Biodiversity within a community

In a nutshell

Biodiversity is the variety of living organisms in an area, and it can be measured at any scale, from a single small local habitat (a pond, a hedgerow) right up to the whole Earth.

This subtopic is about measuring biodiversity within a community: counting the species present (species richness), and using an index of diversity that also takes account of how many individuals belong to each species. It finishes with how farming lowers biodiversity, and the balance between conservation and farming.

Assumed knowledge: Species and taxonomy, Genetic diversity and adaptation.

Core content

Biodiversity and the scale of a habitat

Biodiversity is the variety of living organisms in an area.

It is not tied to one fixed size of area. AQA wants you to know that biodiversity can relate to a range of habitats:

• a small local habitat (for example one pond, one field, one hedgerow), or
• a whole region, or
• the entire Earth (global biodiversity).

Within a single community, biodiversity is measured in two ways: species richness and an index of diversity.

One term has to be clear first, because examiners see it confused constantly:

A community is all the populations of different species living in the same area at the same time.

A population is just the organisms of one species; a community is all the different species together. Species richness and the index of diversity are properties of a community, not a population.

Species richness

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

• It is a simple count: how many different species are present.
• It takes no account of how many individuals there are of each species.

So if you survey a meadow and find 14 different species, its species richness is 14, whether each species has thousands of individuals or just one. That is the weakness species richness has, and it is exactly the gap the index of diversity fills.

The 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.

Unlike species richness, it takes account of both:

• the number of species, and
• the number of individuals of each species (the abundance).

AQA gives you the formula in the exam; you are not asked to recall it, but you must be able to use it:

$d = \frac{N(N-1)}{\sum n(n-1)}$

where:

• N = the total number of organisms of all species;
• n = the total number of organisms of each species.

$\sum n(n-1)$ means: work out $n(n-1)$ for every species, then add the results.

Key points about what the index tells you:

• A higher value of d means greater diversity.
• Because it weights by the number of individuals, a community dominated by one species has a low index even when several species are present. The influence of rare species is reduced, which is why an index of diversity is more useful than species richness when some species are present only in small numbers.
• To calculate it you need the number of individuals of each species and the total number of organisms of all species. You do not need the number of species itself.

The chart below shows why the number of individuals matters. Both communities contain three species and 25 organisms in total, so their species richness is identical, but the individuals are shared out very differently:

Still don't get it? · why the index beats just counting species

Imagine two football stadiums, each holding 25 fans. In the first, the fans support three different teams fairly evenly: 12, 8 and 5. In the second, 22 support one team and the other two teams have just 2 fans and 1 fan. Both stadiums contain supporters of three teams, so "counting the teams" gives both a 3. But the first crowd is genuinely mixed, while the second is really a one-team crowd with a couple of stragglers.

The index of diversity is the number that notices the difference. It does not just ask "how many species?", it asks "how are the individuals shared out between them?". A community where the individuals are spread evenly across the species scores high; a community dominated by one species scores low, because almost every organism you pick belongs to that one species.

In the exam: species richness is only the number of different species in a community, whereas an index of diversity also takes account of the number of individuals in each species. That is exactly why an index of diversity is more useful when some species are present only in small numbers: it stops a single dominant species from hiding the fact that the rest are rare.

Farming techniques reduce biodiversity

Farming is designed to grow as much of one crop or animal as possible. The techniques that raise yield tend to remove the habitats and the variety of plants that wild species depend on, so biodiversity falls.

Notice the chain running through every row: a farming technique removes plant species or habitats, which removes food sources and niches, which reduces the number of animal species. Leading with "fewer plant species" is the point most students miss.

The balance between conservation and farming

The techniques above pull in opposite directions to conservation, so a balance (a compromise) has to be struck:

• Farming must produce enough food, economically, for a growing human population.
• Conservation aims to protect biodiversity, which keeps ecosystems stable and preserves services such as pollination.

Measures that let land stay productive while protecting biodiversity include:

• maintaining or replanting hedgerows, and leaving uncultivated field margins and wildflower strips;
• using pesticides and herbicides more sparingly or more selectively;
• planting hedges and using crop rotation to support a wider range of species.

The balance is between the short-term gain in yield and profit from intensive farming and the long-term value of conserving biodiversity.

Worked examples

Model calculation: compare the diversity of the two communities above.

Use $d = \dfrac{N(N-1)}{\sum n(n-1)}$. Both communities have N = 25, so $N(N-1) = 25 \times 24 = 600$.

Community X (individuals 12, 8, 5):

$d = \frac{25(25-1)}{208} = \frac{600}{208} = 2.88 \;(3\text{ s.f.})$

Community Y (individuals 22, 2, 1):

$d = \frac{25(25-1)}{464} = \frac{600}{464} = 1.29 \;(3\text{ s.f.})$

Interpretation. Community X and Community Y have the same species richness (3) and the same total number of organisms (25), yet Community X has the higher index of diversity (2.88 against 1.29). This is because its individuals are spread more evenly among the three species, whereas Community Y is dominated by Species A. It shows why an index of diversity tells you more than species richness alone.

Model 3-mark answer: "Explain how removing hedgerows reduces the biodiversity of farmland."

1. Removing hedgerows removes a habitat, so there are fewer habitats / niches (and fewer nesting sites).
2. There are therefore fewer plant species and fewer food sources / less variety of food for animals.
3. So fewer animal species can survive, reducing the biodiversity / index of diversity.

Notice the phrasing: "fewer food sources", not "less food"; "fewer plant species", not "fewer plants". The vague versions are not credited.

Common exam mistakes

• Confusing population and community. A population is one species; a community is all the different species in an area. Species richness and the index of diversity describe a community.
• Muddling species richness with an index of diversity. Species richness is only the number of different species; an index of diversity also takes account of the number of individuals of each species.
• In the formula, thinking N is the number of species. N is the total number of organisms of all species; the number of species is not used in this index at all.
• Saying the data needed for the index is the "number of species". You need the number of individuals of each species and the grand total of all organisms, not the number of species.
• Explaining why an index of diversity beats species richness without the key idea: it takes account of the number of individuals in each species, so it is more useful when some species are present only in small numbers.
• Reading an increase in the index of diversity as just "more organisms". It means the number of species has increased and so has the number of individuals in each species.
• Assuming a lower total number of organisms automatically means lower biodiversity. Diversity depends on the number of species and how evenly individuals are spread, not the raw total.
• Farming answers that write "less food" (not credited) instead of "fewer food sources / less variety of food", or "fewer homes" instead of "fewer habitats / niches", or "fewer plants" instead of "fewer plant species".
• Thinking a herbicide kills insects directly. A herbicide kills plants (weeds); animals decline because there are then fewer plant species providing food and habitats.
• Writing about genetic diversity (variation within a species) when the question is about biodiversity within a community (variation between species).
• Quoting the index to 5 or 6 decimal places, or giving an inappropriate number of significant figures. Round to a sensible number of significant figures.

Key definitions

• Biodiversity: the variety of living organisms in an area, which can range from a small local habitat to the whole Earth.
• Community: all the populations of different species living in the same area at the same time.
• Species richness: the number of different species in a community.
• Index of diversity: a measure that describes the relationship between the number of species in a community and the number of individuals in each species.

Specification

• I can state that biodiversity can relate to a range of habitats, from a small local habitat to the whole Earth.
• I can define species richness as the number of different species in a community.
• I can explain that an index of diversity describes the relationship between the number of species in a community and the number of individuals in each species.
• I can calculate an index of diversity using $d = \dfrac{N(N-1)}{\sum n(n-1)}$, where N is the total number of organisms of all species and n is the total number of organisms of each species.
• I can interpret the significance of a calculated index of diversity, including comparing two communities.
• I can explain how farming techniques reduce biodiversity.
• I can discuss the balance between conservation and farming.

Related notes

• Genetic diversity and adaptation
• Species and taxonomy
• Investigating diversity