IB DP Environmental Systems and Societies – Biodiversity, human impacts and conservation Revision Notes

These are condensed revision notes. For full explanations and longer worked examples, read the biodiversity and conservation study guide first.

The notes cover Topic 3, Biodiversity and conservation, of IB Diploma Programme Environmental Systems and Societies. They are aligned to the IB Diploma Programme Subject Brief, Environmental systems and societies, and cover syllabus sections 3.1–3.3: biodiversity and evolution, human impact on biodiversity, and conservation and regeneration. The topic is common to SL and HL (13 hours SL, 26 hours HL), and HL students study some topics in extra depth. They follow the IB ESS subject brief for first assessment 2026 — the course examined in the May and November 2026, 2027 and 2028 sessions.

The brief gives Topic 3 Biodiversity and conservation 13 teaching hours at SL and 26 at HL. It does not list the subtopics or learning outcomes for this topic, so the numbered subtopics and outcomes on this page follow the syllabus numbering used in the printable ESS checklist, not the brief itself.

When you can recall everything here, test yourself with the biodiversity and conservation practice questions. The IB DP ESS course hub and the printable ESS syllabus checklist show where this unit sits. Topic 3 leans on niches and succession from the Topic 2 Ecology revision notes and on value systems from the Topic 1 Foundations revision notes. For paper formats, see the ESS assessment revision notes.

Definitions

• Biodiversity: the variety of life in an area, at three levels.
• Genetic diversity: the range of genes and alleles within a species or population.
• Species diversity: the number of species and their relative abundance.
• Habitat diversity: the range of habitats or ecosystems in an area.
• Species richness: the number of species present.
• Species evenness: how similar the population sizes of the species are.
• Species: organisms that can interbreed to produce fertile offspring.
• Speciation: the formation of new species, usually after populations become isolated.
• Mass extinction: loss of a large share of species over a geologically short time.
• Background extinction rate: the normal, slow rate of extinction between mass extinctions.
• Endemic species: a species found in only one area.
• Invasive species: a non-native species that spreads and harms native species or ecosystems.
• Edge effect: changed conditions near a habitat’s boundary (light, wind, drying, predators, invasive species).
• Wildlife corridor: a strip of habitat linking fragments so organisms can move between them.
• Rewilding: restoring natural processes so an ecosystem regulates itself with less human management, often with reintroductions.

Key formula and calculations

What	Formula or method	Notes
Simpson’s reciprocal index	D = N(N − 1) / Σn(n − 1)	N = total individuals, n = individuals of each species; higher D = more diverse; minimum 1
Percentage decline	(start − end) / start × 100	Use the start value as the base
Mean rate of decline	(start − end) / number of years	Give units, e.g. individuals per year
Core area of a square reserve	(side − 2 × edge depth)²	Edge depth comes off both sides
Core share	core area / total area × 100	Compare shapes with equal total area

Method in steps: Simpson’s reciprocal index

1. Add all counts to get N.
2. Work out N(N − 1).
3. For every species, work out n(n − 1). A species with n = 1 gives 0.
4. Add these to get Σn(n − 1).
5. Divide: D = N(N − 1) / Σn(n − 1).
6. Interpret: compare with the other site, then give a reason
   (evenness, dominance, disturbance, habitat variety).

Small reminder: counts of 5, 5, 5, 5 give N = 20, N(N − 1) = 380 and Σn(n − 1) = 4 × 20 = 80, so D = 380 / 80 = 4.75.

Method in steps: explaining speciation

1. Variation exists in the population.
2. A barrier (geographical) or behaviour/timing (reproductive) stops gene flow.
3. Different selection pressures act on each population.
4. Allele frequencies change separately over many generations.
5. The populations can no longer interbreed to give fertile offspring.

3.1 Biodiversity and evolution: must-know points

• Natural selection: variation → overproduction → competition → survival of better-suited individuals → inheritance of their alleles.
• Selection acts on existing variation. Organisms do not change because they “need” to.
• Plate tectonics: splitting land masses isolates populations; collisions build mountains and new habitats; land bridges allow spread and competition.
• Five major mass extinctions are recorded in the fossil record. Causes include volcanism, climate change, sea-level change and an asteroid impact (end-Cretaceous, about 66 million years ago).
• Many scientists argue that human activity is causing a sixth mass extinction.

3.2 Human impact: must-know points

Cause of loss	Mechanism in one line
Habitat loss and fragmentation	Less space, smaller populations, more edge, gene flow blocked
Overexploitation	Harvest rate exceeds the population’s rate of replacement
Invasive species	Compete with, prey on or infect natives that lack defences
Pollution	Toxins and nutrient enrichment harm organisms and alter habitats
Climate change	Conditions shift faster than species can move or adapt

Features of vulnerable species: small population, small range, low reproductive rate, specialised niche, high trophic level or large body, low genetic diversity, value to people.

Vulnerable ecosystems: tropical rainforest (high diversity, nutrients held in biomass, slow recovery), coral reefs, islands (many endemics).

IUCN Red List categories: Least Concern, Near Threatened, Vulnerable, Endangered, Critically Endangered, Extinct in the Wild, Extinct (plus Data Deficient). Criteria include population size, rate of decline, range size and fragmentation, number of mature individuals, and probability of extinction in the wild.

3.3 Conservation and regeneration: must-know points

Arguments for conservation: ecosystem services, direct use (food, medicine, genetic resources), economic (tourism), ethical, cultural and aesthetic values.

Approach	Strengths	Limitations
Captive breeding and reintroduction	Saves species close to extinction; public education	Expensive; small gene pool; habitat may still be damaged
Seed and gene banks	Stores genetic diversity cheaply and long term	No evolution in storage; ecosystem not protected
Legal protection and trade controls (e.g. CITES)	Tackles overexploitation across borders	Depends on enforcement
Protected areas	Protect whole communities and ecosystem services	Can displace local people; edge effects; need funding
Regeneration and rewilding	Restores processes and food webs	Slow; uncertain; possible conflict with land users

Reserve design reminders

• One large reserve usually gives more core area than several small ones of equal total area.
• Compact shapes have less edge per unit area than long, thin ones.
• Corridors link fragments; buffer zones protect the core.
• Several small reserves can spread risk (fire, disease) and cover more habitat types. This is the SLOSS debate.

Must-know distinctions

• Richness vs evenness: number of species vs how equally individuals are spread.
• Geographical vs reproductive isolation: a physical barrier vs a difference in behaviour, timing or anatomy in the same area.
• Keystone vs flagship vs umbrella species: large effect on the ecosystem vs public appeal vs large area needs that protect many others.
• Species-based vs habitat-based conservation: one target species vs a whole area and its community.
• Regeneration vs rewilding: active restoration of a degraded site vs restoring natural processes with minimal ongoing management.

Quick self-test

1. Name the three levels of biodiversity.
2. Two sites each hold 6 species. Why might their diversity index values differ?
3. Calculate Simpson’s reciprocal index for counts 17, 1, 1, 1.
4. What is the smallest possible value of Simpson’s reciprocal index, and when does it occur?
5. State two ways plate tectonics can increase biodiversity.
6. A population falls from 2400 to 600. Calculate the percentage decline.
7. Give three features that make a species vulnerable to extinction.
8. Why does cleared tropical rainforest recover slowly?
9. A square reserve has sides of 5 km and edge effects reach 1 km inside. Calculate the core area and core share.
10. Define an umbrella species.
11. Give one limitation of seed banks.
12. State one argument for several small reserves rather than one large one.

Answers

1. Genetic, species and habitat (ecosystem) diversity.
2. Evenness differs: one site may be dominated by a few species while the other has individuals spread more equally.
3. N = 20, N(N − 1) = 380; Σn(n − 1) = 272 + 0 + 0 + 0 = 272; D = 380 / 272 = 1.40 (3 s.f.). Low, because one species dominates.
4. 1, when only one species is present.
5. Separating land masses isolates populations (speciation); mountain building creates new habitats. (Land bridges allowing spread is also acceptable.)
6. (2400 − 600) / 2400 × 100 = 75%.
7. Any three: small population or range, low reproductive rate, specialised niche, high trophic level or large body, low genetic diversity, value to people.
8. Most nutrients are held in the biomass, not the soil, so removing the vegetation removes the nutrient store.
9. Core side = 5 − 2 × 1 = 3 km, so core area = 9 km²; core share = 9 / 25 = 36%.
10. A species that needs a large area of habitat, so protecting it also protects many other species there.
11. Stored seeds do not evolve with changing conditions, and the habitat is not protected. (Either.)
12. They spread risk: one fire or disease outbreak is less likely to affect all of them. (Or: they can cover more habitat types.)

Where marks are usually lost

• Writing “biodiversity = number of species”. That is only richness.
• Forgetting that a species with n = 1 contributes 0 to Σn(n − 1), or adding n instead of n(n − 1).
• Stating a higher index “means the site is healthier” without referring to evenness or richness.
• Describing speciation without saying that gene flow between the populations stops.
• Writing “the species adapted to survive” instead of describing selection of existing variation.
• Naming a cause of loss (e.g. “pollution”) without the mechanism or a named example.
• Taking the edge depth off only one side when calculating a reserve’s core area.
• Calling a keystone species “the top predator”. It is defined by its effect, not its trophic level.
• In “evaluate” or “to what extent” answers, giving only benefits of an approach with no limitation and no final judgement.

Official syllabus

International Baccalaureate Organization, Diploma Programme Subject Brief, Environmental systems and societies, first assessment 2026. The brief lists subtopics for Topic 1 only; the subtopic numbering on this page follows the printable ESS checklist.