IB DP Environmental Systems & Societies — Topic 3: Biodiversity & Conservation
Biodiversity
& Evolution
Subtopic 3.1 — New Syllabus (First Assessment 2026)
Standard Level + Higher Level
9 SL Points
3.1.1 – 3.1.9
10 HL Extensions
3.1.10 – 3.1.19
Overview
What You Need to Know
This subtopic defines biodiversity at multiple levels, explains how evolutionary processes generate and maintain it, introduces quantitative measures, and examines the role of human activity in both reducing and reshaping biodiversity across geological time.
A. Biodiversity & Its Importance
Levels of diversity, resilience, ecosystem complexity
Levels of diversity, resilience, ecosystem complexity
B. Evolution & Speciation
Natural selection, mechanisms of evolution, speciation
Natural selection, mechanisms of evolution, speciation
C. Measuring Biodiversity
Richness, evenness, Simpson's reciprocal index
Richness, evenness, Simpson's reciprocal index
D. Human Impacts & Deep Time
Artificial selection, geological epochs, mass extinctions
Artificial selection, geological epochs, mass extinctions
Guiding Questions
- How can diversity be explained and quantified, and why is this important?
- How does the unsustainable use of natural resources impact biodiversity?
Time allocation: minimum 3 hours SL + additional 5–6 hours HL. First subtopic in Topic 3: Biodiversity & Conservation.
3.1.1
Levels of Biodiversity
Key UnderstandingBiodiversity is the total diversity of living systems and it exists at several levels.
🧬 Genetic
Diversity
Diversity
→
🌿 Species
Diversity
Diversity
→
🌍 Habitat / Ecosystem
Diversity
Diversity
| Level | Definition | Example |
|---|---|---|
| Genetic diversity | Variation of genes within individuals and populations of a species | Cheetahs: low genetic diversity → vulnerable to disease. Coral reefs: high genetic diversity → resilient to temperature changes |
| Species diversity | Number of species (richness) and relative abundance (evenness) in a community | Tropical rainforest: high richness (50% of world's species on 7% of land). Temperate forest: lower richness but critical for ecosystem services |
| Habitat diversity | Variety of physical environments (forests, wetlands, deserts, reefs) in a region | Estuaries: transition between marine and freshwater → many niches → high biodiversity |
The three levels are nested — genetic diversity exists within species, species diversity exists within habitats. Loss at any level weakens the entire system.
3.1.2
Biodiversity & Ecosystem Resilience
Key UnderstandingThe components of diversity contribute to the resilience of ecological systems.
Ecosystem resilience — the ability to absorb disturbance and return to equilibrium — depends on biodiversity at all three levels.
| Diversity Level | How It Builds Resilience |
|---|---|
| Genetic | More genetic variation → more chance some individuals survive new threats (disease, climate shift) |
| Species | More species → functional redundancy → if one is lost, others fill its ecological role |
| Habitat | More habitat types → more refugia during disturbance → faster recovery from fire, flood, drought |
Complexity, Feedback & Succession
- Complex food webs: More connections → more alternative energy pathways → greater stability
- Negative feedback loops: Diverse ecosystems have more mechanisms to restore balance after disturbance
- Succession stage: Climax communities (high diversity) are more resilient than pioneer communities (low diversity)
Case study: Wolves reintroduced to Yellowstone → trophic cascade restored ecosystem balance. Sea otters protect kelp forests by controlling sea urchins — a keystone species maintaining resilience.
3.1.3
Evolutionary Processes
Key UnderstandingBiodiversity arises from evolutionary processes.
| Process | Mechanism | Effect on Diversity |
|---|---|---|
| Mutation | Random changes in DNA; most neutral or harmful, some beneficial | Introduces new alleles — raw material for evolution |
| Gene flow | Movement of genes between populations via migration & interbreeding | Increases genetic diversity; reduces differences between populations |
| Genetic drift | Random changes in gene frequencies, especially in small populations | Can reduce diversity; traits may spread or disappear by chance |
| Natural selection | Individuals with advantageous traits survive & reproduce more | Populations adapt to environments; drives speciation over time |
🎲 Mutation
→
🔀 Gene flow + Drift
→
⚔️ Natural selection
→
🌍 Biodiversity
Key distinction: Genetic drift is random; natural selection is directional. Drift can fix harmful alleles in small populations; selection tends to remove them.
3.1.4
Natural Selection
Key UnderstandingNatural selection is the mechanism driving evolutionary change.
Organisms better adapted to their environment survive, flourish, and reproduce more successfully than those less well adapted — "survival of the fittest."
Case Study: The Peppered Moth (Biston betularia)
- Pre-Industrial Revolution: Light-colored moths (typica) dominant — camouflaged against lichen-covered bark. Dark form (carbonaria) rare.
- During Industrial Revolution: Soot darkened tree bark → light moths visible to predators → dark moths favored → population shifted toward carbonaria (industrial melanism)
- Post–Clean Air Acts: Lichen regrew → bark lightened → light moths regained advantage → typica increased again
- Key lesson: When the environment changes, the direction of selection reverses — evidence that natural selection is driven by current conditions.
3.1.5
Mechanisms of Natural Selection
Key UnderstandingEvolution by natural selection involves variation, overproduction, competition for limited resources, and differences in adaptation that affect rates of survival and reproduction.
| Step | What Happens | Consequence |
|---|---|---|
| 1. Variation | Individuals differ in traits (size, color, behavior) due to genetic diversity | Provides raw material for selection |
| 2. Overproduction | Species produce more offspring than can survive | Leads to competition for limited resources |
| 3. Competition | Individuals with advantageous traits are more likely to survive | Differential survival based on fitness |
| 4. Differential reproduction | Survivors reproduce more → pass on advantageous traits | Frequency of beneficial alleles increases |
| 5. Heritability | Advantageous traits are inherited by offspring | Population evolves over generations |
Example — beetles: In a population of beetles on dark tree bark, dark-colored individuals are better camouflaged from birds → survive more → reproduce more → over generations, the population shifts toward dark coloration.
Exam tip: All five steps are required for a complete explanation of natural selection. Omitting heritability or variation loses marks.
3.1.6
Speciation
Key UnderstandingSpeciation is the generation of new species through evolution.
Speciation occurs when populations of a single species become reproductively isolated, evolve separately, and develop enough genetic divergence that they can no longer interbreed.
Case Study: Darwin's Finches (Galápagos)
A single immigrant finch species colonized the Galápagos. Geographic isolation on different islands → different food sources → different beak shapes evolved → 13+ distinct species, each adapted to a specific niche. An example of adaptive radiation.
3.1.7
Species Richness & Evenness
Key UnderstandingSpecies diversity in communities is a product of richness and evenness.
| Component | Definition | What It Measures |
|---|---|---|
| Species richness | The total number of different species in an area | How many species are present (not how common they are) |
| Species evenness | The relative abundance of each species | How evenly individuals are distributed among species |
Why Both Matter
- Two ecosystems can have the same richness (e.g. 20 species) but very different evenness
- An ecosystem where one species dominates (low evenness) is less resilient than one with balanced abundances
- High richness + high evenness = maximum diversity = greatest stability and resilience
Exam skill: Given a data table, always calculate or compare both richness and evenness — never assume high richness = high diversity.
3.1.8
Simpson's Reciprocal Index
Key UnderstandingSimpson's reciprocal index is used to provide a quantitative measure of species diversity, allowing different ecosystems to be compared and monitored over time.
Simpson's Diversity Index Formula
D = Σ(n/N)² | Reciprocal Index = 1/D
n = number of individuals of each species | N = total number of individuals of all species
Higher 1/D value = greater species diversity
Worked Example
A quadrat sample contains: Species A (40), Species B (35), Species C (25). Total N = 100.
D = (40/100)² + (35/100)² + (25/100)² = 0.16 + 0.1225 + 0.0625 = 0.345
1/D = 1/0.345 ≈ 2.90 → moderate diversity
Skill: You must be able to calculate Simpson's index from a data table. Common error: forgetting to square the fractions (n/N)².
3.1.9
Biodiversity Knowledge & Conservation
Key UnderstandingKnowledge of global and regional biodiversity is needed for the development of effective management strategies to conserve biodiversity.
| How Knowledge Is Gathered | Method |
|---|---|
| Citizen science | Public volunteers record species sightings, monitor ecosystems (e.g. bird counts, iNaturalist) |
| Government & NGOs | Remote sensing, field surveys, species monitoring programs (e.g. IUCN Red List assessments) |
| Indigenous & parabiologists | Local ecological knowledge combined with scientific methods; trained community members collect data |
Conservation Strategies Informed by Knowledge
| Strategy | How Knowledge Helps | Example |
|---|---|---|
| Protected areas | Data identifies regions of high ecological value → designated as national parks | Amazon reserves; Great Barrier Reef Marine Park |
| Sustainable management | Biodiversity data informs fishing quotas, forestry practices, water management | MSC-certified sustainable fisheries |
| Species recovery programs | Population & genetic data guides captive breeding and habitat restoration | California condor; black-footed ferret recovery |
Case study: Gorongosa National Park (Mozambique) — collaboration between government and Carr Foundation uses biodiversity monitoring to guide restoration of a war-damaged ecosystem.
HL — 3.1.10
Mutation & Sexual ReproductionHL
Key UnderstandingMutation and sexual reproduction increase genetic diversity.
Mutation
- Random changes in DNA sequence — occur spontaneously during replication or from mutagens
- Beneficial: Improve survival/reproduction → spread over generations
- Neutral / Harmful: Add to gene pool or eliminated by selection
- Ultimate source of all new genetic variation
Sexual Reproduction
- Meiosis: Chromosomes exchange material (recombination) → unique gametes
- Fertilization: Fusion of gametes from 2 parents → unique genotype
- Ensures every individual (except identical twins) is genetically unique
Populations with high genetic diversity are better equipped to survive environmental change. Low diversity (e.g. agricultural monocultures) → vulnerable to extinction.
HL — 3.1.11
Reproductive IsolationHL
Key UnderstandingReproductive isolation can be achieved by geographical separation or, for populations living in the same area, by ecological or behavioural differences.
| Type | Mechanism | Example |
|---|---|---|
| Allopatric (geographic) | Physical barrier (mountain, river, ocean) prevents gene flow between populations | Bonobos & chimpanzees: Separated by the Congo River → evolved independently → different social structures and genetics |
| Sympatric (same area) | No physical barrier; isolation by ecological niche or behavioral differences | Apple maggot fly: Shifted to apple trees → different fruiting times → reproductive isolation within generations |
| Parapatric (adjacent) | Continuous range but individuals mate locally; environmental gradient drives divergence | Grass species along a mine toxicity gradient — tolerant and non-tolerant populations diverge |
Island Endemism
- Isolated islands have high endemism (species found nowhere else)
- Galápagos finches: 13+ species from one ancestor
- Madagascar lemurs: ~100 species, all endemic — evolved in isolation for ~60 million years
HL — 3.1.12
Uneven Distribution of BiodiversityHL
Key UnderstandingBiodiversity is spread unevenly across the planet, and certain areas contain a particularly large proportion of species, especially species that are rare and endangered.
| Factor | Effect |
|---|---|
| Latitude | Tropical regions (0°–23.5°) have higher species diversity due to stable temperatures and high rainfall |
| Area | Larger habitats support more species (species-area relationship) |
| Evolutionary time | Tropical regions have been climatically stable for longer → more time for speciation |
| Productivity | Higher NPP in tropics → more energy → supports more complex food webs |
Biodiversity Hotspots
- Areas with exceptionally high species richness and endemism under significant threat
- Tropical rainforests: ~50% of world's species on ~7% of Earth's surface
- Coral reefs: "Rainforests of the sea" — structural complexity supports vast marine diversity
- 36 recognized hotspots globally (Conservation International criteria)
HL — 3.1.13
Human-Driven Evolutionary ChangeHL
Key UnderstandingHuman activities have impacted the selective forces acting on species within ecosystems, resulting in evolutionary change.
| Human Activity | Selective Pressure | Evolutionary Response |
|---|---|---|
| Hunting / poaching | Targeting individuals with specific traits (e.g. large tusks) | Trait reduction or loss in population |
| Urbanization | New hazards (traffic), altered food sources, novel habitats | Behavioral adaptation (road-crossing, diet shifts) |
| Agriculture | Pesticide use, habitat simplification | Pest resistance to pesticides |
| Climate change | Shifting temperature and precipitation patterns | Range shifts, phenological changes |
Case Study: Tuskless Elephants — Gorongosa, Mozambique
- During civil war poaching, ~90% of elephants were killed → poachers targeted tusked elephants
- Tuskless elephants had survival advantage → tuskless gene frequency increased dramatically
- Demonstrates natural selection driven directly by human selective pressure
HL — 3.1.14
Artificial SelectionHL
Key UnderstandingArtificial selection reduces genetic diversity and, consequently, species resilience.
Humans deliberately breed organisms for desired traits — a process called artificial selection. While commercially useful, it drastically narrows the gene pool.
| Example | What Was Selected | Cost of Reduced Diversity |
|---|---|---|
| Cavendish banana | Size, taste, seedlessness | Genetically identical clones — Panama disease (TR4) threatens global production |
| Commercial wheat | High yield, uniform growth | Vulnerable to new fungal strains; relies on chemical inputs |
| Domestic dog breeds | Appearance, temperament | Inherited health problems (e.g. breathing in bulldogs, hip dysplasia) |
Contrast with natural selection: Natural selection maintains or increases genetic diversity. Artificial selection decreases it by imposing a single, narrow set of criteria.
HL — 3.1.15
Deep TimeHL
Key UnderstandingEarth history extends over a period of 4.5 billion years. Processes that occur over an extended timescale have led to the evolution of life on Earth.
The Scale of Earth History
- 4.5 billion years since Earth formed
- ~3.8 billion years since first life (prokaryotes)
- ~540 million years since the Cambrian Explosion (rapid diversification)
- ~66 million years since the end-Cretaceous mass extinction (dinosaurs extinct)
- ~300,000 years since Homo sapiens appeared — a fraction of a second in geological time
The processes that built Earth's biodiversity took billions of years. Human impacts are altering biodiversity in decades — orders of magnitude faster than natural evolution can respond.
HL — 3.1.16
Geological EpochsHL
Key UnderstandingEarth history is divided up into geological epochs according to the fossil record.
Precambrian
4500–541 Ma
4500–541 Ma
→
Paleozoic
541–252 Ma
541–252 Ma
→
Mesozoic
252–66 Ma
252–66 Ma
→
Cenozoic
66 Ma–present
66 Ma–present
How epochs are defined: Geologists use boundary markers in rock layers — sudden changes in fossil assemblages, chemical signatures (e.g. iridium layer at the K–Pg boundary), or isotopic ratios. The fossil record is the primary archive of Earth's biological history.
HL — 3.1.17
Mass Extinctions & SpeciationHL
Key UnderstandingMass extinctions are followed by rapid rates of speciation due to increased niche availability.
The "Big Five" Mass Extinctions
| Event | When (Ma) | Species Lost | Likely Cause |
|---|---|---|---|
| End-Ordovician | ~443 | ~85% | Glaciation, sea-level drop |
| Late Devonian | ~372 | ~75% | Ocean anoxia, volcanism |
| End-Permian | ~252 | ~96% | Volcanism (Siberian Traps), climate change |
| End-Triassic | ~201 | ~80% | Volcanism (CAMP), climate change |
| End-Cretaceous | ~66 | ~76% | Asteroid impact + Deccan Traps volcanism |
💀 Mass extinction
→
🌿 Empty niches
→
🆕 Rapid speciation
After the end-Cretaceous extinction: Non-avian dinosaurs disappeared → opened niches for mammals → explosive diversification of mammals. Extinction creates opportunity.
HL — 3.1.18
The AnthropoceneHL
Key UnderstandingThe Anthropocene is a proposed geological epoch characterized by rapid environmental change and species extinction due to human activity.
Evidence for the Anthropocene
| Evidence Type | What It Shows |
|---|---|
| Extinction rate | Current extinction rate is 100–1,000× background rate |
| Nuclear signatures | Global fallout from nuclear weapons testing (1945–1963) leaves detectable marker |
| Plastics & concrete | Novel materials now found in sediment layers worldwide |
| CO₂ levels | Atmospheric CO₂ higher than at any point in 800,000+ years |
| Land use | ~75% of ice-free land significantly altered by human activity |
HL — 3.1.19
Planetary-Scale Human ImpactsHL
Key UnderstandingHuman impacts are having a planetary effect, which will be detectable in the geological record.
| Human Impact | Geological Signature | Persistence |
|---|---|---|
| CO₂ emissions | Carbon isotope shift (δ¹³C) in sediments; ocean acidification layers | 100,000+ years |
| Plastic pollution | Plastic fragments in sedimentary layers worldwide | Millions of years |
| Nuclear fallout | Radioactive isotopes (Pu-239) in global sediment layers | Tens of thousands of years |
| Species extinction | Sudden disappearance of species in fossil record | Permanent |
Exam tip: Connect ecological effects (biodiversity loss, climate change) to their geological signatures to show understanding at the systems level.
Key Terms Glossary
Essential Vocabulary
| Term | Definition |
|---|---|
| Biodiversity | The total diversity of living systems — genetic, species, and habitat levels |
| Genetic diversity | Variation of genes within individuals and populations of a species |
| Species diversity | The number of species (richness) and their relative abundance (evenness) |
| Habitat diversity | The variety of physical environments in a region |
| Resilience | An ecosystem's ability to absorb disturbance and recover to equilibrium |
| Natural selection | Individuals with advantageous traits survive and reproduce more |
| Speciation | Generation of new species through evolution and reproductive isolation |
| Species richness | The number of different species in an area |
| Species evenness | The relative abundance of each species in a community |
| Simpson's index (1/D) | Quantitative measure of species diversity; higher = more diverse |
| Anthropocene | Proposed geological epoch defined by human-dominated environmental change |
Exam Preparation
Key Takeaways & Exam Tips
Must-Know Relationships
- Genetic → Species → Habitat diversity: Nested levels; loss at any level weakens the system
- Diversity → Resilience: More species + more genetic variation = more functional redundancy
- 5 steps of natural selection: Variation, overproduction, competition, differential reproduction, heritability
- Isolation → Divergence → Reproductive isolation = speciation
- Richness + Evenness = Species diversity — Simpson's index quantifies both
For 9-mark essays: Always link biodiversity to both ecosystem services and evolutionary processes. Use case studies (peppered moths, Gorongosa elephants, Galápagos finches).
You've covered all 19 syllabus points ✅
3.1.1 – 3.1.9 (SL) + 3.1.10 – 3.1.19 (HL)