AHL: Conservation biology

IB Biology HL· Theme C: Interaction and Interdependence, AHL Topic 10· 20 min read

1. Biodiversity: Components and Measurement★★☆☆☆HL only⏱ 5 min

✓ Calculator OK

📘 Definition

Biodiversity

The total variation of living organisms and their ecosystems, encompassing three interconnected levels: genetic diversity (variation within a species' genome), species diversity (variation of species in a community), and ecosystem diversity (variation of habitats across a region).

Example:

Coral reefs have far higher biodiversity than the open ocean, across all three levels.

Species diversity is the most commonly measured form of biodiversity in IB Biology, and it accounts for two components: species richness (number of different species) and evenness (relative abundance of individuals across species). The standard metric used to quantify this is Simpson's Diversity Index.

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

📐 Worked Example

A survey of a temperate grassland found the following counts of grass species: 15 individuals of Lolium perenne, 10 of Festuca rubra, 4 of Poa pratensis, and 1 of Bromus inermis. Calculate Simpson's Diversity Index for this community.

1
Calculate the total number of individuals $N$ :
$N = 15 + 10 + 4 + 1 = 30$
2
Calculate $n (n - 1)$ for each species, then sum the results: $
3
$\sum n(n-1) = (15 \times 14) + (10 \times 9) + (4 \times 3) + (1 \times 0) = 210 + 90 + 12 + 0 = 312$
4
Calculate $N (N - 1)$ :
$30 \times 29 = 870$
5
Substitute into the formula to get $D$ :
$D = 1 - \frac{312}{870} = 1 - 0.359 = 0.641$
6
Interpret the result: Simpson's $D$ ranges from 0 (no diversity, one species) to 1 (maximum diversity). A value of 0.641 indicates moderate diversity.

Exam tip:

Always show all steps of your calculation in exam questions, even for simple arithmetic, to earn full method marks.

2. Extinction: Risk Factors and Causes★★★☆☆HL only⏱ 6 min

📘 Definition

Extinction

The permanent loss of an entire species globally, when no living individuals remain. Extinction is a natural process, but current rates are 100-1000 times higher than the background rate due to human activity.

Example:

The passenger pigeon Ectopistes migratorius was driven to extinction by human overhunting in 1914.

Certain biological traits make species more vulnerable to extinction: small population size, low genetic diversity, restricted geographic range, specialized habitat requirements, low reproductive rates, and endemism (found only in one location). Human activity is the leading driver of modern extinction.

📐 Worked Example

Explain why endemic island species are disproportionately at risk of extinction compared to widespread continental species.

1
Endemic island species have extremely restricted geographic ranges, so any local disturbance (e.g., deforestation, volcanic eruption) can eliminate the entire global population.
2
Island populations are typically small, so they have low genetic diversity due to founder effects and genetic drift. This reduces their ability to adapt to new threats like climate change or disease.
3
Many island species evolved in the absence of mammalian predators, so they lack defensive behaviors and are extremely vulnerable to introduced predators like rats, cats, and goats.
4
For all these reasons, over 60% of known modern extinctions have been of endemic island species.

3. In Situ Conservation★★★★☆HL only⏱ 7 min

📘 Definition

In situ conservation

Conservation of species in their natural, intact habitat, maintaining all natural ecosystem interactions and evolutionary processes.

Example:

National parks, wilderness reserves, and wildlife corridors are all examples of in situ conservation.

In situ conservation is the preferred primary strategy for long-term biodiversity preservation, because it protects entire ecosystems and all species within them, rather than just individual endangered species. Effective in situ conservation requires protected areas that are large enough to support viable populations, have minimal edge effects, and are connected via corridors to reduce fragmentation.

📐 Worked Example

Evaluate the use of wildlife corridors as a strategy to mitigate habitat fragmentation.

1
Habitat fragmentation splits large continuous populations into small isolated subpopulations, which suffer from reduced gene flow and increased inbreeding depression, raising extinction risk.
2
Wildlife corridors are narrow strips of intact natural habitat that connect fragmented patches, allowing individuals to move between subpopulations.
3
Advantages: Restore gene flow between isolated populations, reduce inbreeding depression, allow species to shift their ranges to track suitable climate as the climate warms, and maintain ecosystem processes like migration.
4
Disadvantages: Can facilitate the spread of invasive species and infectious disease between populations, require additional land that may conflict with agriculture or urban development, and can be expensive to maintain.
5
Conclusion: When appropriately designed, wildlife corridors are a highly effective strategy for reducing the negative impacts of habitat fragmentation.

4. Ex Situ Conservation★★★☆☆HL only⏱ 6 min

📘 Definition

Ex situ conservation

Conservation of endangered species outside their natural habitat, used as a last-resort intervention when in situ conservation cannot prevent immediate extinction.

Example:

Zoos, captive breeding programs, botanic gardens, and global seed banks are all forms of ex situ conservation.

Ex situ conservation allows researchers to preserve genetic diversity, increase population size via captive breeding, and eventually reintroduce individuals to restored wild habitats. Seed banks, for example, preserve genetic diversity of crop plants and endangered plants for decades at low temperature.

📐 Worked Example

Compare the advantages and disadvantages of ex situ and in situ conservation.

1
In situ conservation advantages: Preserves entire ecosystems and all ecological interactions, allows natural selection and evolution to continue, lower long-term cost, supports multiple species at once.
2
In situ conservation disadvantages: Cannot protect species from widespread threats like global climate change or pandemics, requires large intact habitats that may not be available.
3
Ex situ conservation advantages: Prevents immediate extinction of critically endangered species, supports research and public education, can preserve genetic diversity that would otherwise be lost.
4
Ex situ conservation disadvantages: Only preserves a small sample of the original genetic diversity, very expensive for long-term maintenance, cannot preserve natural ecosystem interactions, captive-bred individuals often have low survival rates when reintroduced to the wild.
5
Conclusion: The most effective conservation programs use in situ conservation as the primary strategy, with ex situ conservation as a complementary backup for high-risk species.

5. Common Pitfalls

Wrong move:

Interpreting a low Simpson's D value as high diversity

Why:

The formula for Simpson's index is structured so D increases with diversity, but many students reverse the scale

Correct move:

Remember: D ranges from 0 (only one species, no diversity) to 1 (maximum diversity). Higher D = higher diversity

Wrong move:

Confusing the definitions of in situ and ex situ conservation

Why:

Students often mix up the terms when describing examples in exam responses

Correct move:

Use the simple memory hook: In = In natural habitat, Ex = Exit natural habitat

Wrong move:

Claiming ex situ conservation is always preferable to in situ

Why:

Many students overstate the role of captive breeding, ignoring the benefits of preserving entire ecosystems

Correct move:

In situ is the preferred primary strategy; ex situ is only used as a last-resort complement for critically endangered species

Wrong move:

Defining species diversity as only the number of species

Why:

Students forget to include evenness (relative abundance) in their definitions, which loses exam marks

Correct move:

Species diversity combines two components: species richness (number of species) and evenness (relative abundance of individuals across species)

Wrong move:

Assuming large widespread species have higher extinction risk

Why:

Students incorrectly associate large body size with higher risk, ignoring the role of population size and range

Correct move:

Small endemic populations with restricted ranges have the highest extinction risk due to low genetic diversity and vulnerability to local disturbance

6. Quick Reference Cheatsheet

Concept	Key Definition	Exam Note
Biodiversity	Variation across genetic, species, ecosystem levels	Includes both richness and evenness
Simpson's D	$D = 1 - \frac{\sum n ( n - 1 )}{N ( N - 1 )}$	0 = low diversity, 1 = high diversity
Extinction Risk	Higher for endemic, small, specialist populations	Human activity drives most modern extinctions
HIPPO	Human threats to biodiversity	Habitat loss, Invasive, Pollution, Population, Overexploitation
In situ	Conservation in natural habitat	Preferred primary strategy
Ex situ	Conservation outside natural habitat	Last-resort backup, examples: zoos, seed banks

7. Frequently Asked

What is the difference between species richness and species diversity?

Species richness is just the number of different species in a community. Species diversity combines richness with evenness, the relative abundance of each species. Two communities with the same richness can have very different diversity if one is dominated by a single species.

When is ex situ conservation the right approach?

Ex situ is only used as a last resort when in situ conservation cannot prevent immediate extinction of a critically endangered species. It is always used alongside in situ strategies, not as a replacement.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2024 · Paper 2
Evaluate in situ conservation strategies
2023 · Paper 1
Identify extinction risk factors
2022 · Paper 3
Calculate Simpson's diversity index

Going deeper

What's Next

Conservation biology is an applied AHL topic aligned with IB Biology's focus on real-world global challenges, and it is frequently assessed in extended response questions in Paper 2 and data analysis questions in Paper 3. You will often be asked to evaluate conservation strategies or calculate biodiversity indices, so practicing these skills is key for exam success. This topic builds on core ecological concepts and provides a foundation for understanding how human activity alters global ecological balance, and how we can mitigate biodiversity loss. To continue building your exam preparation, explore the following related topics: