• Disease: myxomatosis (a viral disease) can devastate rabbit populations.
• Food availability: the amount of grass directly affects rabbit reproduction and survival.

1.3 Adaptations

Organisms are adapted to their environment through natural selection. Adaptations can be:

Extremophiles: Organisms adapted to extreme environments, such as:

• Thermophiles: High temperature (hot springs, deep-sea vents). Their enzymes have a higher optimum temperature and are more heat-stable than those of other organisms.
• Halophiles: High salt concentration. They maintain a high internal concentration of solutes to balance the external salt concentration and prevent water loss.
• Acidophiles: Low pH. They maintain a near-neutral internal pH despite living in acidic environments.

1.4 Higher Tier: Adaptations in Detail

Polar bear adaptations to the Arctic:

• Structural: White fur for camouflage; thick layer of blubber (fat) for insulation; large body size (small surface-area-to-volume ratio reduces heat loss); black skin under the fur (absorbs heat); wide, hairy paws (distribute weight on ice and provide grip).
• Physiological: Ability to slow metabolism during periods of food scarcity.
• Behavioural: Hunting in groups; building dens for raising young.

Cactus adaptations to the desert:

• Structural: Thick, waxy cuticle (reduces water loss by evaporation); spines instead of leaves (reduces surface area, deters herbivores); swollen stem (stores water); shallow but extensive root system (quickly absorbs water after rain).
• Physiological: CAM (Crassulacean Acid Metabolism) photosynthesis: stomata open at night to take in \mathrm{CO_2Reducing water loss during the hot day.

Worked Example: Surface-area-to-volume ratio and heat loss.

A small animal (e.g., a mouse) has a larger surface-area-to-volume ratio than a large animal (e.g., An elephant). This means the mouse loses heat more quickly relative to its volume. This is why small Mammals in cold environments need to eat more frequently (to fuel respiration for heat production) And why Arctic animals tend to be large (small SA:V ratio reduces heat loss).

For a cube with side length $s$:

• Surface area = $6s^2$
• Volume = $s^3$
• SA:V ratio = $6/s$

If $s$ doubles from 1 to 2: SA:V decreases from 6 to 3. Larger organisms lose proportionally less Heat.

2. Feeding Relationships

2.1 Food Chains and Food Webs

A food chain shows the transfer of energy from one organism to another:

\mathrm{Producer \to \mathrm{Primary consumer \to \mathrm{Secondary consumer \to \mathrm{Tertiary consumer

• Producer: An organism that makes its own food by photosynthesis (e.g. Plants, algae). Producers are the base of all food chains because they are the organisms that convert light energy into chemical energy.
• Consumer: An organism that eats other organisms.
• Primary consumer: Herbivore (eats plants).
• Secondary consumer: Carnivore that eats herbivores.
• Tertiary consumer: Carnivore that eats secondary consumers.
• Decomposer: Breaks down dead material and waste (e.g. Bacteria, fungi). Decomposers are essential for recycling nutrients; without them, dead organisms and waste would accumulate and nutrients would be locked in dead material indefinitely.

A food web shows many interconnected food chains in an ecosystem. Food webs are more realistic Than food chains because most organisms eat more than one type of food and are eaten by more than One type of predator.

2.2 Trophic Levels

Each step in a food chain is a trophic level:

2.3 Pyramids of Number, Biomass, and Energy

Pyramid of number: Shows the number of organisms at each trophic level. Can be inverted (e.g. One oak tree supporting many insects, because a single producer supports many primary consumers).

Pyramid of biomass: Shows the total mass of living material at each trophic level. A Pyramid shape (never inverted in a natural ecosystem), because biomass is lost at each trophic Level.

Pyramid of energy: Shows the energy available at each trophic level. Always a pyramid shape (never inverted), because energy is always lost as it passes from one trophic level to the next.

Why pyramids of energy are never inverted. Energy cannot be created or destroyed (first law of Thermodynamics). At each trophic level, some energy is always lost (as heat, through respiration), So the energy available at the next level is always less than at the current level.

Worked Example: Constructing a pyramid of biomass.

A field contains the following biomass:

• Grass (producer): 10,000 kg
• Rabbits (primary consumers): 1,000 kg
• Foxes (secondary consumers): 100 kg

Pyramid of biomass:

        Foxes (100 kg)
    Rabbits (1,000 kg)
  Grass (10,000 kg)

The pyramid shape shows that biomass decreases at each trophic level. Only about 10% of biomass is Transferred from one level to the next.

2.4 Energy Transfer

Energy is transferred along a food chain, but much is lost at each trophic level:

Ways energy is lost:

• Respiration (released as heat — this energy is lost to the environment and cannot be used by other organisms)
• Excretion (energy in waste products such as urea and faeces)
• Movement (energy used by the organism for locomotion, which is eventually lost as heat)
• Not all parts of the organism are eaten (bones, teeth, hair are not consumed)
• Egestion (undigested food passes through the digestive system and is lost as faeces)

Only about 10% of energy is transferred from one trophic level to the next. This is why food Chains are short ( 3—5 trophic levels): there is not enough energy left to support Many levels.

Implications:

• Much more biomass is needed at lower trophic levels to support higher levels.
• Eating plants (vegetarian diet) is more energy-efficient than eating meat because there are fewer trophic levels between the producer and the consumer.
• A field of wheat can support more people than the same field used to raise cattle (because the cattle lose energy at each trophic level before the humans eat the beef).

2.5 Calculating Efficiency

\mathrm{Efficiency = \frac{\mathrm{energy available at next level}{\mathrm{energy available at current level} \times 100\%

Worked Example 1. 10,000 kJ of energy is available at the producer level. 1,000 kJ is Transferred to the primary consumer. Calculate the efficiency.

\mathrm{Efficiency = \frac{1000}{10000} \times 100\% = 10\%

Worked Example 2. If a primary consumer has 1,000 kJ of energy and 100 kJ is transferred to the Secondary consumer, the efficiency is 10%. The remaining 900 kJ is lost through the processes Described above.

Worked Example 3: A multi-step calculation.

A food chain: grass $\to$ rabbit $\to$ fox.

Grass contains 20,000 kJ of energy. 10% is transferred to rabbits. 15% of the rabbit’s energy is Transferred to foxes.

Energy in rabbits: $20,000 \times 0.10 = 2,000$ kJ.

Energy in foxes: $2,000 \times 0.15 = 300$ kJ.

Overall efficiency from grass to foxes: $(300 / 20,000) \times 100 = 1.5\%$.

This shows how little energy reaches the top of a food chain.

3. Material Cycles

3.1 The Carbon Cycle

Carbon is constantly recycled between the atmosphere, organisms, and the Earth. The total amount of Carbon on Earth is fixed, but it moves between different stores (reservoirs).

Processes that remove \mathrm{CO_2 from the atmosphere:

• Photosynthesis: Plants convert \mathrm{CO_2 into glucose. This is the main route by which carbon moves from the atmosphere into the biosphere.
• Dissolving in oceans: \mathrm{CO_2 dissolves in seawater to form carbonic acid, which reacts to form carbonate ions. The oceans are a massive carbon store.

Processes that return \mathrm{CO_2 to the atmosphere:

• Respiration: All living organisms release \mathrm{CO_2 during respiration. This includes plants, which respire 24 hours a day.
• Combustion: Burning fossil fuels (coal, oil, natural gas) and biomass (wood, biofuels) releases \mathrm{CO_2 that was previously locked away.
• Decomposition: Decomposers break down dead material and release \mathrm{CO_2 as a by-product of their respiration.

Carbon stores:

• Atmosphere (as \mathrm{CO_2) — approximately 750 billion tonnes.
• Oceans (as dissolved \mathrm{CO_2 and carbonate compounds) — the largest active carbon store.
• Fossil fuels (coal, oil, natural gas) — formed from the remains of organisms that died millions of years ago.
• Limestone (calcium carbonate) — formed from the shells of marine organisms over geological time.
• Living organisms (carbohydrates, lipids, proteins) — a relatively small but actively cycling store.
• Soil (organic matter, peat) — contains significant amounts of carbon.

3.2 The Water Cycle

The water cycle describes the continuous movement of water between the atmosphere, land, and oceans. It is driven by solar energy (which provides the heat for evaporation) and gravity (which pulls Water back down as precipitation).

Key processes:

• Evaporation: Water from oceans and lakes evaporates (liquid to gas). Requires energy from the sun.
• Transpiration: Water evaporates from plant leaves through stomata. Transpiration from forests contributes significantly to the water cycle.
• Condensation: Water vapour cools and forms clouds. This occurs when warm, moist air rises and cools.
• Precipitation: Water falls as rain, snow, sleet, or hail.
• Surface runoff: Water flows over the ground into rivers and streams.
• Infiltration: Water soaks into the ground.
• Percolation: Water moves through porous rocks (aquifers).

3.3 Decomposition

Decomposers (bacteria and fungi) break down dead organic matter, recycling nutrients back into the Soil. This process is essential because without decomposition, nutrients would remain locked in dead Organisms and would be unavailable for producers.

Factors affecting the rate of decomposition:

Why temperature matters. Decomposition is carried out by enzymes in bacteria and fungi. Like all Enzymes, these work faster at higher temperatures (up to their optimum) because the substrate Molecules have more kinetic energy and collide more frequently with the active sites. Above the Optimum, the enzymes denature and decomposition slows.

3.4 Required Practical: Investigating Decomposition

Method:

1. Collect equal masses of organic material (e.g. Leaf litter).
2. Place samples in different conditions (e.g. Warm/moist, cold/dry, warm/dry).
3. Measure the mass of the samples at regular intervals (e.g. Every week for 4 weeks).
4. The sample that loses mass fastest is decomposing most rapidly (the mass loss is due to carbon being released as \mathrm{CO_2 by the decomposers).

Variables:

• Independent variable: environmental condition (temperature, moisture).
• Dependent variable: mass of the sample over time.
• Control variables: type and mass of organic material, volume of container, surface area of material.

3.5 Higher Tier: The Nitrogen Cycle

The nitrogen cycle describes how nitrogen is converted between different chemical forms and Circulates through the ecosystem. Nitrogen is essential for making amino acids and proteins, DNA, And other biomolecules, but most organisms cannot use atmospheric nitrogen (\mathrm{N_2) directly Because the triple bond between the two nitrogen atoms is very strong.

Key processes:

1. Nitrogen fixation: Conversion of atmospheric \mathrm{N_2 to ammonia (\mathrm{NH_3). Carried out by nitrogen-fixing bacteria (e.g. Rhizobium in the root nodules of leguminous plants such as peas, beans, and clover) and by lightning (which provides enough energy to break the \mathrm{N\equiv\mathrm{N bond).
2. Nitrification: Conversion of ammonia to nitrites (\mathrm{NO_2^-) and then nitrates (\mathrm{NO_3^-). Carried out by nitrifying bacteria (Nitrosomonas and Nitrobacter) in the soil. Nitrates are the form of nitrogen that plants can absorb.
3. Assimilation: Plants absorb nitrates through their roots and use them to make amino acids and proteins. Animals obtain nitrogen by eating plants or other animals.
4. Ammonification: Decomposers break down dead organisms and waste (urine, faeces), releasing ammonia back into the soil.
5. Denitrification: Conversion of nitrates back to \mathrm{N_2 gas. Carried out by denitrifying bacteria in waterlogged (anaerobic) soil. This process removes nitrogen from the ecosystem and returns it to the atmosphere, reducing the fertility of the soil.

Summary table: nitrogen cycle bacteria.

4. Biodiversity and Human Impact

4.1 What Is Biodiversity?

Biodiversity is the variety of all the different species of organisms on Earth, or within a Particular habitat. It includes:

• Species diversity (the number of different species and their relative abundance).
• Genetic diversity (the variety of genes within a species).
• Ecosystem diversity (the variety of habitats and ecological processes).

Measuring biodiversity:

• Species richness: The number of different species in an area. A simple count, but it does not account for how common each species is.
• Index of diversity: Takes into account both the number of species and the abundance of each species. It is calculated using the following formula:

$D = \frac{N(N - 1)}{\sum n(n - 1)}$

Where $N$ is the total number of organisms and $n$ is the number of organisms of each species.

A higher index of diversity indicates greater biodiversity.

Worked Example 1. A habitat has three species: A (50 individuals), B (30), C (20).

$N = 50 + 30 + 20 = 100$.

$D = \frac{100 \times 99}{50 \times 49 + 30 \times 29 + 20 \times 19} = \frac{9900}{2450 + 870 + 380} = \frac{9900}{3700} = 2.68$

Worked Example 2. Compare two habitats.

Habitat 1: Species A (90), Species B (5), Species C (5). Total: 100. Species richness: 3.

Habitat 2: Species A (34), Species B (33), Species C (33). Total: 100. Species richness: 3.

Both habitats have the same species richness (3 species), but Habitat 2 has higher biodiversity Because the species are more evenly distributed.

Habitat 1: $D = \frac{100 \times 99}{90 \times 89 + 5 \times 4 + 5 \times 4} = \frac{9900}{8010 + 20 + 20} = \frac{9900}{8050} = 1.23$

Habitat 2: $D = \frac{100 \times 99}{34 \times 33 + 33 \times 32 + 33 \times 32} = \frac{9900}{1122 + 1056 + 1056} = \frac{9900}{3234} = 3.06$

Habitat 2 has a higher index of diversity (3.06 vs 1.23), confirming it has greater biodiversity.

4.2 Threats to Biodiversity

4.3 Maintaining Biodiversity

4.4 Trophic Levels and Toxins

Some substances, such as pesticides and heavy metals, accumulate in organisms. Because only about 10% of biomass is passed on at each trophic level, the concentration of these toxins increases At each level.

This is called bioaccumulation (build-up within a single organism over its lifetime) or biomagnification (increasing concentration at each successive trophic level).

Example: DDT (a pesticide) accumulates in the fatty tissue of organisms. Top predators (e.g. Birds of prey) accumulate the highest concentrations, which can cause thinning of eggshells and Population decline. This is why DDT was banned in many countries.

Why biomagnification occurs. Toxins that are soluble in fat (lipophilic) are not broken Down or excreted. When an organism eats many smaller organisms, it accumulates all the toxins from Its food. At each trophic level, the total concentration increases because a predator eats many prey Items, each containing the toxin.

Worked Example: Biomagnification calculation.

In a food chain: plankton $\to$ small fish $\to$ large fish $\to$ osprey.

The concentration of DDT at each level:

• Plankton: 0.003 ppm
• Small fish: 0.05 ppm (eat many plankton)
• Large fish: 0.5 ppm (eat many small fish)
• Osprey: 5.0 ppm (eat many large fish)

The osprey has a DDT concentration over 1,600 times higher than the plankton. This demonstrates how Dramatically biomagnification increases toxin concentration at higher trophic levels.

4.5 Eutrophication

Eutrophication is the over-enrichment of water bodies with nutrients (nitrates and phosphates), from fertiliser runoff.

Stages:

1. Nitrates/phosphates wash into rivers and lakes (from agricultural fertiliser or sewage).
2. Algae grow rapidly (algal bloom) because the nutrients are no longer limiting.
3. The algal bloom blocks light, killing aquatic plants beneath the surface.
4. Dead algae and plants are decomposed by bacteria.
5. Bacteria use up oxygen during respiration (the biochemical oxygen demand, BOD, increases).
6. Oxygen levels drop (hypoxia), killing fish and other aquatic organisms that require oxygen.
7. The ecosystem is severely damaged; biodiversity is reduced.

4.6 Global Warming

Greenhouse gases (\mathrm{CO_2Methane, water vapour, nitrous oxide) trap heat in the Atmosphere, causing the Earth’s average temperature to rise. This is the greenhouse effect.

Evidence for climate change:

• Rising global temperatures (measured since the 1800s using thermometer records and ice cores).
• Rising sea levels (measured by tide gauges and satellite altimetry).
• Melting ice caps and glaciers (measured by satellite imagery and ground surveys).
• Increased frequency of extreme weather events (floods, droughts, heatwaves).
• Increased atmospheric \mathrm{CO_2 concentration (measured at Mauna Loa Observatory since 1958; the Keeling Curve shows a steady increase from approximately 315 ppm to over 420 ppm).

Consequences:

• Loss of habitats (polar regions, coral reefs).
• Changes to migration patterns (species shift towards the poles or to higher altitudes).
• Rising sea levels threatening coastal areas (due to thermal expansion of seawater and melting of land ice).
• Changes in rainfall patterns (droughts in some regions, increased flooding in others).
• Spread of tropical diseases to temperate regions (as temperatures rise, mosquitoes that carry malaria and dengue fever can survive at higher latitudes).

Reducing climate change:

• Using renewable energy sources instead of fossil fuels (solar, wind, hydroelectric, nuclear).
• Energy efficiency (insulation, LED bulbs, electric vehicles).
• Carbon capture and storage (capturing \mathrm{CO_2 from power stations and storing it underground).
• Reforestation (planting trees to absorb \mathrm{CO_2 through photosynthesis).
• International agreements (Paris Agreement, 2015 — commits countries to limit global warming to well below 2$^{\circ}$C above pre-industrial levels).

5. Required Practical: Sampling Organisms

5.1 Quadrats

A quadrat is a square frame ( 0.5 m $\times$ 0.5 m or 1 m $\times$ 1 m) used to sample Organisms in a habitat.

Method:

1. Place the quadrat randomly in the area (use random numbers for coordinates to avoid bias).
2. Count the number of organisms of each species within the quadrat.
3. Repeat many times (at least 10) to get a reliable estimate.
4. Calculate the mean number per quadrat.
5. Multiply by the total area to estimate the total population.

5.2 Transects

A transect is a line across a habitat. A quadrat is placed at regular intervals along the Transect. This is useful for studying how species distribution changes across an environmental Gradient (e.g., from the shore of a lake into the surrounding field).

Types:

• Line transect: Record organisms touching the line.
• Belt transect: Place quadrats along the line (provides more quantitative data than a line transect).

5.3 Sweep Nets and Pitfall Traps

• Sweep nets: Used for catching insects in long grass. Sweep the net through the vegetation in a standardised way to ensure results are comparable.
• Pitfall traps: Containers set into the ground to catch small animals (insects, spiders). The trap should be covered with a raised lid to prevent rain from filling it and to stop trapped animals from being eaten by predators.

5.4 Estimating Population Size

Mark-release-recapture:

\mathrm{Estimated population = \frac{n_1 \times n_2}{n_3}

Where $n_1$ = number captured and marked first time, $n_2$ = number captured second time, $n_3$ = Number marked in second capture.

Assumptions:

• No birth, death, or migration between captures.
• Marked individuals mix fully with the population.
• Marks do not affect survival or behaviour.
• Each individual has an equal chance of being captured.

Worked Example. 50 woodlice are captured, marked, and released. In a second capture, 40 woodlice Are caught, of which 10 are marked.

\mathrm{Population = \frac{50 \times 40}{10} = 200

Worked Example 2: 30 beetles are captured and marked. In a second capture of 50 beetles, 5 are Marked.

\mathrm{Population = \frac{30 \times 50}{5} = 300

5.5 Higher Tier: Reliability and Validity in Ecological Sampling

Reliability: Can the results be repeated? To improve reliability, use a large sample size (many Quadrats), repeat the sampling, and calculate a mean. Use systematic methods to reduce investigator Bias.

Validity: Do the results measure what they are intended to measure? To improve validity, ensure That the sampling method is appropriate for the organisms being studied (e.g., quadrats for plants, Pitfall traps for ground insects), and control confounding variables (time of day, weather Conditions).

Common Pitfalls

• Confusing species richness and index of diversity. Species richness counts the number of species; the index of diversity also considers the relative abundance of each species. A habitat with 10 equally abundant species has higher diversity than one with 10 species where one species dominates and the others are rare.
• Drawing food chains with arrows in the wrong direction. Arrows show the direction of energy flow (from what is eaten to what eats it), not from predator to prey.
• Confusing bioaccumulation and the water cycle. Bioaccumulation refers to the build-up of toxins in food chains; the water cycle is the movement of water through the environment.
• Forgetting that pyramids of energy are always pyramid-shaped but pyramids of number can be inverted.
• Stating that 90% of energy is “lost” as heat. Energy is lost through several processes (respiration, excretion, movement, egestion, etc.), not just heat. Respiration releases energy as heat, but the energy in undigested food (faeces) and the energy used for movement are also “lost” from the food chain.
• Confusing the carbon cycle with the nitrogen cycle. Focus on the specific processes that move carbon between stores (photosynthesis, respiration, combustion, decomposition).
• Forgetting that plants respire. Plants carry out both photosynthesis and respiration. At night, plants only respire (no photosynthesis because there is no light).
• Forgetting the assumptions of mark-release-recapture. If marked individuals are more likely to be recaptured (e.g., the mark makes them slower), the estimate will be too low.
• Confusing nitrification and denitrification. Nitrification converts ammonia to nitrates (useful for plants); denitrification converts nitrates to nitrogen gas (removes nitrogen from the ecosystem).

Practice Questions

1. Describe the difference between a food chain and a food web.

2. Explain why only about 10% of energy is transferred from one trophic level to the next.

3. Describe the carbon cycle, naming the processes that add and remove \mathrm{CO_2 from the atmosphere.

4. Explain how eutrophication leads to the death of fish in a lake.

5. Describe how a student could use quadrats to estimate the population of daisies in a field.

6. Calculate the index of diversity for an area with the following data: species A (50 individuals), species B (30), species C (15), species D (5).

7. Explain the process of bioaccumulation and why it particularly affects top predators.

8. Describe three ways in which humans are threatening biodiversity and suggest one method to reduce each threat.

9. 60 beetles are captured, marked, and released. In a second capture of 80 beetles, 12 are marked. Estimate the total population.

10. Explain why deforestation contributes to both climate change and loss of biodiversity.

11. (Higher Tier) Describe the nitrogen cycle, naming the bacteria involved in each stage and explain why their activities are important for plants.

12. (Higher Tier) Explain why a pyramid of biomass for a parasitic food chain could be inverted, but a pyramid of energy cannot.

13. A farmer applies fertiliser to a field next to a lake. Describe the process of eutrophication that is likely to occur and explain its effects on the lake ecosystem.

14. Explain why decomposition is faster in a warm, moist, aerobic environment than in a cold, dry, anaerobic environment.

15. Evaluate the effectiveness of using mark-release-recapture to estimate the population size of a mobile animal species. What are the main assumptions, and how could they be violated?

16. Explain why carnivores are generally rarer than herbivores in ecosystems, with reference to energy transfer.

17. Describe how a student could use a transect to investigate the distribution of plant species from the edge of a pond into a nearby field.

18. Explain the role of decomposers in the carbon cycle and nitrogen cycle.

19. A student investigates the effect of temperature on the rate of decomposition of leaf litter. Describe a method they could use, including the variables they should control.

20. Evaluate the claim that “biodiversity loss is as serious a threat to human welfare as climate change.” Support your argument with scientific evidence.

6. Higher Tier: Detailed Carbon Cycle Calculations

Worked Example: Carbon in a food chain.

A food chain consists of grass $\to$ rabbit $\to$ fox. Assume 10% efficiency of energy transfer at Each trophic level. If the grass fixes 50,000 kJ of carbon (as glucose) per day through Photosynthesis:

1. Energy available to rabbits: $50,000 \times 0.10 = 5,000$ kJ/day.
2. Energy available to foxes: $5,000 \times 0.10 = 500$ kJ/day.
3. Total energy lost as heat and waste from the entire food chain per day: $50,000 - 500 = 49,500$ kJ.

This means 99% of the energy fixed by photosynthesis is lost before reaching the fox. This Illustrates why food chains are limited to a few trophic levels and why ecosystems can support far More herbivores than carnivores.

7. Higher Tier: Peat Bogs and Climate Change

Peat bogs are particularly important ecosystems in the context of climate change:

What is peat? Peat is partially decomposed organic matter (mainly sphagnum moss) that Accumulates in waterlogged, acidic conditions. Because the waterlogged conditions are anaerobic, Decomposition is very slow, and carbon accumulates over thousands of years.

Why peat bogs are carbon stores: Peat bogs cover only about 3% of the Earth’s land surface but Store approximately 30% of all soil carbon. This makes them one of the most important carbon stores On the planet.

The problem: When peat bogs are drained for agriculture (e.g., for palm oil plantations in Southeast Asia) or harvested for fuel, the peat is exposed to air. Aerobic decomposition resumes, And the stored carbon is released as \mathrm{CO_2. Drained tropical peatlands are estimated to Release approximately 1.5 billion tonnes of \mathrm{CO_2 per year, making them a major contributor To climate change.

Conservation: Protecting and restoring peat bogs is an important strategy for mitigating climate Change. Rewetting drained peatlands can stop the release of stored carbon and allow the bogs to Begin accumulating carbon again.

8. Higher Tier: Indicator Species

Indicator species are organisms whose presence, absence, or abundance provides information about the Quality of the environment.

Air quality indicators (lichens):

Lichens are symbiotic organisms composed of a fungus and an alga. They are sensitive to air Pollution, particularly sulphur dioxide (\mathrm{SO_2).

If bushy lichens are present, the air is clean. If only crusty lichens are present, the air is Polluted. If no lichens are present at all, the air is very heavily polluted.

Water quality indicators (invertebrates):

The species of invertebrates found in a river or stream indicate the level of water pollution.

If stonefly larvae are found, the water is clean. If only bloodworms and rat-tailed maggots are Found, the water is severely polluted with organic waste (e.g., sewage).

9. Higher Tier: The Impact of Land Use Change on Ecosystems

Land use change is one of the most significant drivers of biodiversity loss worldwide.

Deforestation for agriculture: Tropical rainforests are cleared for cattle ranching and soya Production. Rainforests contain an estimated 50% of all species on Earth, despite covering only About 6% of the land surface. Deforestation destroys habitats, fragments populations (making them More vulnerable to extinction), and releases stored carbon.

Urbanisation: Building cities and roads on natural habitats removes vegetation, increases Surface runoff (leading to flooding), creates heat islands (urban areas are warmer than surrounding Rural areas), and fragments wildlife corridors.

Agricultural intensification: Monoculture farming (growing a single crop over large areas) Reduces habitat diversity. Pesticides kill non-target species (including pollinators such as bees). Fertiliser runoff causes eutrophication in nearby water bodies.

Sustainable alternatives:

• Agroforestry: growing crops alongside trees, maintaining some habitat structure.
• Organic farming: avoiding synthetic pesticides and fertilisers.
• Urban green spaces: parks, green roofs, and wildlife corridors within cities.
• Set-aside schemes: paying farmers to leave areas of land uncultivated for wildlife.

10. Higher Tier: Ecological Succession

Succession is the process by which an ecosystem changes over time, from a bare surface (e.g., After a volcanic eruption or glacier retreat) to a mature, stable community called a climax Community.

Primary succession: Occurs on a surface that has never been colonised before (e.g., bare rock After a volcanic eruption). Stages:

1. Pioneer species (e.g., lichens) colonise the bare rock. They are hardy, tolerate extreme conditions, and can fix nitrogen from the air.
2. Lichens break down the rock, forming a thin layer of soil.
3. Mosses and grasses colonise the soil.
4. Small shrubs and herbaceous plants grow.
5. Larger plants (trees) establish.
6. The climax community (e.g., oak woodland) is reached, which remains relatively stable until disturbed.

Secondary succession: Occurs on a surface where an existing ecosystem has been disturbed (e.g., After a forest fire, flood, or farming). The soil is already present, so succession proceeds faster Than primary succession. It begins with grasses and herbs rather than pioneer lichens.

Key concepts:

• At each stage, the current species modify the environment, making it more suitable for the next group of species.
• Biodiversity generally increases during succession until the climax community is reached.
• The climax community is the most stable and has the highest biodiversity in that environment.

Worked Example: Describing succession on abandoned farmland.

When a field is abandoned:

1. In the first year, fast-growing annual weeds colonise (they produce many seeds that disperse ).
2. Over the next few years, perennial grasses and herbaceous plants outcompete the weeds.
3. Shrubs (e.g., brambles, hawthorn) establish, creating shade.
4. Light-demanding tree species (e.g., birch, willow) grow in the sunny conditions.
5. These trees create shade, and shade-tolerant species (e.g., oak, beech) replace them.
6. Eventually, a mature woodland develops (the climax community in the British climate).

This process can take 100—200 years or more, depending on the climate and soil conditions.

Practice Problems

Worked Examples

Example 1:

A typical exam question on Ecology requires you to apply your knowledge to an unfamiliar context. Read the question carefully, identify the key concept being tested, and structure your answer using the appropriate terminology.

Example 2:

Multi-step problems in Ecology often combine two or more concepts. Break the problem down: identify what you need to find, recall the relevant formula or principle, substitute values, and state your answer with correct units or formatting.

Summary

This topic covers the biological principles of ecology, including key concepts, experimental evidence, and real-world applications.

Key concepts include:

• ecosystems and food webs
• nutrient cycles (carbon, nitrogen)
• population dynamics
• biodiversity and conservation
• succession

Success requires the ability to recall specific factual content, apply knowledge to novel scenarios, and evaluate experimental evidence critically.