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A4.2

Conservation of biodiversity

Theme A · Unity and diversity · Ecosystems · SL and HL · no additional higher level

There is no additional higher level content in this topic: SL and HL study exactly the same material. It is the most case-study-driven topic in Theme A. Examiners want named examples — a lost species, a lost ecosystem, a real survey — and they want the causes of the biodiversity crisis tied to human activity specifically, not to extinctions in general.

🎯What you need to be able to do

  • Define biodiversity at the levels of ecosystem, species and genetic diversity.
  • Compare the number of species alive now with past levels, and explain why the count depends on how species are classified.
  • Explain the causes of anthropogenic extinction using three case studies, including the North Island giant moa and the Caribbean monk seal.
  • Explain the causes of ecosystem loss using two case studies, including mixed dipterocarp forest in Southeast Asia.
  • Evaluate evidence for a biodiversity crisis, including the need for repeated surveys and the role of citizen scientists.
  • Outline the causes of the current biodiversity crisis.
  • Explain why several approaches to conservation are needed: in situ, nature reserves, rewilding, ex situ and seed or tissue banks.
  • Explain the rationale for prioritizing evolutionarily distinct and globally endangered (EDGE) species.

📚The biology

What biodiversity is

Biodiversity is the variety of life in all its forms, levels and combinations. It is measured at three levels:

Ecosystem diversity
The variety of habitats and ecosystems in an area — forest, grassland, wetland, reef, and so on.
Species diversity
The variety of species. It has two components: richness, the number of species, and evenness, how equal their population sizes are.
Genetic diversity
The variety of alleles within a species. A species with high genetic diversity is better able to adapt to change and to resist disease.

Species diversity combining richness and evenness can be measured with Simpson’s reciprocal index, which you are expected to be able to apply:

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

where \( N \) is the total number of organisms of all species and \( n \) is the number of each species. The higher the value of \( D \), the greater the diversity. Its minimum value is 1, when only one species is present.

How many species, now and in the past?

Around two million species have been discovered, named and described, but estimates of the total alive today range from several million to many more, because most insects, fungi, microorganisms and deep-sea species have never been described. Evidence from the fossil record suggests that more species are alive on Earth today than at any time in the past, since biodiversity has, overall, increased over the history of life despite the mass extinctions.

Counting species is itself a matter of judgement. Classification is pattern recognition, and the same observations can be classified in different ways: “splitters” divide a group into many species on the basis of small differences, while “lumpers” combine them into fewer, more variable species. The same group of organisms can therefore be recorded as a different number of species by different taxonomists.

Anthropogenic extinction

There have been five mass extinctions in Earth’s history, caused by natural events such as asteroid impacts and massive volcanic activity. Many biologists argue that a sixth mass extinction is now under way — and that, unlike the other five, it is caused by humans. This topic is about that one. Three case studies illustrate a range of causes:

North Island giant moa (Dinornis novaezealandiae)
A huge flightless bird of New Zealand, part of its terrestrial megafauna. New Zealand had no land mammals, so moas had no defences against ground predators. Within roughly two centuries of Polynesian settlement (around 1300 CE), all moa species were extinct. Causes: hunting of the slow-breeding birds and collection of their eggs, together with habitat clearance by burning.
Caribbean monk seal (Neomonachus tropicalis)
A marine mammal of the Caribbean and Gulf of Mexico. Hunted from the fifteenth century onwards for its oil-rich blubber, and its fish prey were depleted by overfishing. Seals rested on beaches, which made them easy to kill. The last confirmed sighting was in 1952, and it was declared extinct in 2008.
A local example: the Bali tiger (Panthera tigris sondaica, Bali population)
The smallest tiger, found only on Bali. As the human population grew, it was hunted — for sport and as a perceived threat to people and livestock — and its forest habitat was cleared for agriculture. There have been no confirmed records since the 1930s–40s, and it is extinct. If you study elsewhere, choose a species lost from your own region.

The common threads: over-exploitation (hunting, fishing), habitat loss, and species with slow reproduction or no evolved defence against a new threat.

Ecosystem loss

Whole ecosystems can be lost, not just species. The causes are directly or indirectly human.

Mixed dipterocarp forest, Southeast Asia
Lowland rainforest dominated by tall trees of the family Dipterocarpaceae, among the most species-rich forests on Earth, found in Borneo, Sumatra and the Malay Peninsula. Large areas have been lost to commercial logging (dipterocarps are valuable timber), conversion to oil palm and other plantations, and fires, often started to clear land and worsened by drained peat. Logged forest is more fire-prone, and fragments are too small to support species like orangutans.
A local example: mangrove forest in Indonesia
Mangroves line the coasts of Bali and Java, protecting shores from erosion and acting as nurseries for fish. Large areas have been cleared for shrimp and fish ponds, coastal development and tourism infrastructure, and polluted by run-off. Choose a lost ecosystem from your own region if you can.

Evidence for a biodiversity crisis

A claim of crisis needs reliable evidence from surveys across a wide range of habitats around the world. Major sources include the reports of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES), whose 2019 global assessment concluded that around a million species are threatened with extinction, and indices that track many monitored populations over decades.

Two features of good evidence are examinable:

  • Surveys must be repeated. A single survey shows what is present, not whether anything is changing. Only repeated surveys of the same sites, using the same methods, can show a change in species richness or evenness.
  • The evidence should be verifiable. It usually needs to come from a published, peer-reviewed source in which the methodology can be checked.

Citizen scientists — members of the public recording sightings, for example in bird counts or phone apps — contribute data on a scale no team of experts could collect. That brings real benefits: huge geographic coverage and long-term records. It also brings particular concerns: volunteers vary in identification skill, effort is uneven (more records near cities, of attractive species and at weekends), and methods are hard to standardize. Good citizen-science projects control for this with training, photo verification and statistical correction.

Causes of the biodiversity crisis

The overarching cause is human population growth, together with rising consumption per person. It drives a set of specific causes:

Over-exploitation
Hunting, overfishing, and harvesting of plants and animals faster than they can reproduce.
Urbanization
Towns, roads and infrastructure replace and fragment natural habitats.
Deforestation and land clearance for agriculture
The largest single cause of habitat loss; natural ecosystems are replaced by crops and pasture.
Pollution
Pesticides, fertilizer run-off, plastics and other pollutants damage organisms and ecosystems.
Spread of pests, diseases and invasive alien species
Global transport moves organisms to places where they have no natural predators or where native species have no defences.

Approaches to conservation

No single approach is enough, and different species need different measures. The main approaches:

  • In situ conservation — protecting species in their natural habitat. Species stay in the ecosystem they are adapted to, with all their interactions intact, and continue to evolve.
  • Management of nature reserves — reserves need active management: controlling invasive species, preventing poaching, managing grazing or fire, and maintaining corridors between fragments.
  • Rewilding and reclamation of degraded ecosystems — restoring natural processes, for example by reintroducing lost species and letting land return to a natural state (D4.2).
  • Ex situ conservation — keeping species outside their natural habitat in zoos and botanic gardens, often with captive breeding and later reintroduction. It can save species whose habitat is gone, but captive populations are small, lose genetic diversity and may lose natural behaviours.
  • Storage of germ plasm — seed banks (such as the Svalbard Global Seed Vault) and tissue banks store seeds, sperm, eggs or tissue at low temperatures. They preserve genetic diversity cheaply and for a long time, but stored material does not evolve and cannot preserve the ecosystem.

The EDGE of Existence programme

Money and effort for conservation are limited, so choices have to be made about which species to prioritize. The EDGE of Existence programme ranks species by combining two scores:

  • Evolutionarily Distinct (ED) — how much unique evolutionary history a species represents. A species with no close living relatives, on a long branch of the tree of life, scores highly.
  • Globally Endangered (GE) — how threatened it is, based on its IUCN Red List category.

The rationale is that losing a species with no close relatives — such as the aye-aye, the Chinese giant salamander or a pangolin — loses far more unique genetic and evolutionary information than losing one of many similar species. These species are also often overlooked by conservation efforts that focus on familiar, attractive animals.

Decisions about which species to save have ethical, environmental, political, social, cultural and economic dimensions. Should a keystone species with close relatives outrank an evolutionarily unique but ecologically minor one? Should local communities’ needs come first? There is no purely scientific answer, which is why these priorities need to be debated.

✏️Worked example

Two areas of forest were surveyed using the same method. Four species of beetle were found in each.
Site A
species 1: 10
species 2: 10
species 3: 10
species 4: 10
Site B
species 1: 34
species 2: 2
species 3: 2
species 4: 2
(a) Calculate Simpson’s reciprocal index for each site.
(b) Explain why the two sites have different values even though both have four species.
(c) A wider survey of the whole forest recorded 18 beetle species, compared with 24 in the first survey. Calculate the percentage change in species richness, and explain why a repeated survey was needed to reach this conclusion.

(a) Both sites have \( N = 40 \), so \( N(N-1) = 40 \times 39 = 1560 \).

Site A: each species has \( n(n-1) = 10 \times 9 = 90 \), and there are four, so the sum is 360.

\[ D_{A} = \frac{1560}{360} = 4.33 \]

Site B: \( 34 \times 33 = 1122 \); each of the other three species gives \( 2 \times 1 = 2 \), so the sum is \( 1122 + 6 = 1128 \).

\[ D_{B} = \frac{1560}{1128} = 1.38 \]

(b) Both sites have the same richness (four species) but different evenness. At site A all species are equally common. At site B one species makes up 85% of the individuals, so a randomly chosen pair of beetles is very likely to be the same species. Simpson’s index accounts for evenness as well as richness, so site B is less diverse.

(c) Percentage change:

\[ \frac{18 - 24}{24} \times 100 = -25\% \]

Species richness fell by 25%. A single survey only records what is present at one time; it cannot show a trend. Only by repeating the survey at the same sites, with the same methods and effort, can a difference be attributed to a real change rather than to where or how the sampling was done.

Check it. For a large sample in which every species is equally common, Simpson’s reciprocal index comes out almost exactly equal to the number of species; for a small sample the \( n-1 \) terms push it a little higher. Site A has four equally common species and only 40 beetles, so a value just above 4 is right. Site B is dominated by one species, so its value should be close to 1 — and it is. A value for site B larger than for site A would signal an arithmetic slip.
Using \( n^{2} \) instead of \( n(n-1) \), or forgetting the brackets. The index needs \( n(n-1) \) for each species, summed, and \( N(N-1) \) for the total. Squaring gives a different (and here wrong) answer. Also note the sign in (c): a fall is a negative percentage change, and the denominator is always the original value.

📝Practise

Work through these on paper, then reveal the answer.

1. Distinguish between species richness and species evenness, and explain why both matter for species diversity.
Richness is the number of different species in an area. Evenness is how similar the population sizes of those species are. Two areas with the same number of species can differ greatly in diversity: if one species dominates, most of the others are rare and the community behaves almost like a single-species community, and the rare species are more at risk of local extinction. A diversity measure such as Simpson’s reciprocal index includes both.
2. Explain why two taxonomists studying the same group of organisms might report different numbers of species.
Classification involves pattern recognition, and the same observations can be interpreted in different ways. A “splitter” treats populations with small, consistent differences as separate species; a “lumper” treats them as variation within one species. Different species concepts (morphological, biological, based on DNA sequence) can also give different answers, and speciation is gradual, so where to draw the boundary is often arbitrary.
3. Using a named example, outline the causes of the extinction of a terrestrial species as a result of human activity.
Example: the North Island giant moa. It was a large flightless bird in New Zealand, which had no native land mammals, so it had no defences against ground predators. After humans settled New Zealand (around 1300 CE) moas were hunted for meat and their eggs were collected; they reproduced slowly, so populations could not recover. Forest was also cleared by burning, destroying habitat. The species became extinct within about two centuries. (Other valid examples, such as the Bali tiger, should give specific human causes in the same way.)
4. Evaluate the use of data collected by citizen scientists as evidence for a decline in biodiversity.
Strengths: very large numbers of records over wide geographic areas and long periods, which professional scientists could not collect alone; can detect trends across whole countries; engages the public. Weaknesses: identification errors by untrained observers; uneven effort (more records near towns, of conspicuous species, at convenient times), which can create false trends; methods are not standardized; data may not be peer-reviewed. Judgement: citizen-science data are valuable, especially when combined with verification (e.g. photographs), standardized protocols and statistical correction, and when they agree with professional surveys; on their own they should be treated with caution.
5. Compare in situ and ex situ conservation.
In situ conservation protects species in their natural habitat (e.g. nature reserves); ex situ conservation keeps them outside it (e.g. zoos, botanic gardens, seed banks). In situ keeps species in the ecosystem they are adapted to, preserves their interactions with other species and protects many species at once, and populations can go on evolving; but it cannot help if the habitat has been destroyed or the threat cannot be controlled. Ex situ can save species whose habitat is lost and allows captive breeding and later reintroduction; but captive populations are small, lose genetic diversity, may lose natural behaviour, and it is expensive and protects few species. Both are often used together.
6. Explain the rationale for the EDGE of Existence programme.
Conservation resources are limited, so species must be prioritized. EDGE prioritizes species that are both Evolutionarily Distinct — having few or no close living relatives, so they represent a large amount of unique evolutionary history and genetic information — and Globally Endangered, according to their IUCN threat category. Losing such a species removes a whole branch of the tree of life that cannot be replaced by relatives. These species are often neglected by conservation that focuses on well-known animals. The choice is still debatable, because prioritization also involves ethical, economic, social and ecological considerations, such as the importance of keystone species.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • IPBES — the Summary for Policymakers of the Global Assessment on Biodiversity and Ecosystem Services, the standard source of evidence for the crisis.
  • EDGE of Existence (Zoological Society of London) — the ranked list of EDGE species, with a profile of each.
  • IUCN Red List of Threatened Species — search any species for its status, population trend and threats.