Biodiversity and species richness

Biodiversity describes the variety of living organisms in an area. The area being considered can be small, such as a local habitat, or as large as the whole Earth. The scale must therefore be clear when describing biodiversity.

Within a community, species richness is the number of different species present. To find it, count each species once, regardless of how many individuals belong to it.

For example, a community with 90 individuals of one species and 5 individuals of each of two other species has a species richness of 3. A community with 34, 33 and 33 individuals of three species also has a species richness of 3.

The two communities have the same species richness, even though their individuals are distributed differently among the species. Species richness tells us how many species are present, not how individuals are shared among them.

Index of diversity

An index of diversity describes diversity in a community using both the number of species present and the number of individuals belonging to each species. It accounts for two features:

• Species richness: the number of different species.
• Evenness: how evenly individuals are distributed among those species.

A community tends to have a greater index when it contains more species and when individuals are distributed more evenly among them. If one species dominates, the index is lower than it would be for a community with the same number of species but a more even distribution.

Calculating the index of diversity

The index $$d$$ is calculated using:

Here, $$N$$ is the total number of organisms of all species, and $$n$$ is the number of organisms of each species. The symbol $$\sum$$ means that the values of $$n(n - 1)$$ are added across all species.

1. Find $$n(n - 1)$$ for each species.
2. Add these values to obtain the denominator, $$\sum n(n - 1)$$.
3. Calculate $$N(N - 1)$$ and divide it by the denominator.

Index of diversity: worked calculation

Suppose a community contains three species with 4, 3 and 3 individuals. The total is $$N = 10$$. The denominator is $$4(3) + 3(2) + 3(2) = 24$$, and the numerator is $$10(9) = 90$$.

A greater value of $$d$$ indicates greater diversity. When comparing communities, the index reflects both how many species are present and how evenly their individuals are distributed.

Farming and biodiversity conservation

Farmland can support many species, but farming changes the habitats on which they depend. Biodiversity is the variety of living organisms in an area. It can decrease when farming removes habitats or reduces populations of organisms that live in or use the farmland.

For example, removing hedgerows to create larger fields reduces the shelter and food available to some organisms. Pesticides can reduce populations of the organisms they target and may also affect other species, directly or by reducing their food supply. Intensive management can favour a limited range of organisms, such as the crop being grown, over a more varied community.

Balancing farming and conservation

Conservation aims to maintain habitats and species. Keeping hedgerows, for example, can preserve places where organisms find food and shelter. However, land used for habitats may not be available for growing crops, and maintaining those habitats can require resources and work.

Farming must also produce food and remain economically viable. Balancing these aims means weighing the benefits of protecting biodiversity against the land, resources and management needed for agricultural production.