Measuring Biodiversity: GCSE Biology

Why is it not enough to just count species?

Species richness — the number of different species present in an area — is the simplest measure of biodiversity. A meadow with 20 species of flowering plant appears more biodiverse than one with 10.

However, species richness alone can be misleading. Consider two woodland plots, each with 5 species of ground beetle:

• Plot A: 100 individuals total — 96 of one species, 1 each of the other four.
• Plot B: 100 individuals total — 20 individuals of each species.

Both have the same species richness (5 species), but Plot B is intuitively more "balanced" — no single species dominates. Plot A is close to a monoculture in practical terms.

This is why ecologists also measure species evenness — how uniformly individuals are distributed across species — and why a combined diversity index is more informative than species richness alone.

What is Simpson's Diversity Index?

Simpson's Diversity Index (D) is a widely used measure that combines species richness and evenness into a single number. The formula is:

D = 1 − Σ (n/N)²

Where:

• n = the number of individuals of each species in the sample
• N = the total number of individuals of all species combined
• Σ = "sum of" (add up for every species)
• (n/N) = the proportion of the sample made up by each species

D ranges from 0 to (approaching) 1:

• D close to 1 → high diversity (many species, relatively evenly distributed)
• D close to 0 → low diversity (community dominated by one or very few species)

Worked example: calculating Simpson's Diversity Index

A student samples insects in two garden habitats using pitfall traps. Their results are:

Garden A (maintained lawn with some wildflowers):

D = 1 − 0.399 = 0.60

Garden B (unmown wildflower meadow):

D = 1 − 0.206 = 0.79

Conclusion: Garden B (D = 0.79) has higher biodiversity than Garden A (D = 0.60), which is consistent with the expectation that an unmown wildflower meadow supports more evenly distributed and varied insect communities than a maintained lawn.

What factors affect biodiversity in an ecosystem?

Biodiversity tends to be higher when:

• Habitat complexity is greater — more different microhabitats means more niches for different species (e.g. a woodland edge has greater diversity than the interior).
• Productivity is moderate — highly productive but simple ecosystems (like an intensively fertilised field) often have lower biodiversity than moderately productive natural ones.
• Disturbance is intermediate — the Intermediate Disturbance Hypothesis suggests that moderate disturbance prevents any one species from dominating, allowing more species to coexist.
• Human impact is low — removal of habitats, pesticide use, pollution and invasive species all typically reduce biodiversity.

Biodiversity tends to be lower when:

• Intensive agriculture has simplified the landscape (monocultures, herbicide use, removal of hedgerows).
• Pollution or eutrophication occurs (dominant opportunistic species take over).
• An invasive species disrupts established community relationships.

Why does measuring biodiversity matter?

Tracking biodiversity indices over time is essential for conservation:

• Detecting decline: a falling D value in repeated surveys alerts conservationists that a habitat is deteriorating before individual species become endangered.
• Comparing habitats: D values allow objective comparison between habitats — useful for assessing which areas most need protection.
• Evaluating conservation interventions: if a management practice (e.g. wildflower strip planting, grazing management) increases D, it is demonstrably improving biodiversity.
• Monitoring recovery: habitats recovering from pollution or disturbance can be tracked; rising D values confirm recovery is occurring.

In the UK, organisations including the RSPB, Natural England, and the British Trust for Ornithology use diversity measures to report on the state of ecosystems and to assess the effectiveness of conservation spending.

Frequently asked questions

What is the difference between species richness and a diversity index?

Species richness is simply the count of how many different species are present — it gives no information about relative abundance. A diversity index (such as Simpson's D) combines species richness with species evenness. Two communities can have identical species richness but very different diversity indices if one is dominated by a single species. A diversity index is more informative for monitoring ecosystem health because it captures both the number of species and the balance of their populations.

Does a high Simpson's D always mean a healthy ecosystem?

Not necessarily. A diversity index measures one specific aspect of ecosystem health — the variety and balance of species. An ecosystem could have a high D value but still be under threat from an emerging disease, climate change, or invasion by a non-native species whose effects have not yet become apparent. Conversely, some naturally low-diversity ecosystems (e.g. salt marshes, arctic tundra) are healthy and ecologically important. Diversity indices are best used as change indicators — a significant fall in D over time is a warning sign, even if the absolute value seems acceptable.

How does sampling method affect the diversity index calculated?

The index is only as reliable as the sample on which it is based. If the sample is too small or is collected in a non-random way (e.g. all pitfall traps placed in one corner of a large field), rare species may be missed, and the index will underestimate true diversity. Good practice involves using random sampling (random number coordinates to place quadrats or traps), adequate sample size, and repeat sampling at different times of year or across multiple years to account for seasonal variation. Comparing D values between habitats is only valid if the same sampling method was used for both.

Can Simpson's Diversity Index be used for plants as well as animals?

Yes — the same formula applies to any assemblage of countable organisms, whether plants in a quadrat, birds in a survey area, or fish in a river. For plants in a quadrat, the student counts the number of individual plants (or percentage cover) of each species. For organisms that cannot easily be counted as individuals (such as colonial organisms or ground-covering plants), percentage cover of each species may replace the count (n/N becomes the proportion of cover). The interpretation of D is the same regardless of the organism group.

---

For Socratic GCSE biology with Professor Darwin — linking fieldwork, sampling statistics and ecosystem ecology — visit aitutors.me.