IB DP Environmental Systems and Societies – Biodiversity, human impacts and conservation Study Guide

This study guide teaches Topic 3, Biodiversity and conservation, of IB Diploma Programme Environmental Systems and Societies (ESS). It is aligned to the IB Diploma Programme Subject Brief, Environmental systems and societies, and covers syllabus sections 3.1 (biodiversity and evolution), 3.2 (human impact on biodiversity) and 3.3 (conservation and regeneration). The topic is studied at both SL and HL: the brief gives it 13 teaching hours at SL and 26 at HL, so HL students study some topics in extra depth. It follows 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 have worked through this guide, move on to the biodiversity and conservation revision notes and then 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 in the course.

This topic builds directly on the Topic 2 Ecology study guide (niches, food webs, succession) and on the systems and sustainability ideas in the Topic 1 Foundations study guide. The ESS syllabus guide shows how Topic 3 fits the whole course, and the ESS exam preparation guide explains how the papers are set.

What this unit covers

Section	What you must be able to do	SL/HL
3.1 Biodiversity and evolution	Define biodiversity at three levels; separate richness from evenness; calculate and interpret a diversity index; explain natural selection, speciation and mass extinctions	SL and HL
3.2 Human impact on biodiversity	Explain the main human causes of biodiversity loss; explain why some species and ecosystems are more vulnerable; use Red List ideas to judge threat	SL and HL
3.3 Conservation and regeneration	Give arguments for conservation; compare species-based and habitat-based approaches; explain reserve design; evaluate regeneration and rewilding	SL and HL

The public subject brief gives Topic 3’s name and hours only; check your school’s copy of the full subject guide for the exact wording of each understanding.

3.1 Biodiversity and evolution

Three levels of biodiversity

Biodiversity is the variety of life in an area. You need three levels:

• Genetic diversity: the range of different alleles and genes within a species or population.
• Species diversity: the number of different species and how evenly individuals are spread among them.
• Habitat (ecosystem) diversity: the range of different habitats or ecosystems in an area.

The levels are linked: more habitats usually means more species.

Richness and evenness

• Species richness is the number of species present.
• Species evenness is how similar the population sizes of those species are.

Two sites can have the same richness but very different diversity. A site dominated by one species is less diverse than a site where the same species are present in similar numbers.

A diversity index

A diversity index combines richness and evenness in one number. One widely used form is Simpson’s reciprocal index:

D = N(N − 1) / Σn(n − 1)

where N is the total number of individuals of all species and n is the number of individuals of each species. A higher D means higher diversity. The smallest possible value is 1 (only one species present). The index is most useful for comparing similar sites sampled in the same way, not as an absolute measure.

Worked example: comparing two meadows

Two meadow plots are sampled with the same quadrat method. Each has five species and 40 individuals.

Species	Meadow P (n)	Meadow Q (n)
A	12	30
B	10	4
C	8	3
D	6	2
E	4	1

Meadow P
N = 40, so N(N − 1) = 40 × 39 = 1560
Σn(n − 1) = 12×11 + 10×9 + 8×7 + 6×5 + 4×3
          = 132 + 90 + 56 + 30 + 12 = 320
D = 1560 / 320 = 4.875 ≈ 4.88

Meadow Q
N(N − 1) = 1560
Σn(n − 1) = 30×29 + 4×3 + 3×2 + 2×1 + 1×0
          = 870 + 12 + 6 + 2 + 0 = 890
D = 1560 / 890 = 1.752... ≈ 1.75

Both meadows have the same richness (5). Meadow P scores higher because its individuals are spread more evenly. Meadow Q is dominated by species A, perhaps because heavy grazing or fertiliser run-off favours one competitive species. When you interpret an index, always say what it shows and suggest a reason.

Natural selection and speciation

Biodiversity is the product of evolution by natural selection:

1. Individuals in a population vary, and much of this variation is inherited.
2. More offspring are produced than the environment can support, so there is competition.
3. Individuals with variations better suited to the environment are more likely to survive and reproduce.
4. They pass on the helpful alleles, so allele frequencies change over generations.

A species is a group of organisms that can interbreed to produce fertile offspring. Speciation is the formation of new species. It usually needs isolation: two populations stop exchanging genes. Each population then faces different selection pressures and random changes. In time they become so different that they can no longer interbreed.

• Geographical isolation: a physical barrier (sea, mountain range, river) separates the populations.
• Reproductive isolation: populations in the same area stop interbreeding, for example because they breed at different times.

Plate tectonics affects speciation over long time scales. Moving plates split land masses and isolate populations. Collisions build mountains and create new habitats. Land bridges can join areas and let species spread and compete.

Mass extinctions

The fossil record shows five major mass extinctions. In each, a large share of species died out over a geologically short time. Causes include volcanic activity, climate change, changes in sea level and an asteroid impact (the end-Cretaceous event, about 66 million years ago). After each event, surviving groups diversified into the empty niches. Many scientists argue that human activity is now driving a sixth mass extinction, because present extinction rates are far above the background rate seen in the fossil record.

3.2 Human impact on biodiversity

Main causes of loss

Cause	How it reduces biodiversity	Example of the process
Habitat loss and fragmentation	Removes living space; small fragments hold small populations and more edge	Forest cleared for farmland and roads
Overexploitation	Harvesting faster than the population can replace itself	Overfishing; hunting for meat or trade
Invasive species	Out-compete, prey on or bring disease to native species with no evolved defence	A predator introduced to an island
Pollution	Toxins, nutrient enrichment and plastics harm organisms or change habitats	Pesticides reaching streams
Climate change	Shifts temperature and rainfall faster than species can move or adapt	Warming seas bleaching corals

Most real cases combine several causes. Fragmentation plus warming is especially harmful, because species cannot move through cleared land to cooler areas.

Why some species are more vulnerable

A species is more likely to become extinct if it has:

• a small population or a small geographical range
• a low reproductive rate or a long time to reach breeding age
• a specialised niche (narrow diet or habitat)
• a high trophic level or large body size, so it needs a large area
• low genetic diversity, so it cannot adapt to change
• value to people as food, medicine or trade goods.

Ecosystems differ in vulnerability too. Tropical rainforests hold very high biodiversity, but much of their nutrient store is in the living biomass rather than the soil. Once cleared, they recover slowly. Coral reefs and islands are also sensitive, because many species there are found nowhere else.

Judging threat: the IUCN Red List

The International Union for Conservation of Nature (IUCN) publishes the Red List of Threatened Species. It places species in categories: Least Concern, Near Threatened, Vulnerable, Endangered, Critically Endangered, Extinct in the Wild and Extinct (plus Data Deficient). The assessment uses criteria such as:

• population size and rate of decline
• size of the geographical range, and how fragmented it is
• the number of mature individuals
• the estimated probability of extinction in the wild.

A steep decline or a tiny range moves a species into a higher-threat category.

3.3 Conservation and regeneration

Why conserve biodiversity?

Arguments fall into groups. Your answer should use more than one.

• Ecosystem services: pollination, clean water, soil formation, flood control, climate regulation.
• Direct use: food, timber, medicines, genetic resources for crops.
• Economic: tourism and recreation.
• Ethical, cultural and aesthetic: many value systems hold that species have a right to exist, or have spiritual meaning.

Species-based conservation

Species-based approaches focus on one species at a time:

• Captive breeding and reintroduction in zoos or breeding centres.
• Seed banks and gene banks that store genetic material.
• Legal protection and trade controls, such as CITES (the Convention on International Trade in Endangered Species of Wild Fauna and Flora).

Some species are chosen for a reason:

• A flagship species is charismatic and draws public support and funding.
• A keystone species has an effect on its ecosystem far larger than its numbers suggest.
• An umbrella species needs a large area, so protecting it protects many other species in the same habitat.

The weakness is that a species returned to a damaged habitat will decline again. Small captive populations can also lose genetic diversity.

Habitat-based conservation and reserve design

Habitat-based approaches protect whole areas, such as national parks and marine protected areas. This keeps whole communities, food webs and ecosystem services working.

Design matters. Edge effects are changes near a reserve’s boundary: more light, wind, drying, noise, predators and invasive species. The core area away from the edge is the most valuable. In general:

• one large reserve holds more core area than several small ones of the same total area
• compact shapes (near-circular or square) have less edge per unit area than long thin ones
• wildlife corridors link fragments so animals can move, find mates and keep gene flow going
• buffer zones with limited human use surround the core.

Worked example: core area

A reserve is a 4 km × 4 km square. Edge effects reach 0.5 km inside the boundary. Compare it with four separate 2 km × 2 km squares.

One large reserve
Total area = 4 × 4 = 16 km²
Core side  = 4 − 2(0.5) = 3 km, so core area = 3 × 3 = 9 km²
Core share = 9 / 16 = 0.5625 → 56.25 %

Four small reserves
Total area = 4 × (2 × 2) = 16 km²
Core side  = 2 − 2(0.5) = 1 km, so core area = 4 × (1 × 1) = 4 km²
Core share = 4 / 16 = 0.25 → 25 %

The single reserve has more than twice the core area for the same land. Several small reserves are not always worse, though. They can spread risk from a single fire or disease outbreak and may cover more habitat types. That is the basis of the long-running “single large or several small” (SLOSS) debate.

Regeneration and rewilding

Regeneration (restoration) returns a degraded ecosystem towards a more natural state, for example by replanting native species or restoring wetland water levels. It uses the succession you met in Topic 2.

Rewilding aims to restore natural processes and let the ecosystem regulate itself, with less ongoing human management. It often reconnects habitats and reintroduces missing species, such as large herbivores or top predators. Benefits can include restored food webs and more diverse habitats. Problems include conflict with farmers, uncertain outcomes and the long time needed.

Common errors

• Treating species richness as the same as biodiversity. Richness is only one part of species diversity.
• Using the diversity index to compare sites sampled by different methods or at different times of year.
• Saying an organism “adapts because it needs to”. Selection acts on variation that already exists in the population.
• Describing speciation without saying that gene flow stops between the populations.
• Listing causes of loss without explaining the mechanism for a named species or ecosystem.
• Saying “several small reserves are always worse”. Give both sides, then judge.
• Defining keystone species as “the most common species” or “the top predator”. It is about the size of the effect, not numbers or trophic level.

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.