Life on Earth

Higher Biodiversity and Evolution

Classification

Organisms are classified into a hierarchical system:

Domain $\to$ Kingdom $\to$ Phylum $\to$ Class $\to$ Order $\to$ Family $\to$ Genus $\to$ Species

The Three Domains:

• Bacteria: Prokaryotic, unicellular
• Archaea: Prokaryotic, extremophiles
• Eukarya: Eukaryotic (animals, plants, fungi, protists)

The Five Kingdoms (traditional):

Kingdom	Cell type	Organisation	Nutrition
Animalia	Eukaryotic	Multicellular	Heterotrophic
Plantae	Eukaryotic	Multicellular	Autotrophic (photosynthesis)
Fungi	Eukaryotic	Multicellular (mostly)	Heterotrophic (absorption)
Protoctista	Eukaryotic	Mostly unicellular	Both
Prokaryotae	Prokaryotic	Unicellular	Various

Binomial nomenclature: Two-part Latin name: Genus species (e.g., Homo sapiens).

Worked Example: Understanding the classification hierarchy.

The classification of the domestic cat is:

Domain: Eukarya Kingdom: Animalia Phylum: Chordata Class: Mammalia Order: Carnivora Family: Felidae Genus: Felis Species: Felis catus

The domestic cat and the lion (Panthera leo) share the same kingdom, phylum, class, and order, but Differ at the family level. This tells us they are related but diverged evolutionarily at the family Level.

Biodiversity

Biodiversity is the variety of life at all levels of biological organisation.

Species richness: The number of different species in a habitat.

Species evenness: How evenly individuals are distributed among species.

Simpson’s Diversity Index:

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

Where $n$ is the number of individuals of a particular species and $N$ is the total number of Individuals.

Worked Example 1. A habitat has three species with 10, 20, and 30 individuals respectively.

$N = 60$.

$D = 1 - \left[\frac{10(9)}{60(59)} + \frac{20(19)}{60(59)} + \frac{30(29)}{60(59)}\right]$

$= 1 - \left[\frac{90}{3540} + \frac{380}{3540} + \frac{870}{3540}\right]$

$= 1 - \left[0.0254 + 0.1073 + 0.2458\right]$

$= 1 - 0.3785 = 0.6215$

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

Worked Example 2. Compare two habitats.

Habitat 1: Species A (80), B (10), C (10). $N = 100$. Habitat 2: Species A (34), B (33), C (33). $N = 100$.

Habitat 1: $D = 1 - [80(79) + 10(9) + 10(9)] / [100(99)] = 1 - [6320 + 90 + 90] / 9900 = 1 - 6500/9900 = 1 - 0.6566 = 0.343$

Habitat 2: $D = 1 - [34(33) + 33(32) + 33(32)] / [100(99)] = 1 - [1122 + 1056 + 1056] / 9900 = 1 - 3234/9900 = 1 - 0.3267 = 0.673$

Habitat 2 has higher diversity (0.673 vs 0.343) despite having the same species richness (3), Because the species are more evenly distributed.

Sampling Techniques

Random sampling: Used to avoid bias. Use a quadrat at random coordinates.

Systematic sampling: Samples taken at regular intervals (e.g., along a transect line).

Quadrats: Used for plants and slow-moving animals. Count individuals or estimate percentage Cover.

Pitfall traps: Used for ground-dwelling invertebrates.

Sweep netting: Used for insects in tall vegetation.

Kick sampling: Used for aquatic invertebrates in rivers.

Evolution

Darwin’s Theory of Natural Selection:

1. Variation exists within populations
2. More offspring are produced than can survive (competition)
3. Individuals with advantageous characteristics are more likely to survive and reproduce
4. These advantageous alleles are passed on to the next generation
5. Over many generations, the frequency of advantageous alleles increases

Evidence for Evolution:

• Fossil record: Shows gradual changes in species over time and transitional forms
• Comparative anatomy: Homologous structures (same origin, different function, e.g., pentadactyl limb)
• Comparative biochemistry: Similar DNA and protein sequences in related species
• Embryology: Similar early embryonic development in vertebrates
• Geographical distribution: Species on islands resemble mainland species (e.g., Darwin’s finches)

Speciation

Speciation is the formation of new species. A species is a group of organisms that can Interbreed to produce fertile offspring.

Allopatric speciation: Speciation due to geographical separation.

1. Populations become geographically isolated
2. Different selection pressures act on each population
3. Allele frequencies change independently in each population
4. Reproductive isolation develops (they can no longer interbreed)

Sympatric speciation: Speciation without geographical separation. Often due to:

• Polyploidy (common in plants)
• Behavioural isolation
• Ecological isolation
• Temporal isolation (different breeding seasons)

Adaptation

Structural adaptations: Physical features (e.g., thick fur in Arctic animals).

Behavioural adaptations: Actions (e.g., migration, hibernation, courtship displays).

Physiological adaptations: Internal processes (e.g., venom production, antifreeze proteins in Fish).

Example: The camel is adapted to desert life with: a hump for fat storage (energy and water from Metabolism), concentrated urine, wide feet for walking on sand, and long eyelashes to protect Against sand.

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Ecosystems

Energy Flow

Producers convert light energy to chemical energy via photosynthesis.

Consumers obtain energy by eating other organisms.

Trophic levels:

1. Producers (autotrophs)
2. Primary consumers (herbivores)
3. Secondary consumers (carnivores)
4. Tertiary consumers (top carnivores)

Ecological efficiency: Only about 10% of energy is transferred between trophic levels. The rest Is lost as:

• Heat from respiration
• Undigested material (faeces)
• Excretion (urine)
• Not all biomass is consumed

Pyramid of numbers: Shows the number of organisms at each trophic level. Can be inverted (e.g., One tree supporting many insects).

Pyramid of biomass: Shows the total biomass at each trophic level. Pyramid-shaped.

Pyramid of energy: Always pyramid-shaped; shows energy transfer.

Nutrient Cycles

Carbon Cycle:

1. Photosynthesis: \mathrm{CO_2 \to organic compounds (producers)
2. Feeding: Carbon passed along food chains
3. Respiration: Organic compounds \to \mathrm{CO_2 (all organisms)
4. Decomposition: Dead organisms \to \mathrm{CO_2 (decomposers)
5. Combustion: Fossil fuels \to \mathrm{CO_2
6. Fossilisation: Dead organisms $\to$ fossil fuels (over millions of years)

Nitrogen Cycle:

1. Nitrogen fixation: \mathrm{N_2 \to \mathrm{NH_3 (by nitrogen-fixing bacteria in root nodules or lightning)
2. Nitrification: \mathrm{NH_3 \to \mathrm{NO_2^- \to \mathrm{NO_3^- (by nitrifying bacteria)
3. Assimilation: Plants absorb \mathrm{NO_3^-; animals obtain nitrogen by eating plants
4. Ammonification: Dead organisms/urea/faeces \to \mathrm{NH_3 (by decomposers)
5. Denitrification: \mathrm{NO_3^- \to \mathrm{N_2 (by denitrifying bacteria in anaerobic conditions)

Summary table: nitrogen cycle bacteria.

Process	Bacteria	Product	Conditions
Nitrogen fixation	Rhizobium, Azotobacter	Ammonia (\mathrm{NH_3)	Aerobic
Nitrification	Nitrosomonas, Nitrobacter	Nitrite, nitrate	Aerobic
Denitrification	Pseudomonas	\mathrm{N_2 gas	Anaerobic
Ammonification	Decomposers	Ammonia	Both

Human Impact on Ecosystems

Deforestation: Loss of habitat, reduced biodiversity, increased \mathrm{CO_2.

Eutrophication:

1. Excess nutrients (nitrates/phosphates) enter water (from fertilisers)
2. Algal bloom occurs
3. Algae block light, killing aquatic plants
4. Dead organisms decomposed by bacteria, using up oxygen
5. Fish and other organisms die

Climate change:

• Increased \mathrm{CO_2 and other greenhouse gases enhance the greenhouse effect
• Consequences: rising temperatures, sea level rise, changes in species distribution, increased extreme weather

Sustainable resource management:

• Selective logging instead of clear-felling
• Conservation programmes
• Captive breeding
• Protected areas (national parks, nature reserves)
• Sustainable fishing quotas

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Worked Examples

See the examples integrated throughout the sections above.

Common Pitfalls

1. Biodiversity measures: Species richness and species evenness are different. A habitat with 2 species (50 each) and one with 2 species (90 and 10) have the same richness but different evenness.

2. Simpson’s Index: Higher values indicate greater diversity.

3. Energy pyramids: Pyramids of numbers and biomass can be inverted; pyramids of energy cannot.

4. Speciation: Allopatric speciation requires geographic isolation; sympatric does not.

5. Nitrogen fixation: Not all bacteria can fix nitrogen. Only specific species (e.g., Rhizobium in legume root nodules) can convert \mathrm{N_2 to \mathrm{NH_3.

6. Confusing nitrification and denitrification. Nitrification converts ammonia to nitrate (useful for plants); denitrification converts nitrate to nitrogen gas (removes nitrogen).

7. Forgetting that decomposers are essential for nutrient cycling. Without decomposers, nutrients would remain locked in dead organisms.

8. Confusing habitat and niche. Habitat is where an organism lives; niche is its role in the ecosystem (how it interacts with other organisms and the environment).

9. Forgetting that plants respire. Plants carry out both photosynthesis and respiration. At night, they only respire (net release of \mathrm{CO_2). During the day, photosynthesis exceeds respiration (net uptake of \mathrm{CO_2).

10. Assuming all energy at one trophic level is transferred to the next. Only about 10% is transferred; the rest is lost. This limits food chains to 4-5 trophic levels.

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Practice Questions

1. Calculate Simpson’s Diversity Index for a habitat with the following species: Species A (40), Species B (30), Species C (20), Species D (10).

2. Describe the evidence for evolution from (a) the fossil record and (b) comparative anatomy.

3. Explain how geographical isolation can lead to the formation of new species.

4. Draw a labelled diagram of the carbon cycle.

5. Explain why only about 10% of energy is transferred between trophic levels.

6. Describe the process of eutrophication and its effects on an aquatic ecosystem.

7. Compare allopatric and sympatric speciation.

8. Explain why a pyramid of biomass for a tree ecosystem is often inverted, but a pyramid of energy is not.

9. A student uses quadrats to estimate the population of daisies in a 100 m $\times$ 50 m field. The quadrat is 1 m $\times$ 1 m. The mean number of daisies per quadrat is 12. Estimate the total population.

10. Explain the role of nitrogen-fixing bacteria in the nitrogen cycle and describe the conditions under which they are most active.

11. Describe three structural, three behavioural, and three physiological adaptations of an organism of your choice to its environment.

12. Explain why conservation of biodiversity is important, giving three reasons.

13. A lake receives runoff from a nearby farm that uses nitrate fertilisers. Describe the sequence of events that leads to the death of fish in the lake.

14. Compare the carbon cycle and the nitrogen cycle, focusing on the role of microorganisms in each.

15. Explain how deforestation contributes to both biodiversity loss and climate change.

16. Describe how a transect can be used to study the distribution of organisms along an environmental gradient.

17. Explain the concept of an ecological niche and why two species cannot occupy the same niche indefinitely (competitive exclusion principle).

18. Describe the evidence from comparative biochemistry (DNA and protein sequences) that supports the theory of evolution.

19. Explain why captive breeding programmes are used for endangered species and describe two potential problems with releasing captive-bred animals into the wild.

20. A forest has Simpson’s Diversity Index of 0.85. After deforestation, the index drops to 0.35. Explain what this means and describe the likely consequences for the ecosystem.

21. Explain the role of mycorrhizae in plant nutrition and describe how this is an example of mutualism.

22. Describe how the mark-release-recapture method is used to estimate animal population sizes, including the assumptions and potential sources of error.

23. Explain why intraspecific competition is more intense than interspecific competition.

24. Describe the process of succession in a previously barren habitat (primary succession) and explain how it leads to a climax community.

25. Explain how bioaccumulation and biomagnification affect organisms at different trophic levels, using DDT as an example.

Review: Evolution — Darwin’s Theory in Detail

Darwin’s observations (from his voyage on HMS Beagle):

1. Organisms produce more offspring than can survive (overproduction).
2. There is variation within populations.
3. Some variation is heritable (passed from parent to offspring).
4. There is competition for limited resources (struggle for existence).
5. Individuals with advantageous variations are more likely to survive and reproduce (natural selection).
6. Over many generations, advantageous characteristics become more common in the population.

Key insight: Natural selection acts on the phenotype (the observable characteristics), but the Alleles that produce those phenotypes are what are passed to the next generation. Therefore, natural Selection causes changes in allele frequencies in a population over time.

Worked Example: Peppered moths as evidence for natural selection.

Before industrialisation in England, the majority of peppered moths (Biston betularia) were light- Coloured (typica morph), well camouflaged against lichen-covered trees. A rare dark morph (carbonaria) was occasionally seen. During the Industrial Revolution, soot darkened tree bark and Killed the lichen. The light morph became conspicuous to bird predators, while the dark morph became Better camouflaged. The frequency of the dark allele increased dramatically through natural Selection. After clean-air legislation reduced pollution, tree bark lightened again, and the Frequency of the light morph began to increase. This demonstrates that natural selection is not Directional in a fixed sense but responds to changes in the environment.

Review: Evidence for Evolution

Fossil record: Fossils provide a record of past life. The sequence of fossils in rock layers (strata) shows gradual changes in organisms over millions of years. Transitional fossils (e.g., Archaeopteryx, which has features of both reptiles and birds) provide evidence for evolutionary Transitions.

Comparative anatomy:

• Homologous structures: Same basic structure and evolutionary origin but different functions (e.g., the pentadactyl limb in humans, bats, whales, and horses). Evidence for common ancestry.
• Analogous structures: Similar function but different evolutionary origin (e.g., wings of birds and insects). Evidence for convergent evolution (similar selection pressures leading to similar adaptations).
• Vestigial structures: Remnants of structures that were functional in ancestors but have lost their function (e.g., the human appendix, pelvic bones in whales, wings in flightless birds).

Comparative embryology: Early embryos of vertebrates (fish, amphibians, reptiles, birds, Mammals) look remarkably similar, suggesting a common ancestry. Features such as pharyngeal pouches And a tail are present in all vertebrate embryos early in development.

Molecular evidence: DNA and protein sequence comparisons show that more closely related species Have more similar sequences. The universality of the genetic code (the same codons code for the same Amino acids in almost all organisms) is powerful evidence for common ancestry.

Review: Speciation

Allopatric speciation: Populations are separated by a geographical barrier (mountain range, River, ocean). The populations experience different selection pressures and accumulate different Mutations. Over time, they become so different that they can no longer interbreed to produce fertile Offspring.

Sympatric speciation: Speciation occurs without geographical separation. This can happen through Polyploidy (common in plants, where a mistake in meiosis produces offspring with extra sets of Chromosomes, making them reproductively isolated from the parent population).

Reproductive isolation mechanisms:

• Pre-zygotic: Habitat isolation, temporal isolation (different breeding seasons), behavioural isolation (different courtship displays), mechanical isolation (incompatible reproductive structures), gametic isolation (sperm and egg cannot fuse).
• Post-zygotic: Hybrid inviability (hybrid offspring do not survive), hybrid sterility (hybrid offspring are sterile, e.g., mules), hybrid breakdown (first-generation hybrids are fertile but subsequent generations are not).

Review: Sampling Techniques and Statistics

Random sampling: Quadrats are placed at random coordinates (generated using random number Tables) to avoid bias. At least 10 quadrats should be sampled to obtain a representative estimate.

Systematic sampling: A transect is laid across a habitat, and quadrats are placed at regular Intervals. This is useful for studying how species distribution changes along an environmental Gradient (e.g., from dry land into a pond).

Mark-release-recapture assumptions:

• The population is closed (no immigration, emigration, birth, or death between captures).
• Marks are not lost and do not affect survival.
• Marked individuals mix randomly with the population.
• All individuals have an equal probability of being captured.

Worked Example: Using the Lincoln index.

40 snails are captured, marked, and released. One week later, 50 snails are captured, 8 of which are Marked.

Estimated population $= (40 \times 50) / 8 = 250$ snails.

Worked Example: Estimating population from quadrat data.

A student places ten 1 m $\times$ 1 m quadrats randomly in a 100 m $\times$ 50 m field. The mean Number of daisies per quadrat is 12.

Mean density $= 12$ daisies per m$^2$.

Total area $= 100 \times 50 = 5000$ m$^2$.

Estimated population $= 12 \times 5000 = 60,000$ daisies.

Review: Human Impact on the Environment

Deforestation: Removes habitats, releases stored carbon (contributing to climate change), Disrupts the water cycle, and causes soil erosion. Tropical rainforests are particularly important Because they contain an estimated 50% of all terrestrial species.

Pollution:

• Eutrophication: Caused by nitrate and phosphate fertiliser runoff. Algal blooms block light, dead algae are decomposed by bacteria that use up oxygen, killing aquatic organisms.
• Bioaccumulation: Persistent toxins (e.g., DDT, mercury) accumulate in fatty tissue and increase in concentration at each trophic level (biomagnification).
• Acid rain: Sulphur dioxide and nitrogen oxides from burning fossil fuels dissolve in atmospheric moisture to form acids. Acid rain damages forests, acidifies lakes, and corrodes buildings.

Climate change: Rising atmospheric \mathrm{CO_2 from burning fossil fuels enhances the Greenhouse effect. Consequences include rising sea levels (thermal expansion of water and melting Ice caps), more frequent extreme weather events, and shifts in species distributions.

Worked Example: Bioaccumulation of DDT.

DDT is a pesticide that was widely used in the mid-20th century. It is persistent in the environment And accumulates in fatty tissue. In an aquatic food chain: phytoplankton (0.003 ppm DDT) $\to$ Zooplankton (0.04 ppm) $\to$ small fish (0.5 ppm) $\to$ large fish (2.0 ppm) $\to$ osprey (25.0 Ppm). The concentration increases by a factor of over 8,000 from phytoplankton to osprey. Top Predators are most affected because they are at the highest trophic level and consume many Contaminated organisms.

Review: Ecological Relationships

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

Competitive exclusion principle: Two species cannot occupy exactly the same ecological niche Indefinitely. One will eventually outcompete the other. In practice, species with similar niches Coexist by resource partitioning.

Review: Succession

Primary succession: Colonisation of a previously lifeless area (e.g., bare rock after a volcanic Eruption or glacial retreat). Pioneer species (lichens, mosses) are the first to colonise. They Break down rock and contribute organic matter to the soil, allowing grasses and then shrubs to Establish. Over time, trees colonise, and a climax community is reached.

Secondary succession: Recovery of an area where an existing community has been disturbed (e.g., After a forest fire or abandoned farmland). The soil is already present, so succession proceeds more Rapidly than primary succession.

Worked Example: Succession on a sand dune.

1. Pioneer species (e.g., marram grass) colonise the bare sand, stabilising it with their roots.
2. Organic matter accumulates as plants die and decompose, improving soil quality.
3. Larger plants (e.g., gorse, brambles) establish, further improving the soil.
4. Trees (e.g., birch, then oak) colonise, shading out the earlier species.
5. A climax community of oak woodland is established, which remains stable until the next major disturbance.

Review: Classification and Phylogeny

Modern Classification Systems

Three-domain system (Carl Woese, 1990): Based on ribosomal RNA (rRNA) sequence analysis, all Organisms are classified into three domains:

• Domain Bacteria: True bacteria. Single-celled prokaryotes. Cell walls contain peptidoglycan. Found in nearly all environments on Earth.
• Domain Archaea: Prokaryotes that are more closely related to eukaryotes than to bacteria based on rRNA analysis. Often extremophiles (living in extreme conditions such as hot springs, salt lakes, deep-sea vents). Cell walls do not contain peptidoglycan.
• Domain Eukarya: All eukaryotic organisms. Includes kingdoms Animalia, Plantae, Fungi, and Protista.

Why the three-domain system replaced the five-kingdom system: Molecular evidence (particularly RRNA sequencing) revealed that the differences between bacteria and archaea are as great as the Differences between either prokaryotic group and eukaryotes. The old five-kingdom system grouped all Prokaryotes together in one kingdom (Prokaryotae), which did not reflect their evolutionary Divergence.

Phylogenetic Trees

A phylogenetic tree shows the evolutionary relationships among species based on shared derived Characteristics. Modern phylogenetic trees are constructed using molecular data (DNA and protein Sequences) rather than just morphological data.

Reading a phylogenetic tree:

• The root represents the most recent common ancestor of all species on the tree.
• Branch points (nodes) represent speciation events.
• The length of branches may represent the amount of genetic change or time since divergence.
• Species that share a more recent common ancestor are more closely related.

Worked Example: Interpreting a phylogenetic tree.

A phylogenetic tree shows three species: human, chimpanzee, and gorilla. The human and chimpanzee Lineages diverge most recently (approximately 6 million years ago), while the gorilla lineage Diverged earlier (approximately 10 million years ago). This tells us that humans are more closely Related to chimpanzees than to gorillas. All three species share a common ancestor at the root of The tree (approximately 15-20 million years ago).

Review: The Carbon Cycle in Detail

The carbon cycle describes the movement of carbon between the atmosphere, biosphere, oceans, and Geosphere.

Key processes and the carbon reservoirs they connect:

1. Photosynthesis: Atmospheric \mathrm{CO_2 is fixed into organic compounds by plants, algae, and cyanobacteria. Approximately 120 gigatonnes of carbon are fixed by photosynthesis each year.
2. Respiration: Organic compounds are oxidised to release \mathrm{CO_2Returning carbon to the atmosphere. This occurs in all living organisms.
3. Decomposition: Decomposers (bacteria and fungi) break down dead organic matter, releasing \mathrm{CO_2.
4. Combustion: Burning fossil fuels (coal, oil, natural gas) releases stored carbon as \mathrm{CO_2. This is the primary cause of the recent increase in atmospheric \mathrm{CO_2.
5. Ocean absorption: The oceans absorb approximately 25% of anthropogenic \mathrm{CO_2 emissions. Dissolved \mathrm{CO_2 forms carbonic acid, leading to ocean acidification.
6. Sedimentation: Over millions of years, dead marine organisms sink to the ocean floor and are compressed into sedimentary rocks and fossil fuels, locking carbon away.
7. Weathering: Silicate and carbonate rocks absorb \mathrm{CO_2 very slowly through chemical weathering.

Worked Example: The impact of deforestation on the carbon cycle.

When a tropical rainforest is cleared:

1. The trees are either burned (releasing \mathrm{CO_2 immediately) or left to decompose (releasing \mathrm{CO_2 gradually).
2. The carbon stored in the trees (approximately 200 tonnes of carbon per hectare for tropical rainforest) is released to the atmosphere.
3. Photosynthesis no longer removes \mathrm{CO_2 from the atmosphere in the cleared area.
4. Soil carbon is also lost as the exposed soil is eroded and organic matter decomposes.
5. The net effect is a significant increase in atmospheric \mathrm{CO_2.

Review: Ecological Succession in Detail

Stages of Primary Succession

1. Bare substrate: No soil present. The substrate could be bare rock, sand, or volcanic ash.
2. Pioneer community: Lichens are often the first colonisers. Lichens are a mutualistic association between a fungus and an alga. They can grow on bare rock because they secrete acids that break down the rock surface, beginning the process of soil formation.
3. Soil development: As lichens die and decompose, organic matter accumulates, forming a thin layer of soil. Mosses and liverworts colonise this thin soil, adding more organic matter.
4. Herbaceous stage: Grasses and small herbaceous plants establish in the improving soil. Their roots help to bind the soil and further improve its structure and water-holding capacity.
5. Shrub stage: Shrubs and small trees colonise, outcompeting the grasses and herbs for light.
6. Tree stage: Larger trees establish, forming a canopy that shades out the shrubs. The community becomes increasingly complex with more species and more trophic levels.
7. Climax community: A stable, self-sustaining community is reached. In temperate regions, this is oak or beech woodland. In tropical regions, it is tropical rainforest. The climax community remains relatively stable until a major disturbance (fire, storm, disease) resets the succession process.

Characteristics of a Climax Community

• Relatively stable species composition.
• Complex food webs with many trophic levels.
• High biodiversity.
• Efficient nutrient cycling.
• Net primary productivity is balanced by the rate of decomposition.

Worked Example: Secondary succession after a forest fire.

After a forest fire:

1. The fire destroys above-ground vegetation but leaves the soil intact (with seeds, root systems, and soil organisms).
2. Herbaceous plants and shrubs sprout from surviving roots and seeds within weeks to months.
3. Fast-growing, light-loving tree species (e.g., birch, pine) colonise within a few years.
4. Slower-growing, shade-tolerant species (e.g., oak, beech) establish beneath the pioneer trees.
5. Over decades to centuries, the climax community is re-established.

Secondary succession is faster than primary succession because the soil is already present, and Seeds and root systems survive the disturbance.

Factors affecting the rate of succession:

Several factors influence how quickly succession proceeds towards a climax community:

• Climate: Warmer, wetter climates generally support faster succession because plant growth rates are higher.
• Soil quality: Richer soils with higher nutrient content support faster colonisation and growth.
• Proximity to seed sources: Areas closer to existing vegetation colonise more quickly because seeds and spores can disperse more .
• Frequency of disturbance: If disturbances occur too frequently, the community may never reach the climax stage.
• Type of disturbance: Some disturbances (e.g., fire) may leave the soil mostly intact, while others (e.g., mining) may remove or contaminate the soil, slowing succession.

Biodiversity during succession:

Biodiversity changes during the course of succession:

• Early stages: Low species diversity but high abundance of a few pioneer species. These species are often r-selected (fast-growing, producing many offspring, short-lived).
• Middle stages: Highest species diversity as both early and late successional species coexist. Niche diversity increases as the habitat becomes more structurally complex.
• Climax community: Species diversity may stabilise or even decrease slightly compared to the middle stages, but the community is more stable and self-sustaining.

Worked Example: Predicting the outcome of succession.

A bare rock face is exposed after a landslide. Predict the stages of primary succession:

1. Stage 1 (0—5 years): Lichens and mosses colonise the rock surface. Weathering of the rock by lichen acids begins to form a thin soil layer.
2. Stage 2 (5—20 years): Herbaceous plants (grasses, ferns) establish in the thin soil. Their roots further break up the rock and add organic matter when they die.
3. Stage 3 (20—50 years): Shrubs and small woody plants (e.g., gorse, heather) colonise. The deeper root systems accelerate soil formation.
4. Stage 4 (50—150 years): Fast-growing trees (e.g., birch, Scots pine) establish, forming a woodland canopy that shades the pioneer species.
5. Stage 5 (150+ years): Slower-growing, shade-tolerant trees (e.g., oak, beech) replace the pioneer trees, forming the climax community.

Nitrogen cycle:

Nitrogen is essential for the synthesis of amino acids, proteins, and nucleic acids. Although Nitrogen gas (\mathrm{N_2) makes up approximately 78% of the atmosphere, most organisms cannot use Atmospheric nitrogen directly. The nitrogen cycle converts nitrogen between different chemical Forms:

Key processes in the nitrogen cycle:

1. Nitrogen fixation: Atmospheric \mathrm{N_2 is converted into ammonia (\mathrm{NH_3) by nitrogen-fixing bacteria (e.g., Rhizobium in root nodules of legumes, or free-living Azotobacter in soil). Lightning also fixes nitrogen by converting \mathrm{N_2 to nitrogen oxides.
2. Nitrification: Ammonia is converted to nitrite (\mathrm{NO_2^-) by Nitrosomonas bacteria, then to nitrate (\mathrm{NO_3^-) by Nitrobacter bacteria. Nitrates are the form of nitrogen most readily absorbed by plants.
3. Absorption: Plants absorb nitrates (and some ammonium ions) through their roots and incorporate them into amino acids and nucleic acids.
4. Feeding: Animals obtain nitrogen by consuming plants or other animals.
5. Decomposition (ammonification): When organisms die, decomposers (bacteria and fungi) break down proteins and urea into ammonia, returning nitrogen to the soil.
6. Denitrification: Denitrifying bacteria (e.g., Pseudomonas) convert nitrates in waterlogged, anaerobic soil back into nitrogen gas (\mathrm{N_2), returning it to the atmosphere.

Human impact on the nitrogen cycle:

• Synthetic fertilisers: The Haber process converts \mathrm{N_2 and \mathrm{H_2 into ammonia for fertilisers. Excess fertiliser runs off into waterways, causing eutrophication.
• Combustion of fossil fuels: Releases nitrogen oxides into the atmosphere, contributing to acid rain and photochemical smog.
• Deforestation: Removes plants that absorb nitrates, increasing soil nitrate leaching.

Worked Example: Tracing a nitrogen atom through the nitrogen cycle.

A nitrogen atom in the atmosphere as \mathrm{N_2 is fixed by Rhizobium bacteria in the root nodules Of a clover plant, converting it to \mathrm{NH_3. The clover incorporates the ammonia into amino Acids. A rabbit eats the clover and digests the proteins, using the amino acids to make its own Proteins. When the rabbit dies, decomposer bacteria break down its proteins, releasing \mathrm{NH_3 Back into the soil. Nitrosomonas converts the \mathrm{NH_3 to \mathrm{NO_2^-And Nitrobacter Converts \mathrm{NO_2^- to \mathrm{NO_3^-. A grass plant absorbs the nitrate through its roots.

Summary

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

Key concepts include:

• Mendelian inheritance
• gene expression and regulation
• mutations and genetic variation
• genetic engineering (PCR, gel electrophoresis)
• genome projects

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