Classification, Biodiversity & Conservation
Classification organises the diversity of life into a hierarchical system using binomial nomenclature and evidence from morphology and molecules. Biodiversity — measured at genetic, species, and ecosystem levels — is quantified using Simpson's index and is under threat from human activity. Conservation strategies from national parks to seed banks aim to protect what remains. This topic brings together taxonomy, ecology, and practical skills in a synoptic capstone.
Classification
Classification organises the diversity of life into a hierarchical system based on evolutionary relationships. The 9700 syllabus requires knowledge of the classification hierarchy, binomial nomenclature, three species concepts, and molecular vs morphological evidence for relationships.
Classification hierarchy
The Linnaean hierarchical system groups organisms at increasingly specific levels. Each level is a taxon (plural: taxa). Each taxon is nested within the one above it.
| Rank | Human example | Cat example | Memory hint |
|---|---|---|---|
| Domain | Eukarya | Eukarya | Broadest grouping (above kingdom); based on cell type |
| Kingdom | Animalia | Animalia | Animals, Plants, Fungi, Protoctista, Prokaryotae |
| Phylum | Chordata | Chordata | Body plan level |
| Class | Mammalia | Mammalia | |
| Order | Primates | Carnivora | |
| Family | Hominidae | Felidae | |
| Genus | Homo | Felis | First word of binomial name |
| Species | sapiens | catus | Second word of binomial name |
Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species
Mnemonic: Dear King Philip Came Over For Good Soup
Binomial nomenclature
Each species has a two-part Latin name (binomial) consisting of the genus name followed by the species epithet. Rules are strict and must be reproduced exactly in exams:
- Written in italics when typed; underlined when handwritten
- Genus name: capital first letter, e.g. Homo
- Species epithet: all lower case, e.g. sapiens
- Full example: Homo sapiens (human), Felis catus (domestic cat), Panthera leo (lion)
Latin is a dead language — it does not change over time, ensuring the names remain stable and unambiguous across countries and languages. Common names (e.g. "robin") can refer to entirely different species in different countries; binomial names are universal.
Three species concepts
The concept of a "species" is not straightforward to define. The 9700 syllabus requires three specific definitions:
| Concept | Definition | Criterion | Limitation |
|---|---|---|---|
| Biological species concept | A group of organisms capable of interbreeding to produce fertile offspring, reproductively isolated from other groups | Reproductive compatibility | Cannot apply to asexual organisms; ring species and hybrid zones are problematic |
| Morphological species concept | A group of organisms sharing a set of observable structural characteristics (morphological traits) that distinguish them from other groups | Physical appearance | Convergent evolution can make unrelated species look similar; does not reflect evolutionary history |
| Ecological species concept | A group of organisms sharing the same ecological niche — the same role and set of resources in the environment | Ecological role / niche | Difficult to define niche boundaries precisely; two species can share very similar niches |
Do not confuse the biological species concept with asexual organisms — bacteria and many plants reproduce asexually, so the biological species concept cannot apply. In such cases, morphological or molecular criteria are used. The exam may ask you to identify which concept is most suitable for a given scenario.
Two populations of birds look identical (same morphology) but do not interbreed even when their ranges overlap. Which species concept would classify them as different species, while the morphological concept alone would classify them as the same species?
- (A) Ecological species concept only
- (B) Biological species concept
- (C) Morphological species concept
- (D) All three concepts equally
Molecular evidence for classification
Modern classification increasingly uses molecular evidence alongside morphological evidence to determine evolutionary relationships. The more similar the sequences, the more recently two species shared a common ancestor.
Direct comparison of nucleotide sequences in homologous genes. Species that are closely related share a high percentage of identical base sequences. Differences accumulate at a roughly constant rate over time (“molecular clock”), allowing estimation of when lineages diverged.
Method: extract DNA from both species, sequence the same gene, align sequences, count percentage differences. Lower percentage difference = more closely related.
Comparison of the amino acid sequences of homologous proteins (e.g. cytochrome c, haemoglobin). Since amino acid sequence is determined by DNA sequence, closely related species have more similar protein sequences.
Advantage over morphology: proteins reflect genotype directly; convergent evolution of body shape does not affect protein sequence in the same way.
- Morphological: based on observable structural features; accessible without lab equipment; but can be misleading (convergent evolution, sexual dimorphism)
- Molecular: based on DNA or protein sequences; more objective; reflects evolutionary history more accurately; but requires laboratory techniques
- When the two conflict, molecular evidence is generally considered more reliable for establishing true evolutionary relationships
Given a table of amino acid differences between species, identify the most closely related pair (fewest differences). Or given cytochrome c comparisons, place species on a phylogenetic tree in order of relatedness to a reference species.
Key principle: fewer differences in sequence = more recent common ancestor = more closely related.
Phylogenetic trees
A phylogenetic tree is a branching diagram showing the evolutionary relationships between species (or other taxa), inferred from molecular or morphological evidence. The branching points (nodes) represent common ancestors.
- Species sharing a more recent branching point (node) are more closely related than species whose lineages diverged earlier
- The length of branches may represent time or degree of change (check the scale/legend)
- A clade is any group consisting of an ancestor and all of its descendants
- Trees can be constructed from DNA sequences, amino acid sequences, or morphological characteristics — molecular data is generally preferred for accuracy
- Outgroup: a taxon outside the main group of interest, used to root the tree
① Given a table of sequence differences, draw or complete a phylogenetic tree. ② Identify which two species are most closely related from a tree. ③ State which type of evidence (molecular or morphological) is more reliable and explain why. ④ Explain why the binomial naming system is useful.
Ecology terms & biodiversity
Core ecology terminology
| Term | Definition | Example |
|---|---|---|
| Habitat | The place where an organism lives; defined by physical and chemical characteristics | An oak woodland; a rocky intertidal zone; a freshwater pond |
| Ecological niche | The role of an organism in its ecosystem — including what it eats, what eats it, its habitat, its activity patterns, and its tolerance ranges for abiotic factors. No two species can occupy exactly the same niche in the same area (competitive exclusion principle) | A great tit feeds on insects in tree canopy; a blue tit feeds on insects in tree tips — overlapping but distinct niches in the same woodland |
| Population | All the individuals of one species living in the same area at the same time, capable of interbreeding | All rabbits living on a particular heath |
| Community | All the populations of different species living in the same area at the same time — biotic components only | All plants, animals, fungi, and microorganisms in an oak woodland |
| Ecosystem | A community together with all its abiotic (non-living) environmental factors — both biotic and abiotic components, and the interactions between them | The oak woodland community plus soil, light, temperature, water, nutrients |
| Abiotic factors | Non-living physical and chemical components of the environment | Temperature, pH, salinity, light intensity, water availability, mineral content |
| Biotic factors | Living components of the environment; interactions between organisms | Predation, competition, symbiosis, parasitism, disease |
The habitat is where an organism lives — the address. The niche is what it does there — its role, diet, activity times, and relationships with other species. Two species can share the same habitat (e.g. both live in oak woodland) but must occupy different niches to coexist stably. If niches overlap completely, competitive exclusion occurs and one species outcompetes the other.
Biodiversity
Biodiversity has three dimensions, all of which can be tested:
The number and relative abundance of species in an area. High species diversity = many different species present AND roughly equal numbers of individuals across species. A community with 100 species each represented by 1 individual has higher species diversity than one with 3 species, two of which are dominant.
The range of alleles within a species or population. High genetic diversity = many different alleles at many loci. Low genetic diversity (e.g. in a bottlenecked or highly inbred population) reduces the population's ability to adapt to environmental change.
The range of different habitats and ecosystems within a region. High ecosystem diversity provides more niches for species and increases overall biodiversity at landscape scale.
Species richness
Species richness is simply the number of different species present in an area. It is the simplest measure of biodiversity. It does not account for the relative abundance of each species — a site with 10 species all in equal numbers is considered equally species-rich as a site where one species makes up 90% of individuals and 9 other species make up 10%.
Simpson's diversity index
Simpson's diversity index (D) accounts for both the number of species (richness) and the relative abundance of each species (evenness). It is a more informative measure of biodiversity than species richness alone:
D = 1 − [∑(n/N)²]
or equivalently: D = 1 − [∑n(n−1) / N(N−1)]
- n = number of individuals of one particular species
- N = total number of all individuals of all species
- ∑ = sum across all species
- D ranges from 0 to 1: D close to 1 = high diversity; D close to 0 = low diversity (one species dominates)
A grassland survey records three species:
| Species | n (individuals) | n/N | (n/N)² |
|---|---|---|---|
| Grass A | 40 | 40/80 = 0.500 | 0.2500 |
| Grass B | 25 | 25/80 = 0.313 | 0.0977 |
| Grass C | 15 | 15/80 = 0.188 | 0.0353 |
| Total | N = 80 | ∑ = 0.3830 |
D = 1 − 0.3830 = 0.617
Interpretation: a D value of 0.617 indicates moderate diversity. If one species dominated (e.g. 78 individuals of grass A, 1 each of B and C), ∑(n/N)² would approach 1 and D would be close to 0 — low diversity.
Two meadows both have 8 species (equal species richness). Meadow 1 has roughly equal numbers of each species (D ≈ 0.88). Meadow 2 has one dominant grass making up 95% of all individuals and 7 rare species each at 1% (D ≈ 0.11). Species richness gives the same answer (8) but Simpson's D correctly identifies Meadow 1 as more diverse. The evenness component matters for ecosystem function — a community where one species dominates is more vulnerable if that species is lost.
A pond is surveyed and four invertebrate species are found: Species A = 30 individuals, Species B = 20, Species C = 45, Species D = 5. Total N = 100.
Calculate Simpson's diversity index D. Show all working.
∑(n/N)²:
- A: (30/100)² = 0.09
- B: (20/100)² = 0.04
- C: (45/100)² = 0.2025
- D: (5/100)² = 0.0025
- ∑ = 0.09 + 0.04 + 0.2025 + 0.0025 = 0.335
D = 1 − 0.335 = 0.665
This indicates moderate to moderately high diversity. The dominant species (C, 45%) depresses D relative to a fully even distribution. If all four species had 25 individuals each: ∑(n/N)² = 4 × (0.25)² = 0.25; D = 0.75.
Conservation & climate change
Why conserve biodiversity?
Biodiversity underpins services that support human life: pollination of crop plants (bees, other insects), water purification (wetlands filter pollutants), carbon sequestration (forests absorb CO₂), soil formation (decomposers), flood regulation, and climate moderation. These services cannot easily be replaced by technology.
Biodiversity provides food crops, timber, medicines (many drugs derived from natural compounds: penicillin, aspirin precursors, anticancer compounds from plants), and ecotourism income. Wild relatives of crops contain alleles for disease resistance and stress tolerance vital for future crop breeding.
Diverse ecosystems are more resilient to disturbance — if one species is lost, others with overlapping roles can compensate. Low-diversity systems (monocultures, degraded habitats) are vulnerable to disease, invasive species, and climate shifts.
Many argue that species have intrinsic value regardless of their utility to humans. Future generations have a right to inherit a biologically diverse world. Many cultures derive identity, wellbeing, and spiritual value from natural environments.
In situ conservation
In situ conservation means protecting species in their natural habitat. It is the preferred approach because it preserves entire ecosystems and the evolutionary processes operating within them:
Areas designated for biodiversity protection; human activity is restricted. Effective if large enough (edge effects are minimised in large reserves), well-managed, and legally protected. Corridor habitats between isolated reserves allow species movement and gene flow, reducing inbreeding.
Strips of habitat connecting fragmented reserves. Allow individuals to disperse, find mates, and colonise new areas. Important for species with large home ranges (large mammals, migratory birds) and for maintaining gene flow between isolated populations.
Designated zones in the ocean where fishing, extraction, or other disturbance is restricted. Allow fish populations to recover, protect coral reefs and seagrass beds. Spillover effects can benefit adjacent unprotected areas.
Captive-bred or transferred individuals are released into restored habitats. Successful examples: wolves in Yellowstone (with knock-on trophic cascade effects), red kites in UK, beavers in Scotland. Requires habitat restoration and management of introduced individuals.
Ex situ conservation
Ex situ conservation means protecting species outside their natural habitat. It is a complementary strategy, particularly for critically endangered species where in situ protection is insufficient:
| Method | Description | Advantages | Limitations |
|---|---|---|---|
| Captive breeding | Breeding endangered animals in zoos or specialist facilities with genetic management to maintain diversity | Can prevent extinction; builds up numbers for reintroduction; detailed monitoring | Expensive; limited space; captive animals may lose wild behaviours; risk of inbreeding if founder population is small |
| Seed banks | Seeds of plant species stored at low temperature and humidity for long-term preservation (e.g. Svalbard Global Seed Vault) | Very cost-effective; can preserve genetic diversity of thousands of species; long storage life; insurance against extinction | Seeds must be periodically germinated and replaced; cannot preserve whole ecosystems or animal species; not useful for species with recalcitrant seeds |
| Botanic gardens | Living collections of plant species, including many rare or threatened taxa | Living specimens can be studied, propagated, and used for reintroduction; educational value | Limited space; plants adapted to garden conditions may not thrive on reintroduction; not effective for all plant types |
In situ is generally preferred because it preserves the full ecosystem context, evolutionary processes, ecological interactions, and genetic adaptation to local conditions. Ex situ is essential as a safety net when habitats are too degraded or threats too severe. In practice, effective conservation uses both: captive breeding + habitat restoration + reintroduction. The Svalbard Seed Vault is the largest insurance policy in biological conservation.
Sustainable resource use
Sustainable use of natural resources means using them at a rate that allows natural systems to replenish — meeting present needs without compromising future generations' ability to meet theirs (Brundtland definition). Examples:
- Sustainable fisheries: fishing quotas below the maximum sustainable yield; gear restrictions to reduce bycatch; closed seasons to allow spawning
- Sustainable forestry: selective logging; replanting; certification schemes (FSC) that verify responsible management
- Sustainable agriculture: crop rotation; integrated pest management; reducing fertiliser runoff
Climate change and ecosystems
Anthropogenic climate change is altering ecosystems globally. Key mechanisms and consequences:
Species track their optimal climate conditions as temperature rises, moving poleward or to higher altitudes. Mismatches occur when species that depend on each other shift at different rates (e.g. insect emergence timing no longer matches peak flower bloom — affects pollinators and plants).
Rising sea temperatures cause corals to expel their symbiotic zooxanthellae algae, turning white (bleaching). Without zooxanthellae, the coral loses its primary food source and dies if temperatures don't recover. Widespread bleaching events are increasing in frequency and severity (synoptic with Topic 1.3).
Increased atmospheric CO₂ dissolves in seawater forming carbonic acid — lowering pH. This dissolves calcium carbonate shells of corals, molluscs, and pteropods, threatening marine food webs (synoptic with Topic 1.4 carbon cycle).
Thermal expansion of water and melting ice sheets raise sea levels. Coastal and low-lying ecosystems (mangroves, salt marshes, coral islands) are threatened by inundation. Saltwater intrusion into freshwater systems alters community composition.
Warming of arctic regions thaws permafrost — releasing stored organic carbon as CO₂ and CH₄ (methane), creating a positive feedback loop amplifying warming. Alters tundra and boreal forest ecosystems (synoptic with Topic 1.4).
A species of bird is critically endangered due to habitat destruction. Conservation scientists propose both in situ and ex situ strategies.
(a) Describe ONE in situ conservation strategy that could benefit this species and explain why it would be effective. [3]
(b) Describe ONE ex situ conservation strategy and explain ONE limitation compared with in situ conservation. [3]
(c) State TWO reasons why high biodiversity is important to maintain. [2]
(a) In situ strategy [3 marks]
- Establish or expand a nature reserve protecting the bird's remaining habitat [1]
- This prevents further habitat destruction by restricting human activity in the area [1]
- The species can continue to behave naturally, interact with its prey and competitors, and maintain wild-type behaviour; evolutionary processes continue; no costly infrastructure needed to sustain the population [1]
(b) Ex situ strategy + limitation [3 marks]
- Captive breeding programme in a zoo: endangered individuals are collected and bred in a controlled environment; numbers are built up for potential reintroduction [1]
- Limitation: captive individuals may lose wild foraging, predator avoidance, or social behaviours; reintroduced birds may not survive in the wild without extensive post-release support [1]
- Additional: does not address the original cause (habitat destruction); expensive; limited space constrains population size [1]
(c) Two reasons for biodiversity [2 marks]
- Ecosystem services: biodiversity underpins pollination, water purification, carbon sequestration, and other services on which human life depends [1]
- Ecological resilience: diverse ecosystems are more resistant to disturbance; loss of one species has less impact if others can fill its role [1]
- Medical/economic value: wild species provide sources of new medicines and genetic diversity for crop improvement [1]
Nitrogen cycle & human impacts
The nitrogen cycle is fully covered in the AS Part (Topic 5 of the A level is the equivalent of Topic 1.5 APES context, but for 9700 AS it maps to Topic 14 of the original CIE structure). For the A Level ecology context, Topic 18.3 focuses specifically on human disruption of nitrogen cycling and its ecosystem consequences. The underlying cycle processes are:
| Process | Transformation | Organisms / agent |
|---|---|---|
| Nitrogen fixation | N₂ → NH₃ | Rhizobium (legume root nodules); Azotobacter; Haber-Bosch process (industrial) |
| Nitrification | NH₃ → NO₂⁻ → NO₃⁻ | Nitrosomonas, Nitrobacter (aerobic soil bacteria) |
| Assimilation | NO₃⁻ → organic N | Plants; then consumers |
| Ammonification | Organic N → NH₃/NH₄⁺ | Decomposer bacteria and fungi |
| Denitrification | NO₃⁻ → N₂ | Anaerobic bacteria (e.g. Pseudomonas); waterlogged soils |
Human impacts on the nitrogen cycle
The Haber-Bosch process produces synthetic nitrogen fertilisers, massively increasing reactive nitrogen entering ecosystems. Excess nitrates leach from agricultural land into waterways (leaching).
Eutrophication chain (must know every step):
- Excess NO₃⁻ and PO₄³⁻ from fertiliser runoff enter the water body
- Algal bloom: rapid growth of algae and cyanobacteria covers the water surface
- Light blocked from submerged plants → submerged macrophytes die
- Algae die (bloom crashes)
- Decomposer bacteria multiply and respire aerobically, consuming dissolved O₂
- Hypoxia / anoxia: dissolved O₂ falls below the threshold for aerobic life
- Fish and invertebrate kill; aquatic biodiversity collapses
- Dead zone established
Burning fossil fuels and biomass releases nitrogen oxides (NOx) into the atmosphere. In the atmosphere, NOx reacts with water vapour to form nitric acid (HNO₃), which dissolves in rainwater — producing acid rain.
Effects of acid rain on ecosystems:
- Acidification of lakes and rivers → reduced biodiversity; aluminium ions mobilised from soil, toxic to fish gills
- Leaching of mineral nutrients (Ca²⁺, Mg²⁺) from soils → reduced soil fertility
- Direct damage to leaves and tree bark → forest dieback
Clearing and burning forests releases nitrogen stored in biomass and soil organic matter back into the atmosphere and waterways. Reduces biological nitrogen fixation in root-nodule associations. Exposes mineral soil to erosion, further reducing the capacity to retain and cycle nitrogen.
Animal waste (urine and faeces) and decomposition of slurry release large quantities of ammonia (NH₃) into the atmosphere. Ammonia can be deposited on ecosystems far from the source (nitrogen deposition), fertilising sensitive habitats and causing nitrogen-tolerant species to outcompete specialist low-nitrogen species — reducing biodiversity of heaths and moorlands.
Reducing the impact of agricultural nitrogen
- Precision agriculture: apply only the amount of fertiliser the crop can absorb, at the right time; reduces excess runoff
- Buffer strips: plant strips of vegetation along waterway banks to absorb leaching nitrates before they reach the water
- Constructed wetlands: denitrifying bacteria in wetland soils remove NO₃⁻ from agricultural water before it reaches rivers
- Crop rotation with legumes: nitrogen-fixing legumes restore soil N naturally, reducing synthetic fertiliser demand
- Slurry management: store and apply animal waste at agronomically appropriate times; cover stores to reduce ammonia volatilisation
Topic 18 Practice — Comprehensive
Mixed practice across ecology, biodiversity, conservation, and nitrogen cycling.
Two species of finch live in the same island habitat. Species X feeds on small hard seeds in the low scrub; Species Y feeds on insects in the tree canopy. Which correctly describes their ecological relationship?
- A. They occupy the same habitat and the same niche; competitive exclusion will occur
- B. They occupy the same habitat but different niches; they can coexist without competitive exclusion
- C. They occupy different habitats because they have different niches
- D. They must compete because they share the same island
Two woodland sites are surveyed for tree species:
Site 1: Oak = 50, Ash = 45, Birch = 5 (N = 100)
Site 2: Oak = 35, Ash = 33, Birch = 32 (N = 100)
(a) Calculate D for each site (show working). [4]
(b) Explain which site has higher biodiversity and why Simpson's D is a better measure than species richness alone for comparing these sites. [2]
(a) Calculations [4 marks]
Site 1:
- ∑(n/N)² = (50/100)² + (45/100)² + (5/100)² = 0.25 + 0.2025 + 0.0025 = 0.455 [2]
- D₁ = 1 − 0.455 = 0.545
Site 2:
- ∑(n/N)² = (35/100)² + (33/100)² + (32/100)² = 0.1225 + 0.1089 + 0.1024 = 0.3338 [2]
- D₂ = 1 − 0.3338 = 0.666
(b) Interpretation [2 marks]
- Site 2 has higher biodiversity (D = 0.666 vs 0.545); although both sites have the same species richness (3 species), Site 2 has more even distribution of individuals across species [1]
- Species richness alone gives the same value (3) for both sites and cannot distinguish them; Simpson's D captures evenness as well as richness, giving a more accurate picture of community diversity [1]
A farmer applies excess nitrogen fertiliser near a river. Explain the sequence of events that leads to the death of fish in the river, using your knowledge of the nitrogen cycle and decomposition. [7]
- Excess nitrate (NO₃⁻) from fertiliser leaches from the soil and enters the river via surface runoff or drainage [1]
- Nitrate acts as a limiting nutrient; its increase causes rapid proliferation of algae and cyanobacteria — algal bloom [1]
- The algal bloom covers the water surface, blocking light from reaching submerged aquatic plants [1]
- Submerged plants cannot photosynthesize and die; they no longer produce oxygen or provide habitat [1]
- The algal bloom eventually dies (the bloom crashes as nutrients become limiting again) [1]
- Decomposer bacteria (aerobic) multiply rapidly and break down the dead algal and plant material [1]
- Aerobic decomposition consumes dissolved oxygen (O₂) from the water rapidly [1]
- Dissolved O₂ falls below the threshold for aerobic organisms — hypoxia or anoxia develops [1]
- Fish and other aerobic aquatic organisms suffocate and die (fish kill); a “dead zone” is established [1]
Seed banks are used to conserve plant biodiversity.
(a) Explain how a seed bank conserves genetic diversity. [2]
(b) State TWO limitations of seed banks as a conservation strategy. [2]
(c) Suggest why maintaining genetic diversity within a species is important for its long-term survival. [2]
(a) Seed banks and genetic diversity [2 marks]
- Seeds of many different varieties and wild relatives of a species are stored; each seed carries a different combination of alleles (different genotype) [1]
- The collection therefore preserves a wide range of alleles from diverse populations and geographical origins, maintaining the genetic diversity of the species even if wild populations decline or habitats are destroyed [1]
(b) Two limitations [2 marks]
- Seeds must be periodically germinated and regenerated; if not, viability declines and stored alleles are lost [1]
- Some species have recalcitrant seeds (cannot survive drying and freezing); seed banks cannot be used for these [1]
- Seed banks preserve genetic material but not the species’ ecological interactions, behaviours, or evolutionary relationships [1]
(c) Why genetic diversity important for survival [2 marks]
- A genetically diverse population contains many different alleles; if the environment changes (new disease, climate shift), at least some individuals are likely to carry alleles conferring resistance or tolerance, allowing the population to survive [1]
- Low genetic diversity (e.g. after a bottleneck) means all individuals are similar; a single disease or environmental change could kill all individuals; inbreeding depression further reduces fitness [1]
Topic 18 — Common Mistakes
- 🌍Confusing habitat and nicheHabitat = where the organism lives (the address). Niche = what it does there (role, diet, activity times, interactions). Two species can share a habitat but not a niche. Many students write "niche" when they mean "habitat" or vice versa — examiners look for precision.
- 📊Using D close to 0 as "high diversity" in Simpson's indexIn Simpson's index D = 1 − ∑(n/N)²: D close to 1 = high diversity; D close to 0 = low diversity (one species dominates). This is the opposite of some other indices. Always state the interpretation alongside the value.
- 🧮Stopping the eutrophication chain too earlyThe chain must continue all the way to O₂ depletion and fish kill. Stopping at "algal bloom" or "algae block light" loses most of the available marks. The key mechanism is decomposer bacteria consuming dissolved O₂ — this is what kills the fish, not the algae or lack of light directly.
- 🔎Saying species richness is the same as biodiversitySpecies richness is only one component of biodiversity (it counts species without considering relative abundance). Biodiversity also includes evenness (captured by Simpson's D), genetic diversity, and ecosystem diversity. Two communities can have the same species richness but very different biodiversities.
- 📄Saying ex situ conservation replaces in situ conservationEx situ is a safety net, not a replacement. In situ is generally preferred because it preserves evolutionary processes, ecological interactions, and adaptation to natural environments. Captive animals often lose natural behaviours and have difficulty surviving when reintroduced. The ideal is both strategies working together.
- 🌿Confusing nitrogen fixation and nitrificationNitrogen fixation: N₂ gas (from atmosphere) → NH₃ (bringing nitrogen INTO the biological cycle). Nitrification: NH₃ → NO₂⁻ → NO₃⁻ (conversion within the soil). Both are bacterial processes but with completely different inputs and outputs. Fixation requires Rhizobium/Azotobacter; nitrification requires Nitrosomonas/Nitrobacter.
- 🔥Acid rain — saying it is caused only by SO₂Acid rain is caused by both sulphur dioxide (SO₂) and nitrogen oxides (NOx), both from combustion. In the context of nitrogen cycling, NOx from combustion reacts with water to form nitric acid (HNO₃). Don't omit the nitrogen-based contribution to acid rain.
- 🏭Saying seed banks preserve species "in the wild"Seed banks are ex situ conservation — seeds are stored outside the natural habitat. In situ conservation preserves species in the wild. Seed banks are a complement to in situ protection, not a substitute. They preserve genetic material but not the ecological context or evolutionary processes of wild populations.
Topic 18 covers classification, biodiversity and conservation — the three pillars of how biology understands the diversity of life and our responsibility to protect it. It connects ecology to all previous topics. Highest-yield items: habitat vs niche precise distinction, Simpson's D formula and calculation (D = 1 − ∑(n/N)²; D near 1 = high diversity), in situ vs ex situ comparison table, eutrophication complete 8-step chain (fertiliser runoff → algal bloom → submerged plants die → algae die → decomposers consume O₂ → hypoxia → fish kill), acid rain from NO₃ + SO₂ combustion products, 5 nitrogen cycle processes with organisms, why genetic diversity matters for survival (adaptation to change), seed bank limitations, climate change effects (range shifts / coral bleaching / ocean acidification / permafrost feedback). Synoptic links: Topic 1.4 (carbon cycle / permafrost), Topic 1.5 (nitrogen cycle processes), Topic 17 (selection on genetic variation), Topic 18 (genetic screening preserves diversity information).