Ecology

Ecology is the study of the relationships between living organisms and their environment. This topic Covers ecosystems, food chains, nutrient cycles, population dynamics, and human impact on the Environment.

Ecosystems

Definitions (OL/HL)

• Ecosystem: a community of organisms interacting with each other and their physical environment.
• Habitat: the place where an organism lives.
• Niche: the role and position of a species within its ecosystem.
• Population: all organisms of the same species in a given area.
• Community: all populations in a given area.
• Biosphere: all ecosystems on Earth.

Biotic and Abiotic Factors (OL/HL)

Biotic factors: living components — predators, prey, competitors, parasites, disease.

Abiotic factors: non-living components — temperature, light, water, pH, soil type, wind, Mineral availability.

Feeding Relationships

Trophic Levels (OL/HL)

Level	Organisms
Producer	Plants, algae (autotrophs)
Primary consumer	Herbivores
Secondary consumer	Carnivores that eat herbivores
Tertiary consumer	Top carnivores
Decomposer	Bacteria, fungi

Food Chains and Food Webs (OL/HL)

A food chain shows the flow of energy from producers through consumers.

Example (OL): Grass $\to$ Rabbit $\to$ Fox $\to$ Hawk.

A food web consists of many interconnected food chains, showing the complex feeding Relationships in an ecosystem.

Ecological Pyramids (OL/HL)

Pyramid of numbers: number of organisms at each trophic level (may be inverted).

Pyramid of biomass: total mass of organisms at each trophic level (rarely inverted).

Pyramid of energy: energy at each trophic level (always upright; never inverted).

Energy Flow (OL/HL)

• Only about 10% of energy is transferred from one trophic level to the next.
• The rest is lost as heat through respiration, excretion, and uneaten parts.
• This limits the number of trophic levels in a food chain ( 4-5).

Example (OL): If producers have 10,000 kJ of energy, primary consumers have about 1,000 kJ, Secondary consumers about 100 kJ, and tertiary consumers about 10 kJ.

Worked Example: Calculating ecological efficiency.

A meadow contains 50,000 kJ of energy in grass. Rabbits (primary consumers) contain 5,000 kJ. Foxes (secondary consumers) contain 500 kJ.

Efficiency from grass to rabbits: $(5000/50000) \times 100 = 10\%$.

Efficiency from rabbits to foxes: $(500/5000) \times 100 = 10\%$.

Overall efficiency from grass to foxes: $(500/50000) \times 100 = 1\%$.

This demonstrates that most energy is lost at each trophic level.

Biogeochemical Cycles

Carbon Cycle (OL/HL)

Key processes:

1. Photosynthesis: \mathrm{CO_2 + \mathrm{H_2\mathrm{O \to \mathrm{C_6\mathrm{H_{12}\mathrm{O_6 + \mathrm{O_2
2. Respiration: \mathrm{C_6\mathrm{H_{12}\mathrm{O_6 + \mathrm{O_2 \to \mathrm{CO_2 + \mathrm{H_2\mathrm{O + \mathrm{energy
3. Combustion: burning fossil fuels releases \mathrm{CO_2.
4. Decomposition: decomposers break down organic matter, releasing \mathrm{CO_2.

Carbon is stored in:

• Atmosphere (as \mathrm{CO_2)
• Oceans (dissolved \mathrm{CO_2Carbonate rocks)
• Fossil fuels (coal, oil, natural gas)
• Living organisms (biomass)

Nitrogen Cycle (OL/HL)

Key processes:

1. Nitrogen fixation: conversion of atmospheric \mathrm{N_2 to ammonia (\mathrm{NH_3).

• By lightning (small amount).
• By nitrogen-fixing bacteria (e.g., Rhizobium in root nodules of legumes).
• By industrial process (Haber process).

2. Nitrification: conversion of ammonia to nitrites (\mathrm{NO_2^-) and then nitrates (\mathrm{NO_3^-).

• By nitrifying bacteria (Nitrosomonas, Nitrobacter).

3. Absorption: plants absorb nitrates through their roots.

4. Assimilation: plants use nitrates to make amino acids and proteins.

5. Feeding: animals obtain nitrogen by eating plants or other animals.

6. Decomposition: decomposers break down dead organisms and waste, returning ammonia to the soil.

7. Denitrification: conversion of nitrates back to \mathrm{N_2 gas.

• By denitrifying bacteria (in waterlogged, anaerobic soil).

Water Cycle (OL)

Key processes: evaporation, transpiration, condensation, precipitation, percolation.

Population Dynamics

Population Growth (OL/HL)

Populations grow when births + immigration exceed deaths + emigration.

Exponential growth: $N = N_0 e^{rt}$ (unlimited resources).

Logistic growth: population growth slows as it approaches the carrying capacity $K$.

The carrying capacity is the maximum population size that the environment can sustain.

Worked Example: Exponential growth calculation.

A population of bacteria starts with 1000 cells and has a growth rate $r = 0.5$ per hour. What is The population after 10 hours?

$N = N_0 e^{rt} = 1000 \times e^{0.5 \times 10} = 1000 \times e^5 \approx 1000 \times 148.4 = 148,413$ Cells.

This shows how rapidly populations can grow when resources are unlimited.

Factors Affecting Population Size

Density-dependent factors: competition, predation, disease, parasitism (effects increase with Population density).

Density-independent factors: natural disasters, temperature extremes (effects do not depend on Density).

Comparison of density-dependent and density-independent factors:

Feature	Density-Dependent	Density-Independent
Relationship to density	Effect increases with density	Effect independent of density
Examples	Disease, competition	Natural disasters, fire
Regulation type	Intrinsic (self-regulating)	Extrinsic (external)
Predictability	More predictable	Less predictable

Human Impact on the Environment (OL/HL)

Pollution

• Air pollution: \mathrm{CO_2$$\mathrm{SO_2$$\mathrm{NO_xParticulate matter.
• Water pollution: sewage, fertilisers (eutrophication), heavy metals, oil spills.
• Soil pollution: pesticides, herbicides, industrial waste.

Eutrophication (OL/HL)

1. Excess nitrates/phosphates enter water bodies (from fertiliser runoff or sewage).
2. Algal bloom occurs (rapid growth of algae).
3. Algae die and decompose.
4. Decomposers use up oxygen (BOD increases).
5. Fish and other aquatic organisms die.

Worked Example: Biochemical Oxygen Demand (BOD).

A river has a BOD of 2 mg/L upstream of a sewage outfall. Downstream, the BOD rises to 8 mg/L.

The increased BOD means that decomposers in the water are using more oxygen to break down organic Matter from the sewage. This reduces the dissolved oxygen available for fish and other aquatic Organisms. If the dissolved oxygen drops below about 5 mg/L, fish begin to die.

Deforestation (OL/HL)

• Loss of biodiversity.
• Increased \mathrm{CO_2 (fewer trees for photosynthesis).
• Soil erosion.
• Disruption of the water cycle.

Climate Change (HL)

• Enhanced greenhouse effect due to increased atmospheric \mathrm{CO_2$$\mathrm{CH_4 \mathrm{N_2\mathrm{O.
• Rising global temperatures, sea level rise, extreme weather events.
• Impact on ecosystems, agriculture, and human health.

Conservation (OL/HL)

Strategies for conservation include:

• Establishing nature reserves and national parks.
• Captive breeding programmes.
• Sustainable resource management.
• Legislation and international agreements (e.g., CITES, Kyoto Protocol).
• Environmental impact assessments.

Ecological Field Studies (OL/HL)

Sampling Techniques

Quadrat sampling: for plants and slow-moving animals in a uniform habitat.

Place a quadrat ( 1\mathrm{ m \times 1\mathrm{ m) randomly and count organisms. Repeat and Calculate mean density.

Transect sampling: study changes along an environmental gradient (e.g., from sea shore inland).

Place quadrats at regular intervals along a line.

Pitfall traps: for ground-dwelling invertebrates.

Kick sampling: for aquatic invertebrates in rivers.

Measuring Biodiversity (HL)

Simpson’s Diversity Index:

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

Where $n_i$ is the number of individuals of species $i$ and $N$ is the total number of individuals.

$D$ ranges from 0 (no diversity) to 1 (infinite diversity).

Worked Example (HL): In a sample of 100 organisms: species A = 50, B = 30, C = 20.

$$D = 1 - \frac{50(49) + 30(29) + 20(19)}{100(99)} = 1 - \frac{2450 + 870 + 380}{9900} = 1 - \frac{3700}{9900} = 1 - 0.374 = 0.626$$

Worked Examples

See the examples integrated throughout the sections above.

Common Pitfalls

1. Energy pyramids — the pyramid of energy is the only one that is never inverted.
2. 10% rule — it is approximate; actual transfer varies.
3. Nitrogen cycle — distinguish between nitrification and denitrification.
4. Eutrophication — the critical step is oxygen depletion due to decomposition.
5. Simpson’s index — a higher value means greater biodiversity.
6. Confusing habitat and niche. Habitat is where an organism lives; niche is its role in the ecosystem.
7. Forgetting that plants respire. Plants carry out both photosynthesis and respiration.
8. Confusing population size and population density. Population size is the total number; density is the number per unit area.
9. Mark-release-recapture assumptions. The population must be closed (no migration) and marks must not affect the organism’s behaviour.

Practice Questions

Ordinary Level

1. Define ecosystem, habitat, and niche.
2. Draw a food chain with four trophic levels and label each level.
3. Explain the processes of photosynthesis and respiration in the carbon cycle.
4. Describe the effect of eutrophication on a lake ecosystem.

Higher Level

1. Explain the role of bacteria in the nitrogen cycle, naming specific bacteria where possible.

2. Calculate Simpson’s Diversity Index for a community with the following species: A (40), B (25), C (15), D (10), E (10).

3. Describe how a population of rabbits might show logistic growth after being introduced to an island with limited resources.

4. Evaluate the impact of deforestation on the carbon cycle and biodiversity.

5. A pond receives runoff from a nearby farm. Describe the process of eutrophication and calculate the expected decrease in dissolved oxygen at each stage.

6. Compare density-dependent and density-independent factors affecting population size, giving two examples of each.

7. Explain why the pyramid of energy is always pyramid-shaped but the pyramid of numbers can be inverted.

8. Describe three sampling techniques used in ecological field studies and explain when each is most appropriate.

9. A population of insects has a carrying capacity of 10,000. The current population is 2,000. Using the logistic growth equation, explain what will happen to the population over time.

10. Evaluate the effectiveness of international agreements (such as the Paris Agreement) in reducing the impact of climate change on biodiversity.

Review: Ecological Relationships

Organisms within a community interact with each other in various ways. Understanding these Relationships is important for the Irish Leaving Certificate.

Symbiosis: A close, long-term interaction between two different species.

Type	Effect on A	Effect on B	Example
Mutualism	+	+	Nitrogen-fixing bacteria in legume root nodules; mycorrhizae and plant roots
Commensalism	+	0	Barnacles on a whale; epiphytes on trees
Parasitism	+	-	Tapeworms in mammals; fleas on dogs
Predation	+	-	Foxes eating rabbits; owls eating mice
Competition	-	-	Two species competing for the same resource

Competition: Occurs when two organisms require the same limited resource (food, light, space, Mates). Intraspecific competition (between individuals of the same species) is more Intense than interspecific competition (between different species) because individuals of the same Species have identical niche requirements.

Competitive exclusion principle: Two species cannot occupy exactly the same ecological niche Indefinitely. One will eventually outcompete the other, leading to the exclusion of the weaker Competitor. In practice, species with similar niches coexist by resource partitioning (dividing the Resource to reduce direct competition).

Worked Example: Resource partitioning.

Two species of warbler (birds) live in the same tree. Species A feeds on insects near the top of the Tree canopy, while species B feeds on insects near the trunk. Although both species eat insects from The same tree, they partition the resource spatially (different parts of the tree). This reduces Direct competition and allows both species to coexist.

Review: Human Impact on Ecosystems — Detailed Case Studies

Case study: Peat bogs in Ireland.

Ireland has a significant proportion of Europe’s remaining peat bogs. Peat bogs are important carbon Stores (they contain more carbon per hectare than forests) and provide unique habitats for Specialist species (sphagnum moss, insectivorous plants such as sundews).

Threats: Peat has been harvested for fuel and horticulture for centuries. Drainage for Agriculture destroys the waterlogged conditions needed for peat formation. When peat bogs are Drained, the stored carbon is released as \mathrm{CO_2Contributing to climate change.

Conservation: Raised bogs and blanket bogs are protected under EU law (Habitats Directive). Rewetting drained bogs can restore their function as carbon stores. Bord na Mona (the Irish peat Board) has begun to transition away from peat harvesting towards renewable energy.

Case study: Overfishing.

Overfishing occurs when fish are caught faster than they can reproduce. This depletes fish stocks And disrupts marine food webs.

Example: Atlantic cod stocks in the Grand Banks (off Newfoundland, Canada) collapsed in the Early 1990s after decades of overfishing. The cod population has not recovered despite a fishing Moratorium, demonstrating that ecosystems can be pushed beyond a tipping point from which recovery Is very slow or impossible.

Solutions: Fishing quotas, minimum mesh sizes (to allow young fish to escape), marine protected Areas where fishing is banned, and sustainable certification schemes (e.g., Marine Stewardship Council).

Review: The Nitrogen Cycle in Detail

The nitrogen cycle involves the conversion of nitrogen between different chemical forms:

1. Nitrogen fixation: Atmospheric \mathrm{N_2 (very unreactive due to the triple bond) is converted to ammonia (\mathrm{NH_3) by nitrogen-fixing bacteria (Rhizobium in legume root nodules, Azotobacter in soil) or by lightning.
2. Nitrification: Ammonia is converted to nitrites (\mathrm{NO_2^-) by Nitrosomonas, then to nitrates (\mathrm{NO_3^-) by Nitrobacter. Nitrates are the form most absorbed by plants.
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 proteins in dead organisms and urea in urine, releasing ammonia back into the soil.
5. Denitrification: Denitrifying bacteria (Pseudomonas) convert nitrates back to \mathrm{N_2 gas in waterlogged (anaerobic) soil. This removes nitrogen from the ecosystem and returns it to the atmosphere.

Why waterlogged soil reduces fertility: In waterlogged soil, the lack of oxygen means that Denitrifying bacteria are active but nitrifying bacteria are not. Nitrates are converted to \mathrm{N_2 gas and lost to the atmosphere, while no new nitrates are produced. The soil becomes Nitrogen-deficient, reducing plant growth.

Review: Population Growth Curves

Exponential growth: When resources are unlimited, populations grow exponentially ($N = N_0 e^{rt}$Where $r$ is the intrinsic growth rate). A graph of population size against time Shows a J-shaped curve.

Logistic growth: In reality, resources are limited. As the population approaches the carrying Capacity ($K$), the growth rate slows. The graph shows an S-shaped (sigmoidal) curve.

$\frac{dN}{dt} = rN\left(1 - \frac{N}{K}\right)$

When $N$ is small compared to $K$Growth is nearly exponential. When $N$ approaches $K$Growth Slows and the population stabilises.

Worked Example: Interpreting a population growth graph.

A population of bacteria is grown in a flask with a fixed amount of nutrient. The population grows Rapidly at first (exponential phase), then the growth rate slows as nutrients are depleted and waste Products accumulate (deceleration phase). Eventually, the population stabilises (stationary phase) As the birth rate equals the death rate. The population size at this point is the carrying capacity For the flask. If the nutrients are not replenished, the population will decline (death phase) as Waste products become toxic.

Review: Ecological Succession

Primary Succession

Primary succession occurs on previously bare, lifeless surfaces such as bare rock, sand dunes, or Volcanic lava.

Stages:

1. Pioneer species: Lichens and mosses are the first colonisers. Lichens secrete acids that break down rock, beginning soil formation. They can survive harsh conditions with minimal nutrients and water.
2. Soil development: As pioneer species die and decompose, organic matter accumulates, forming a thin layer of soil. This allows more species to colonise.
3. Herbaceous stage: Grasses and small herbaceous plants establish, further improving soil quality with their root systems.
4. Shrub stage: Shrubs and small trees outcompete the herbaceous plants for light.
5. Tree stage: Larger trees establish, forming a canopy that shades out smaller plants.
6. Climax community: A stable, self-sustaining community is reached (e.g., oak woodland in temperate regions).

Secondary Succession

Secondary succession occurs where an existing community has been disturbed (e.g., after a forest Fire, abandoned farmland, or clear-felling). The soil is already present, so succession proceeds More rapidly than primary succession.

Worked Example: Succession on a sand dune (common in Irish ecology).

1. Marram grass colonises the bare sand at the high-water mark, stabilising it with its extensive root system.
2. Organic matter from dead marram grass accumulates, improving soil quality and water retention.
3. Other grasses and herbs colonise the improving soil.
4. Gorse and brambles establish, further enriching the soil with nitrogen (gorse is a legume with nitrogen-fixing bacteria in its root nodules).
5. Birch and willow trees colonise, followed by oak and other climax species.
6. A climax community of oak woodland is eventually established.

Review: Biomes and Climate

A biome is a large geographical area with a characteristic climate and distinctive plant and animal Communities. Climate (particularly temperature and rainfall) is the primary factor determining the Type of biome.

Major biomes:

Biome	Climate	Vegetation
Tropical rainforest	Hot and wet all year	Dense, broadleaved evergreen trees
Temperate deciduous	Moderate, four seasons	Broadleaved deciduous trees
Boreal forest (taiga)	Cold winters, moderate summers	Coniferous trees (pine, spruce)
Grassland/savanna	Seasonal rainfall	Grasses, scattered trees
Desert	Hot and dry / cold and dry	Sparse, drought-resistant plants
Tundra	Very cold, low rainfall	Mosses, lichens, low shrubs

Biome characteristics relevant to the Irish Leaving Certificate:

Ireland’s temperate maritime climate (mild winters, cool summers, high rainfall) supports temperate Deciduous woodland as the natural climax community. However, much of Ireland’s original woodland has Been cleared for agriculture, and grassland is now the dominant vegetation type. Raised bogs and Blanket bogs are significant Irish ecosystems that are rare internationally.

Worked Example: Adaptations of plants to different biomes.

• Xerophytes (desert plants): Thick waxy cuticle to reduce water loss; sunken stomata to reduce transpiration; deep roots to access groundwater; succulent stems to store water; spines instead of leaves to reduce surface area.
• Hydrophytes (water plants): Air spaces in tissues for buoyancy; stomata on upper leaf surface only; reduced xylem tissue; flexible stems to resist water currents.
• Halophytes (salt-tolerant plants): Salt glands to excrete excess salt; succulent leaves to dilute salt concentration; specialised root membranes to exclude salt.

Review: The Phosphorus Cycle

The phosphorus cycle differs from the carbon and nitrogen cycles in that phosphorus has no Significant gaseous phase. It cycles mainly between the land and water.

Key processes:

1. Weathering: Phosphorus is released from rocks by chemical weathering (slow process).
2. Absorption: Plants absorb phosphate ions (\mathrm{PO_4^{3-}) from the soil through their roots.
3. Feeding: Animals obtain phosphorus by eating plants or other animals.
4. Decomposition: Decomposers return phosphorus to the soil when organisms die.
5. Sedimentation: Phosphorus is washed into rivers and oceans, where it accumulates in sedimentary rocks. Over millions of years, geological uplift returns these rocks to the surface, restarting the cycle.

Human impact on the phosphorus cycle: Mining of phosphate rock for fertiliser has greatly Accelerated the rate at which phosphorus enters the ecosystem. Excess phosphate in water bodies Contributes to eutrophication. Phosphate is a non-renewable resource on human timescales, and Reserves are being depleted.

Worked Example: Phosphorus and aquatic ecosystems.

Phosphorus is often the limiting nutrient in freshwater ecosystems. When excess phosphate enters a Lake from fertiliser runoff or sewage, it stimulates rapid algal growth (algal bloom). The algae Block light, killing aquatic plants. Dead algae and plants are decomposed by bacteria, which use up Dissolved oxygen. Fish and other aquatic organisms die due to oxygen depletion. This is another Pathway to eutrophication, alongside nitrogen enrichment.

Review: Indicator Species and Pollution Monitoring

Indicator species are organisms whose presence, absence, or abundance provides information about The environmental quality of a habitat.

Air quality indicators:

• Lichens: Sensitive to sulphur dioxide (\mathrm{SO_2). Crustose lichens are the most tolerant; foliose lichens are moderately tolerant; fruticose lichens are the least tolerant. Areas with high \mathrm{SO_2 pollution have few or no lichens.
• Blackspot fungus on roses: More common in areas with cleaner air (the fungus is sensitive to \mathrm{SO_2).

Water quality indicators:

• Mayfly nymphs: Found only in clean, well-oxygenated water. Their presence indicates low pollution levels.
• Bloodworms (Chironomidae): Tolerant of low oxygen levels. Their presence indicates organic pollution.
• Sludgeworms (Tubificidae): Tolerant of very low oxygen and high organic pollution. Their abundance indicates severe pollution.

Biological Oxygen Demand (BOD): The amount of oxygen used by decomposers to break down organic Matter in a water sample over 5 days at 20$\degree$C. High BOD indicates high levels of organic Pollution. Clean water has a BOD of approximately 2 mg/L; polluted water can have a BOD of over 10 Mg/L.

Worked Example: Using indicator species to assess water quality.

A student samples macroinvertebrates from two sites on a river:

• Site A (upstream): Mayfly nymphs (abundant), stonefly nymphs (present), caddisfly larvae (present), no sludgeworms. BOD = 2 mg/L. Conclusion: clean water, low pollution.
• Site B (downstream, near a sewage outfall): No mayfly nymphs, no stonefly nymphs, few caddisfly larvae, abundant sludgeworms and bloodworms. BOD = 12 mg/L. Conclusion: organically polluted water, low dissolved oxygen.

The absence of sensitive species and the presence of tolerant species at Site B indicate that the Water quality has deteriorated downstream of the sewage outfall.

Review: Conservation Strategies in Ireland

Protected areas: Ireland has a network of national parks (e.g., Killarney National Park, Glenveagh National Park), nature reserves, and Special Areas of Conservation (SACs) designated under EU law. These areas protect important habitats and species.

Legislation:

• Wildlife Acts: Protect endangered species and regulate hunting.
• EU Habitats Directive: Requires member states to maintain or restore habitats of community importance.
• EU Birds Directive: Protects all wild bird species and their habitats.
• Flora Protection Order: Protects certain plant species from being picked or destroyed.

Captive breeding and reintroduction:

• The golden eagle was reintroduced to Ireland in 2001 after being extinct there for nearly 100 years.
• The red kite has been successfully reintroduced in several locations.
• The European bison has been introduced to a reserve in County Wicklow.

Sustainable practices:

• Organic farming reduces the use of synthetic fertilisers and pesticides, protecting soil and water quality.
• Agri-environment schemes pay farmers to manage their land in ways that benefit biodiversity (e.g., maintaining hedgerows, leaving field margins uncultivated).
• Sustainable forestry practices include selective logging, replanting, and maintaining deadwood habitats for invertebrates and fungi.

Worked Example: Evaluating a conservation strategy.

A proposed wind farm development would be located on an area of blanket bog that is designated as a Special Area of Conservation. The wind farm would generate renewable energy (reducing \mathrm{CO_2 Emissions and helping to combat climate change) but would destroy the bog habitat and fragment the Populations of specialist bog species.

This is an example of a conflict between two conservation goals: reducing carbon emissions and Protecting biodiversity. An environmental impact assessment would be required to evaluate the Trade-offs and propose mitigation measures (e.g., routing turbine access roads to avoid the most Sensitive areas, creating compensatory habitat elsewhere).

Census techniques for estimating population size:

When it is impractical to count every individual in a population, biologists use sampling methods:

1. Quadrat sampling: Used for plants or slow-moving animals. A quadrat (a known area frame) is placed randomly, and organisms within it are counted. The population is estimated by multiplying the mean number per quadrat by the total area.

   \mathrm{Estimated population = \frac{\mathrm{Mean number per quadrat}{\mathrm{Area of quadrat} \times \mathrm{Total area

2. Mark-release-recapture (Lincoln index): Used for mobile animals. Animals are captured, marked, released, and then a second sample is captured. The proportion of marked animals in the second sample equals the proportion of marked animals in the total population:

   $N = \frac{M \times C}{R}$

where $N$ = estimated population size, $M$ = number marked in first sample, $C$ = total captured in second sample, $R$ = number of marked individuals recaptured.

3. Transect sampling: A line or belt transect is laid across a habitat, and organisms touching the line (or within the belt) are recorded. This is useful for studying how species distribution changes across an environmental gradient (e.g., from low tide to high tide on a rocky shore).

Assumptions of mark-release-recapture:

• The marked individuals mix randomly back into the population.
• The marks are not lost and do not affect the organism’s survival or behaviour.
• The population size does not change between the two sampling events (no births, deaths, immigration, or emigration).
• The probability of capture is equal for all individuals.

Worked Example: Estimating a population using the Lincoln index.

A biologist captures and marks 80 woodlice. The next day, she captures 100 woodlice, of which 20 are Marked. What is the estimated population size?

$N = \frac{80 \times 100}{20} = \frac{8000}{20} = 400$

The estimated population is 400 woodlice.

Ecological niche and competitive exclusion:

Each species occupies a unique ecological niche — its role in the ecosystem, including the Resources it uses, its interactions with other species, and the environmental conditions it Requires. The competitive exclusion principle states that two species cannot coexist indefinitely if They occupy exactly the same niche. One species will outcompete the other, leading to competitive Exclusion.

In practice, closely related species often coexist because they have slightly different niches (resource partitioning). For example, different species of warblers in a forest may feed at Different heights in the canopy, reducing direct competition.

The carbon cycle:

Carbon is the backbone of all organic molecules. The carbon cycle describes the movement of carbon Between the atmosphere, biosphere, oceans, and geosphere.

Key processes in the carbon cycle:

1. Photosynthesis: Plants and algae absorb \mathrm{CO_2 from the atmosphere and convert it into organic molecules (glucose, carbohydrates, lipids, proteins). This removes carbon from the atmosphere.
2. Respiration: All living organisms break down organic molecules to release energy, producing \mathrm{CO_2 as a by-product. This returns carbon to the atmosphere.
3. Feeding: Carbon is transferred between trophic levels when organisms consume other organisms.
4. Decomposition: Decomposers break down dead organic matter, releasing \mathrm{CO_2 through respiration. If decomposition occurs in anaerobic conditions (e.g., waterlogged soil or deep ocean sediments), carbon can be stored as fossil fuels over millions of years.
5. Combustion: Burning fossil fuels (coal, oil, natural gas) and biomass releases stored carbon as \mathrm{CO_2 back into the atmosphere.
6. Ocean absorption: The oceans absorb approximately 25% of human \mathrm{CO_2 emissions. Dissolved \mathrm{CO_2 forms carbonic acid, which can react to form bicarbonate and carbonate ions.

Human impact on the carbon cycle:

Since the Industrial Revolution, the burning of fossil fuels and deforestation have significantly Increased atmospheric \mathrm{CO_2 concentration from approximately 280 ppm to over 420 ppm. This Enhanced greenhouse effect is the primary driver of current climate change.

Deforestation and the carbon cycle:

Trees store large amounts of carbon in their biomass (trunks, branches, roots). When forests are Cleared:

• The stored carbon is released as \mathrm{CO_2 if the wood is burned or left to rot.
• The loss of trees reduces the rate of photosynthesis, decreasing the rate at which \mathrm{CO_2 is removed from the atmosphere.
• Deforestation accounts for approximately 10-15% of global \mathrm{CO_2 emissions.

Worked Example: Calculating net carbon flux.

A forest absorbs 5000 tonnes of \mathrm{CO_2 per year through photosynthesis. The plants and Animals in the forest release 3000 tonnes of \mathrm{CO_2 per year through respiration. Decomposers Release 800 tonnes per year. What is the net carbon flux?

\mathrm{Net flux = \mathrm{Photosynthesis - \mathrm{Respiration - \mathrm{Decomposition \mathrm{Net flux = 5000 - 3000 - 800 = 1200 \mathrm{ tonnes of \mathrm{CO_2 \mathrm{ absorbed per year

The forest is a net carbon sink, absorbing 1200 tonnes of \mathrm{CO_2 per year.

Eutrophication:

Eutrophication is the over-enrichment of water bodies with nutrients (particularly nitrates and Phosphates), leading to excessive growth of algae and subsequent oxygen depletion.

Stages of eutrophication:

1. Nutrient enrichment: Nitrates from agricultural fertilisers (or phosphates from detergents) are washed into rivers and lakes by rain (leaching and surface runoff).
2. Algal bloom: The increased nutrient concentration stimulates rapid growth of algae on the water surface, forming a dense green layer (algal bloom).
3. Light blockage: The dense algal layer prevents light from reaching submerged aquatic plants, which can no longer photosynthesise and die.
4. Decomposition and oxygen depletion: Bacteria decompose the dead algae and dead plants, using up dissolved oxygen in the water through aerobic respiration.
5. Hypoxia and death: Dissolved oxygen levels drop (hypoxia). Fish and other aquatic organisms die due to lack of oxygen. Anaerobic decomposers produce toxic substances (e.g., hydrogen sulphide), making the water foul-smelling and further reducing biodiversity.

Methods to reduce eutrophication:

• Using fertilisers more carefully (precise application, avoiding excess).
• Planting buffer strips of vegetation along waterways to absorb runoff nutrients.
• Treating sewage to remove nitrates and phosphates before discharge.
• Using phosphate-free detergents.
• Reducing livestock density near waterways.

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.