Biodiversity within a community

What you'll learn

What biodiversity means, from a tiny local habitat to the whole Earth.
How species richness differs from diversity.
How to calculate and interpret the AQA index of diversity, $d$ .
Why farming often reduces biodiversity, and how conservation can be balanced with food production.

The starting point: organisms, habitats and communities

A habitat is the place where an organism lives. A habitat can be very small, such as a rotting log, or very large, such as a tropical rainforest.

A community is all the populations of different species living and interacting in the same habitat at the same time.

A population is all the organisms of one species in a particular area at a particular time.

Definition

Biodiversity

Biodiversity is the variety of living organisms in an area. In this topic, you focus mainly on biodiversity within a community, especially the number of species present and the number of individuals in each species.

Biodiversity can be considered at many scales. You might compare biodiversity in two quadrats in a school field, two woodlands, two countries, or even the whole Earth.

Key Idea

Scale matters

When you describe biodiversity, always say the scale you are talking about: a small local habitat, a whole ecosystem, a country, or the Earth.

Species richness

Definition

Species richness

Species richness is the number of different species in a community.

Species richness does not tell you how many individuals of each species there are. It only counts how many different species are present.

For example, a pond containing five species of invertebrate has a higher species richness than a pond containing two species of invertebrate.

But species richness alone can be misleading. Two communities can have the same number of species but very different distributions of individuals.

The diagram below shows why species richness is only part of the story.

Two communities with the same species richness but different evenness and different diversity

Evenness: how evenly individuals are shared between species

Evenness means how similar the abundances of different species are. A community has high evenness if individuals are spread fairly evenly between species.

A community has low evenness if one species is very common and the others are rare.

Example

Comparing richness and evenness

Two grassland samples contain the following species:

Sample A: 10 daisies, 10 clovers, 10 buttercups, 10 grasses
Sample B: 37 daisies, 1 clover, 1 buttercup, 1 grass

Count the number of different species in each sample. Both samples contain daisies, clovers, buttercups and grasses, so both have a species richness of 4.
Compare how the individuals are distributed. In Sample A, each species has 10 individuals, so abundance is evenly shared.
In Sample B, daisies dominate the sample and the other species are rare, so evenness is low.
Therefore, Sample A has greater diversity even though both samples have the same species richness.

Common Mistake

Richness is not the same as diversity

Do not say a community is more diverse just because it has more individuals overall. Diversity depends on the number of species and how individuals are distributed between those species.

The index of diversity

Species richness gives a simple count, but ecologists often need a more useful measure that includes both:

the number of different species
the number of individuals in each species

This is where the index of diversity comes in.

Definition

Index of diversity

An index of diversity describes the relationship between the number of species in a community and the number of individuals in each species.

For AQA A-Level Biology, you need to calculate the index of diversity using:

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

where:

$d$ = index of diversity
$N$ = total number of organisms of all species
$n$ = total number of organisms of each species
$∑\sum$ means “the sum of”, so you add together the $n (n - 1)$ value for every species

Counts of organisms are treated as dimensionless counts, so the index of diversity has no units.

Key Idea

Interpreting d

A higher value of $d$ means greater species diversity. This usually means there are more species, a more even distribution of individuals, or both.

Calculating the index of diversity

A reliable method is:

Add all the organisms to find $N$ .
For each species, calculate $n (n - 1)$ .
Add all the $n (n - 1)$ values to find $∑n(n−1)\sum n(n-1)$ .
Substitute into the formula.
State that $d$ has no units.

Example

Calculating an index of diversity

A student samples a meadow and records the following results:

Species	Number of organisms
Daisy	18
Clover	12
Grass	15
Buttercup	5

Calculate the index of diversity.

Find the total number of organisms:
$N = 18 + 12 + 15 + 5 = 50$
Calculate $n (n - 1)$ for each species:
$Daisy:18(18−1)=18×17=306Clover:12(12−1)=12×11=132Grass:15(15−1)=15×14=210Buttercup:5(5−1)=5×4=20\begin{aligned} \text{Daisy} &: 18(18-1) = 18 \times 17 = 306 \\ \text{Clover} &: 12(12-1) = 12 \times 11 = 132 \\ \text{Grass} &: 15(15-1) = 15 \times 14 = 210 \\ \text{Buttercup} &: 5(5-1) = 5 \times 4 = 20 \end{aligned}$
Add these values to find the denominator:
$∑n(n−1)=306+132+210+20=668\sum n(n-1) = 306 + 132 + 210 + 20 = 668$
Substitute into the formula:
$d=N(N−1)∑n(n−1)d=50(50−1)668d=2450668d=3.67\begin{aligned} d &= \frac{N(N-1)}{\sum n(n-1)} \\ d &= \frac{50(50-1)}{668} \\ d &= \frac{2450}{668} \\ d &= 3.67 \end{aligned}$
The index of diversity is 3.67, with no units. This value can be compared with other samples collected using a similar method.

Tip

Check the denominator carefully

For each species, calculate $n (n - 1)$ first, then add the answers. Do not add all the $n$ values first and then calculate $n (n - 1)$ for the total — that would just repeat the numerator.

Common Mistake

Using n squared instead of n(n - 1)

The formula uses $n (n - 1)$ , not $n^2$ . Similarly, the numerator is $N (N - 1)$ , not $N^2$ .

Interpreting index values

The index of diversity is most useful when comparing communities.

If two habitats are sampled in a valid, comparable way:

the habitat with the higher value of $d$ has greater diversity
the habitat with the lower value of $d$ has lower diversity

A low index of diversity may mean:

there are very few species
one species dominates the community
the habitat has been disturbed or simplified

A high index of diversity may mean:

there are many species
individuals are more evenly spread between species
the habitat contains many niches and food sources

Definition

Niche

A niche is the role of a species in its ecosystem, including where it lives, what it eats, and how it interacts with other organisms.

Example

Interpreting diversity data

Two fields are sampled using the same method.

Field A has an index of diversity of 9.8.
Field B has an index of diversity of 2.1.

Compare the values directly because the same sampling method was used. Field A has the higher value of $d$ .
Link the value to biodiversity. Field A has greater species diversity than Field B.
Explain what this suggests biologically. Field A probably has more species, a more even spread of individuals between species, or both.
Avoid overclaiming. The index alone does not prove the exact cause of the difference; it only describes the sampled diversity.

Common Mistake

Only compare like with like

Index values are most meaningful when samples are collected using similar methods, similar sampling effort, and similar-sized areas. Poor sampling can make biodiversity estimates unreliable.

How farming techniques reduce biodiversity

Farming aims to maximise food production. This often simplifies ecosystems, which can reduce biodiversity.

Monoculture

Definition

Monoculture

Monoculture is the growth of one crop species over a large area.

Monoculture reduces species richness because many wild plant species are removed. Fewer plant species means fewer food sources and habitats for insects, birds and other animals.

Removal of hedgerows and woodland

Hedgerows provide:

nesting sites for birds
shelter for small mammals
food sources such as berries, flowers and insects
corridors that allow animals to move between habitats

Removing hedgerows creates larger fields for machinery, but it also removes habitats and reduces biodiversity.

Pesticides and herbicides

Pesticides are chemicals used to kill pests, such as insects that feed on crops.

Herbicides are chemicals used to kill unwanted plants, often called weeds.

These can reduce biodiversity because:

pesticides may kill non-target insects
fewer insects means less food for birds and bats
herbicides reduce wild plant species
fewer wild plants means fewer niches for animals

Fertilisers

Fertilisers increase crop growth by adding mineral ions such as nitrate ions. However, fertilisers can reduce biodiversity if they favour a small number of fast-growing species.

If fertiliser runs into ponds, streams or rivers, it can cause eutrophication. This may lead to algal blooms, reduced light penetration, death of aquatic plants, increased decomposition, and reduced oxygen availability for aquatic animals.

Key Idea

Farming simplifies ecosystems

Many farming techniques increase yield by making conditions better for the crop, but worse for many other species. This usually reduces species richness and the index of diversity.

Balancing conservation and farming

Farmers need to produce enough food and make a living. Conservation aims to protect biodiversity and maintain ecosystems. The challenge is to balance both.

Conservation-friendly farming methods include:

leaving or replanting hedgerows
creating wildflower field margins
using pesticides only when necessary
using biological control or integrated pest management
rotating crops to maintain soil fertility
reducing fertiliser runoff using buffer strips near water
maintaining ponds, woodland patches and uncultivated areas

These methods can increase biodiversity by providing more habitats, food sources and breeding sites.

However, they may also:

reduce the area available for crops
increase costs for farmers
require more labour or planning
sometimes reduce short-term yield

Key Idea

The balance

The aim is not simply “farming versus nature”. Good management can support food production while also protecting habitats, pollinators, soil quality and long-term ecosystem stability.

Example

Evaluating a conservation strategy

A farmer leaves a 3 m wildflower strip around a wheat field instead of planting wheat right up to the edge.

Identify the biodiversity benefit. The wildflower strip provides nectar, seeds and shelter, so it can increase plant, insect and bird diversity.
Link this to the index of diversity. More plant species and a more even distribution of animal species could increase the value of $d$ in the field margin.
Consider the farming cost. Some land is no longer used for wheat, so the farmer may produce slightly less crop from that field.
Balance the judgement. The strategy may be worthwhile if benefits such as pollination, natural pest control and conservation payments help offset the loss of crop area.

Exam technique

In the exam

When defining species richness, say it is the number of different species in a community, not the number of individuals.
For the index of diversity, calculate $N$ , then each $n (n - 1)$ value, then $∑n(n−1)\sum n(n-1)$ before substituting into the formula.
State that a higher value of $d$ means greater diversity, but only make valid comparisons when sampling methods are comparable.
When explaining farming effects, link the technique to habitats, food sources, species richness or evenness — avoid vague phrases like “it harms nature”.
For conservation questions, discuss both sides: food production and farmer income as well as biodiversity and ecosystem protection.

Self review

Check yourself

What is the difference between species richness and an index of diversity?
Why can two communities with the same species richness have different biodiversity?
How can removing hedgerows reduce the index of diversity in farmland?

What you'll learn

What biodiversity means, from a tiny local habitat to the whole Earth.
How species richness differs from diversity.
How to calculate and interpret the AQA index of diversity, $d$ .
Why farming often reduces biodiversity, and how conservation can be balanced with food production.

The starting point: organisms, habitats and communities

A habitat is the place where an organism lives. A habitat can be very small, such as a rotting log, or very large, such as a tropical rainforest.

A community is all the populations of different species living and interacting in the same habitat at the same time.

A population is all the organisms of one species in a particular area at a particular time.

Definition

Biodiversity

Biodiversity can be considered at many scales. You might compare biodiversity in two quadrats in a school field, two woodlands, two countries, or even the whole Earth.

Key Idea

Scale matters

When you describe biodiversity, always say the scale you are talking about: a small local habitat, a whole ecosystem, a country, or the Earth.

Species richness

Definition

Species richness

Species richness is the number of different species in a community.

Species richness does not tell you how many individuals of each species there are. It only counts how many different species are present.

For example, a pond containing five species of invertebrate has a higher species richness than a pond containing two species of invertebrate.

But species richness alone can be misleading. Two communities can have the same number of species but very different distributions of individuals.

The diagram below shows why species richness is only part of the story.

Two communities with the same species richness but different evenness and different diversity

Evenness: how evenly individuals are shared between species

Evenness means how similar the abundances of different species are. A community has high evenness if individuals are spread fairly evenly between species.

A community has low evenness if one species is very common and the others are rare.

Example

Comparing richness and evenness

Two grassland samples contain the following species:

Sample A: 10 daisies, 10 clovers, 10 buttercups, 10 grasses
Sample B: 37 daisies, 1 clover, 1 buttercup, 1 grass

Count the number of different species in each sample. Both samples contain daisies, clovers, buttercups and grasses, so both have a species richness of 4.
Compare how the individuals are distributed. In Sample A, each species has 10 individuals, so abundance is evenly shared.
In Sample B, daisies dominate the sample and the other species are rare, so evenness is low.
Therefore, Sample A has greater diversity even though both samples have the same species richness.

Common Mistake

Richness is not the same as diversity

Do not say a community is more diverse just because it has more individuals overall. Diversity depends on the number of species and how individuals are distributed between those species.

The index of diversity

Species richness gives a simple count, but ecologists often need a more useful measure that includes both:

the number of different species
the number of individuals in each species

This is where the index of diversity comes in.

Definition

Index of diversity

An index of diversity describes the relationship between the number of species in a community and the number of individuals in each species.

For AQA A-Level Biology, you need to calculate the index of diversity using:

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

where:

$d$ = index of diversity
$N$ = total number of organisms of all species
$n$ = total number of organisms of each species
$∑\sum$ means “the sum of”, so you add together the $n (n - 1)$ value for every species

Counts of organisms are treated as dimensionless counts, so the index of diversity has no units.

Key Idea

Interpreting d

A higher value of $d$ means greater species diversity. This usually means there are more species, a more even distribution of individuals, or both.

Calculating the index of diversity

A reliable method is:

Add all the organisms to find $N$ .
For each species, calculate $n (n - 1)$ .
Add all the $n (n - 1)$ values to find $∑n(n−1)\sum n(n-1)$ .
Substitute into the formula.
State that $d$ has no units.

Example

Calculating an index of diversity

A student samples a meadow and records the following results:

Species	Number of organisms
Daisy	18
Clover	12
Grass	15
Buttercup	5

Calculate the index of diversity.

Find the total number of organisms:
$N = 18 + 12 + 15 + 5 = 50$
Calculate $n (n - 1)$ for each species:
$Daisy:18(18−1)=18×17=306Clover:12(12−1)=12×11=132Grass:15(15−1)=15×14=210Buttercup:5(5−1)=5×4=20\begin{aligned} \text{Daisy} &: 18(18-1) = 18 \times 17 = 306 \\ \text{Clover} &: 12(12-1) = 12 \times 11 = 132 \\ \text{Grass} &: 15(15-1) = 15 \times 14 = 210 \\ \text{Buttercup} &: 5(5-1) = 5 \times 4 = 20 \end{aligned}$
Add these values to find the denominator:
$∑n(n−1)=306+132+210+20=668\sum n(n-1) = 306 + 132 + 210 + 20 = 668$
Substitute into the formula:
$d=N(N−1)∑n(n−1)d=50(50−1)668d=2450668d=3.67\begin{aligned} d &= \frac{N(N-1)}{\sum n(n-1)} \\ d &= \frac{50(50-1)}{668} \\ d &= \frac{2450}{668} \\ d &= 3.67 \end{aligned}$
The index of diversity is 3.67, with no units. This value can be compared with other samples collected using a similar method.

Tip

Check the denominator carefully

For each species, calculate $n (n - 1)$ first, then add the answers. Do not add all the $n$ values first and then calculate $n (n - 1)$ for the total — that would just repeat the numerator.

Common Mistake

Using n squared instead of n(n - 1)

The formula uses $n (n - 1)$ , not $n^2$ . Similarly, the numerator is $N (N - 1)$ , not $N^2$ .

Interpreting index values

The index of diversity is most useful when comparing communities.

If two habitats are sampled in a valid, comparable way:

the habitat with the higher value of $d$ has greater diversity
the habitat with the lower value of $d$ has lower diversity

A low index of diversity may mean:

there are very few species
one species dominates the community
the habitat has been disturbed or simplified

A high index of diversity may mean:

there are many species
individuals are more evenly spread between species
the habitat contains many niches and food sources

Definition

Niche

A niche is the role of a species in its ecosystem, including where it lives, what it eats, and how it interacts with other organisms.

Example

Interpreting diversity data

Two fields are sampled using the same method.

Field A has an index of diversity of 9.8.
Field B has an index of diversity of 2.1.

Compare the values directly because the same sampling method was used. Field A has the higher value of $d$ .
Link the value to biodiversity. Field A has greater species diversity than Field B.
Explain what this suggests biologically. Field A probably has more species, a more even spread of individuals between species, or both.
Avoid overclaiming. The index alone does not prove the exact cause of the difference; it only describes the sampled diversity.

Common Mistake

Only compare like with like

Index values are most meaningful when samples are collected using similar methods, similar sampling effort, and similar-sized areas. Poor sampling can make biodiversity estimates unreliable.