What is Biodiversity?

Biodiversity

The variety of life on Earth, including the variety of species, the genetic differences within species, and the variety of ecosystems in which organisms live.

Biodiversity is one of the most important concepts in biology and environmental science. It describes not just how many different living things exist, but the richness and complexity of life at every level — from the genes inside a single cell to the variety of entire habitats across the planet.

Scientists recognise three main levels of biodiversity:

Genetic diversity — the variety of genes within a single species (e.g. different coat colours in dogs)
Species diversity — the variety of different species in an area (e.g. a rainforest containing thousands of plant, animal, and fungal species)
Ecosystem diversity — the variety of different habitats, communities, and ecological processes on Earth (e.g. deserts, coral reefs, grasslands, and tundra)

Analogy

Think of biodiversity like a library. Genetic diversity is the variety of words used in books. Species diversity is the variety of different book titles. Ecosystem diversity is the variety of different libraries and reading rooms across the world. A healthy library — and a healthy planet — needs all three levels to function well.

Note

In MYP Sciences, biodiversity is closely linked to the key concept of Change and the related concept of Consequences. As biodiversity changes — often declining due to human activity — there are cascading consequences for ecosystems, economies, and human wellbeing. Keep these ideas in mind as you work through this subtopic.

Measuring Biodiversity: Species Richness and Species Evenness

To compare biodiversity in different habitats, scientists use specific measurements.

Species Richness

The total number of different species found in a given area.

Species Evenness

How equally individuals are distributed among the different species in an area.

A habitat with high species richness has many different species. But richness alone does not tell the whole story. Consider two meadows:

Meadow A: 100 organisms — 50 daisies, 49 buttercups, 1 clover
Meadow B: 100 organisms — 34 daisies, 33 buttercups, 33 clovers

Both meadows have the same species richness (3 species), but Meadow B has higher species evenness — its populations are more balanced. Meadow B is considered more biodiverse.

Exam Tip

When describing biodiversity in an exam, always mention both richness and evenness. A single dominant species in an area does not necessarily mean high biodiversity, even if the total number of individuals is large.

Simpson's Diversity Index

The Simpson's Diversity Index (D) is a formula scientists use to combine richness and evenness into a single number. A higher value means greater biodiversity.

The formula is:

$D = 1 - \sum (\frac{n}{N})^{2}$

Where:

$n$ = the number of individuals of each species
$N$ = the total number of individuals of all species
$\sum$ means "sum of" (add up the values for every species)

The index ranges from 0 (no diversity) to 1 (maximum diversity).

Worked Example — using the meadow data above:

For Meadow A ( $N = 100$ ):

$D = 1 - [(\frac{50}{100})^{2} + (\frac{49}{100})^{2} + (\frac{1}{100})^{2}]$

$D = 1 - [0.25 + 0.2401 + 0.0001] = 1 - 0.4902 = 0.51$

For Meadow B ( $N = 100$ ):

$D = 1 - [(\frac{34}{100})^{2} + (\frac{33}{100})^{2} + (\frac{33}{100})^{2}]$

$D = 1 - [0.1156 + 0.1089 + 0.1089] = 1 - 0.3334 = 0.67$

Meadow B has a higher Simpson's Index (0.67 vs 0.51), confirming it is more biodiverse — even though both meadows have the same number of species. This is because Meadow B's individuals are more evenly spread across species.

Note

You may be asked to calculate or interpret Simpson's Diversity Index in MYP science investigations (Criterion B and C). Practise the formula using the meadow example above, then try applying it to your own data.

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