BiologyCBSE Class 12 · Biodiversity and Conservation

Patterns of Biodiversity

Updated 15 October 2026 · Fact-checked

Biodiversity generally increases from the poles towards the equator (latitudinal gradient) because the tropics have had more evolutionary time, a more constant environment and more solar energy. Species richness also rises with area up to a limit, following log S = log C + Z log A, with Z about 0.1-0.2 within regions and 0.6-1.2 across continents.

Species richness climbs from the poles to the tropics, and on the log-log graph a larger area gives more species, with a steeper slope Z across continents.

Illustration drawn for QuestionBuddy.

Understand Patterns of Biodiversity

Biodiversity and its levels (term popularised by sociobiologist Edward Wilson):

Genetic diversity: e.g. the medicinal plant *Rauwolfia vomitoria* in different Himalayan ranges differs in the potency and concentration of reserpine; in India, rice exists as over 50,000 genetically distinct strains and mango as about 1,000 varieties.
Species diversity: e.g. the Western Ghats have greater amphibian species diversity than the Eastern Ghats.
Ecological diversity: India, with deserts, rain forests, mangroves, coral reefs, wetlands, estuaries and alpine meadows, has greater ecosystem diversity than a Scandinavian country like Norway.

How many species? About 1.5 million species have been described (IUCN, 2004); Robert May estimated about 7 million globally. More than 70 per cent of recorded species are animals; plants (including algae, fungi, bryophytes, gymnosperms, angiosperms) are no more than 22 per cent. Among animals, insects make up more than 70 per cent: 7 out of every 10 animals are insects. India has only 2.4 per cent of the world's land area but 8.1 per cent of global species diversity, making it one of 12 megadiversity countries; nearly 45,000 plant species and twice as many animal species have been recorded.

Pattern 1: Latitudinal gradient. Diversity generally increases from the poles towards the equator; the tropics (23.5° N to 23.5° S) have more species than temperate or polar areas.

Birds: Colombia (near the equator) about 1,400 species; New York (41° N) 105; Greenland (71° N) 56. India, largely tropical, has more than 1,200 bird species.
A tropical forest such as in Ecuador has up to 10 times as many vascular plant species as an equal area of temperate forest in the Midwest USA.
The Amazonian rain forest has the greatest biodiversity on earth: more than 40,000 plant species, 3,000 fishes, 1,300 birds, 427 mammals, 427 amphibians, 378 reptiles and more than 1,25,000 invertebrates.

Why the tropics are richer:
(a) Time: speciation is a function of time; temperate regions had frequent glaciations, while the tropics remained relatively undisturbed for millions of years, giving long evolutionary time for diversification.
(b) Constancy: tropical environments are less seasonal, more constant and predictable, promoting niche specialisation and more species.
(c) Energy: more solar energy in the tropics gives higher productivity, which may indirectly support greater diversity.

Pattern 2: Species-area relationship. Alexander von Humboldt observed that within a region, species richness increases with the explored area, but only up to a limit. For many taxa (angiosperms, birds, bats, freshwater fishes) the relation is a rectangular hyperbola; on a log scale it becomes a straight line:
log S = log C + Z log A, i.e. ** $S = C A^{Z}$ **,
where S = species richness, A = area, Z = slope (regression coefficient), C = Y-intercept.

Within regions, Z = 0.1 to 0.2, whatever the taxon or region (plants in Britain, birds in California, molluscs in New York state).
For very large areas such as whole continents, the slope is much steeper: Z = 0.6 to 1.2; e.g. frugivorous birds and mammals in tropical forests of different continents, Z = 1.15.

Why diversity matters to the ecosystem.

Communities with more species tend to be more stable: less year-to-year variation in productivity, resistance or resilience to disturbance, and resistance to invasion by alien species.
David Tilman's outdoor plot experiments: plots with more species showed less year-to-year variation in total biomass, and higher diversity contributed to higher productivity.
Rivet popper hypothesis (Paul Ehrlich): an ecosystem is like an aeroplane held together by thousands of rivets (species). Removing a few may not affect flight at first, but as more are removed the plane becomes dangerously weak; and losing rivets on the wings (key species) is far more serious than losing those on seats or windows.

Flow chart: latitude ↓ (towards equator) → more time undisturbed + constant climate + more solar energy → more speciation, niche specialisation, productivity → more species.

Draw and label: species-area curves. Checklist: graph 1, S on y-axis, A on x-axis, a rising curve that flattens (rectangular hyperbola); graph 2, log S vs log A, a straight line with slope Z and intercept log C. Labels: species richness (S), area (A), log S, log A, slope Z, intercept log C, 'log S = log C + Z log A'.

Patterns of Biodiversity: key facts

Species-area relationship: $lo g S = lo g C + Z lo g A ⟺ S = C A^{Z}$; S species richness, A area, Z slope, C intercept.
Values of Z: $Z \approx 0.1 - 0.2 (regions); 0.6 - 1.2 (continents); 1.15 (tropical frugivores)$; Steeper slope for very large areas.
Effect of area change: $\frac{S _{2}}{S _{1}} = (\frac{A _{2}}{A _{1}})^{Z}$; Useful in HOTS numericals.
Bird species by latitude: $Colombia \approx 1400; New York (41° N) = 105; Greenland (71° N) = 56$; India: more than 1200.
India's share: $2.4% land area \to 8.1% of species$; One of 12 megadiversity countries.

How to answer Patterns of Biodiversity questions

Pattern questions are explanations (why tropics are richer) or calculations with the species-area equation.

1For the latitudinal gradient, state the pattern, give one data example (birds in Colombia, New York, Greenland), then the three reasons (time, constancy, energy).
2For species-area, write log S = log C + Z log A and define every symbol.
3To compare two areas, use $S_{2} / S_{1} = (A_{2} / A_{1})^{Z}$ ; take logs if needed.
4Quote the Z ranges: 0.1-0.2 within regions; 0.6-1.2 for continents (1.15 for tropical frugivores).
5For stability, cite Tilman's plots and explain the rivet popper analogy, stressing key species (wing rivets).
6For levels of biodiversity, give the NCERT example for each level.

Patterns of Biodiversity in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Species-area relationship

Suppose a forest reserve of 100 km² holds 200 bird species, and a highway project will leave only 10 km² of connected forest. Assume Z = 0.2 for birds in this region.

Estimate the number of bird species the 10 km² fragment can support in the long run. By what percentage does species richness fall?

Try it first, then show the solutionSolution

S₂/S₁ = (A₂/A₁)^Z = (10/100)^0.2 = 0.1^0.2.
0.1^0.2 = 10^(−0.2) ≈ 0.631.
S₂ ≈ 200 × 0.631 ≈ 126 species.
Fall ≈ (200 − 126)/200 × 100 ≈ 37 per cent, even though area fell by 90 per cent.

Answer: About 126 species remain (≈37% fewer) when the area shrinks to one-tenth.

Memory hook: Cut the area by ten, lose more than a third of the species.

Tricks to remember Patterns of Biodiversity

Short memory aids. Say each one aloud once, then use it on a question the same day.

T-C-E: why tropics win
Time (no glaciations), Constancy (less seasonal), Energy (more sunlight).
Use it when: Reasons for the latitudinal gradient.
1400-105-56
Birds fall from Colombia (1400) to New York (105) to Greenland (56) as latitude rises.
Use it when: Data example.
Z small in a region, big for continents
0.1-0.2 nearby; 0.6-1.2 across continents.
Use it when: Z values.

Patterns of Biodiversity: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Species-area equation · 2 marks · Very short answerBoard level

Write the equation of the species-area relationship on a log scale and name each term. What are the typical values of Z?

Try it first, then show the solutionSolution

log S = log C + Z log A; S = species richness, A = area, Z = slope (regression coefficient), C = Y-intercept (1 mark).
Z = 0.1-0.2 within regions; 0.6-1.2 for very large areas such as continents (1 mark).

Answer: log S = log C + Z log A; Z 0.1-0.2 (regions), 0.6-1.2 (continents).

Example 2: Why the tropics are richer · 3 marks · Short answerBoard level

Give three reasons why tropical latitudes have greater biodiversity than temperate ones.

Try it first, then show the solutionSolution

Tropics remained relatively undisturbed for millions of years (no frequent glaciations), giving more evolutionary time for speciation (1 mark).
Tropical environments are less seasonal, more constant and predictable, promoting niche specialisation (1 mark).
More solar energy gives higher productivity, which may indirectly support greater diversity (1 mark).

Answer: More time, constant environment, more solar energy.

Example 3: Rivet popper · 3 marks · Short answerHarder board level

Explain the rivet popper hypothesis. What does the difference between rivets on the wings and on the seats represent?

Try it first, then show the solutionSolution

Proposed by Paul Ehrlich: an ecosystem is like an aeroplane held together by thousands of rivets (species) (1 mark).
Losing a few rivets (species) may not affect flight (ecosystem functioning) at first, but continued loss makes the plane dangerously weak (1 mark).
Rivets on the wings represent key species that drive major ecosystem functions; their loss is far more serious than losing rivets on seats or windows (less critical species) (1 mark).

Answer: Species = rivets; gradual loss weakens the ecosystem; key species (wing rivets) matter most.

Example 4: Levels of biodiversity · 3 marks · Short answerHarder board level

Explain the three levels of biodiversity with one Indian example of each.

Try it first, then show the solutionSolution

Genetic diversity: variation within a species, e.g. Rauwolfia vomitoria in different Himalayan ranges varies in reserpine; over 50,000 rice strains and 1,000 mango varieties (1 mark).
Species diversity: variety of species, e.g. Western Ghats have more amphibian species than the Eastern Ghats (1 mark).
Ecological diversity: variety of ecosystems, e.g. India's deserts, rain forests, mangroves, coral reefs, wetlands, estuaries and alpine meadows (1 mark).

Answer: Genetic (Rauwolfia, rice, mango), species (Western Ghats amphibians), ecological (India's many ecosystems).

Example 5: Using Z for continents · 3 marks · Short answerHOTS / competency

Suppose a continental-scale analysis of fruit-eating birds gives Z = 1.15. If one region has twice the area of another, how many times as many species would it have? Compare with Z = 0.15 for a small region.

Try it first, then show the solutionSolution

S₂/S₁ = 2^Z (1 mark).
Z = 1.15: 2^1.15 ≈ 2.22 times as many species (1 mark).
Z = 0.15: 2^0.15 ≈ 1.11 times; doubling area adds far more species at continental scale because different continents hold different sets of species (1 mark).

Answer: About 2.22× for Z = 1.15 vs about 1.11× for Z = 0.15.

Example 6: Species number and stability · 2 marks · Very short answerHarder board level

Two grassland plots of equal area are studied for ten years: plot X has 16 plant species and plot Y has 2. Using Tilman's findings, predict which plot will show steadier total biomass from year to year and which is likely to be more productive. State one more attribute of a stable community.

Try it first, then show the solutionSolution

Plot X: Tilman's long-term outdoor plots with more species showed less year-to-year variation in total biomass, and increased diversity contributed to higher productivity (1 mark).
A stable community is also resistant or resilient to occasional disturbances and resists invasion by alien species (1 mark).

Answer: Plot X (16 species): steadier biomass and higher productivity; a stable community also resists or recovers from disturbance and resists alien invasion.

Where students lose marks in Patterns of Biodiversity

The slips examiners see most often, and what to write instead.

Mistake: Wrong Z ranges
Two ranges are swapped.
Wrong working
Z is 0.6-1.2 for small regions.
Right working
Z is 0.1-0.2 within regions; 0.6-1.2 for continents.
Do this: Small area, small Z.
Usually costs: 1 mark.
Mistake: Equation without definitions
Students write only the formula.
Wrong working
log S = log C + Z log A.
Right working
...where S = species richness, A = area, Z = slope, C = Y-intercept.
Do this: Always define symbols.
Usually costs: 1 mark.
Mistake: Only one reason for tropical richness
Students say 'more sunlight' only.
Wrong working
Tropics get more sunlight, so more species.
Right working
More evolutionary time (no glaciations), constant environment, and more solar energy.
Do this: T-C-E.
Usually costs: 1-2 marks.
Mistake: Wrong person for an idea
Names are mixed.
Wrong working
Rivet popper hypothesis was given by Tilman.
Right working
Rivet popper: Paul Ehrlich; stability plots: David Tilman; species-area: Humboldt; biodiversity term: Edward Wilson; 7 million estimate: Robert May.
Do this: Learn the five names with their ideas.
Usually costs: 1 mark.

Exam tips

Quote the bird numbers (1400, 105, 56) as evidence for the latitudinal gradient.
Draw both species-area graphs (curve and log-log straight line) for 3-mark questions.
Give three reasons for tropical richness.
Explain the rivet popper hypothesis with the wing vs seat rivets distinction.
Learn India's numbers: 2.4% land, 8.1% species, one of 12 megadiversity countries.

Practice questions on Patterns of Biodiversity

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
In the species-area relationship, for very large areas like continents, the value of Z lies between:
(a) 0.1 and 0.2 (b) 0.2 and 0.4 (c) 0.6 and 1.2 (d) 1.5 and 2.0
Hint
Steeper slope.
Answer
(c) 0.6 and 1.2.
Q2 · 1 mark · Assertion-reasonBoard level
Assertion (A): For frugivorous birds and mammals in tropical forests of different continents, the slope Z of the species-area line is about 1.15.
Reason (R): Within a region, Z lies between 0.1 and 0.2 whatever the taxonomic group.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Two different scales.
Answer
(b). Both statements are true, but R describes small regions; the steep slope in A comes from comparing very large areas such as whole continents (Z = 0.6 to 1.2).
Q3 · 2 marks · Very short answerHarder board level
India has 2.4 per cent of the world's land area and 8.1 per cent of global species diversity. How many times its 'fair share' of species does India hold? What status does this give India?
Hint
Divide.
Answer
8.1/2.4 ≈ 3.4 times its share by area. India is one of the 12 megadiversity countries of the world.
Q4 · 3 marks · Short answerHOTS / competency
Suppose an island of 50 km² has 40 species of land snails, and Z = 0.15 for this group. Estimate the number of species on a similar island of 500 km². Show working.
Hint
S₂ = S₁ × (A₂/A₁)^Z.
Answer
A₂/A₁ = 10; 10^0.15 ≈ 1.413; S₂ ≈ 40 × 1.413 ≈ 56.5, i.e. about 57 species.
Q5 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a table shows the number of bird species: Colombia (near the equator) about 1400, New York (41° N) 105, Greenland (71° N) 56.
(a) What pattern does this show? (1)
(b) Give two reasons for it. (1)
(c) Name the place with the greatest biodiversity on earth and give any two of its species counts. (2)
Hint
Latitudinal gradient; Amazon.
Answer
(a) Latitudinal gradient: diversity decreases from the equator towards the poles. (b) Tropics had long undisturbed evolutionary time (no glaciations), have a less seasonal, more constant environment and receive more solar energy (any two). (c) The Amazonian rain forest: more than 40,000 plant species, 3,000 fishes, 1,300 birds, 427 mammals, 427 amphibians, 378 reptiles, over 1,25,000 invertebrates (any two).
Q6 · 2 marks · Very short answerHarder board level
Who estimated the global species diversity at about 7 million? If only 22 per cent of species have been described, what does this imply for India?
Hint
Robert May.
Answer
Robert May. Applying this proportion to India's recorded species (about 45,000 plants and about twice as many animals) suggests that India probably still has over 1,00,000 undescribed plant species and over 3,00,000 undescribed animal species.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Who popularised the term biodiversity?Show answer
Edward Wilson.
2.Who observed the species-area relationship?Show answer
Alexander von Humboldt.
3.Value of Z for frugivorous birds and mammals across continents?Show answer
1.15.
4.Number of bird species in India?Show answer
More than 1,200.
5.Who proposed the rivet popper hypothesis?Show answer
Paul Ehrlich.
6.What fraction of animals are insects?Show answer
More than 70 per cent (7 of every 10).

Patterns of Biodiversity: frequently asked questions

What is the latitudinal gradient in biodiversity?

The increase in species diversity from the poles towards the equator; tropical regions have the most species.

Why do the tropics have more species?

They have had long undisturbed evolutionary time without glaciations, a less seasonal and more constant environment, and more solar energy giving higher productivity.

What is the species-area relationship?

Species richness increases with area up to a limit; on a log scale log S = log C + Z log A, with Z about 0.1-0.2 within regions and 0.6-1.2 across continents.

What is the rivet popper hypothesis?

Paul Ehrlich's analogy that species are like rivets of an aeroplane; losing more of them weakens the ecosystem, and losing key species (wing rivets) is most serious.

Why are species-rich communities more stable?

They show less year-to-year variation in productivity, resist or recover from disturbances, and resist invasion, as Tilman's experiments showed.