Biodiversity in the Community

Why does it matter how many different species live in an area, and how do you actually measure it? This page covers what biodiversity is, how to measure it with species richness and the index of diversity, and how farming affects it.

What is Biodiversity?

Biodiversity is the number and variety of living organisms in a particular area. It can be measured at different scales:

• A single field, pond, or hedgerow
• A country or biome
• The whole Earth

Biodiversity has several components, but at this level you need to be able to measure it using:

• Species Richness
• Index of Diversity

Species richness is simply a count of species: it doesn’t tell you how many individuals of each species are present.

Index of Diversity

So unlike species richness, it takes into account both how many species there are and how evenly the individuals are spread between them.

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

Where:

• N = total number of organisms of all species
• n = number of organisms of each species
• Σ = sum of (n(n−1)) calculated for every species

Note: You’re usually given this formula in the exam, but you need to know how to use it and what N and n mean.

A higher value of d = greater biodiversity. A community where many species each have roughly equal numbers of individuals will have a much higher d than a community dominated by one species.

Worked Example

Below is a worked example using three plant species found in a field. To calculate the index of diversity:

1. Work out n(n − 1) for each species
2. Add these up to get Σn(n − 1)
3. Work out N(N − 1), using the total number of organisms
4. Divide N(N − 1) by Σn(n − 1)

$$d = \frac{40 \times 39}{610} = \frac{1560}{610} \approx 2.56$$

On its own, a d value of 2.56 doesn’t tell you much. The index of diversity is most useful for comparing communities: for example, if a second field had d = 1.4, the first field would have the higher biodiversity.

Why biomass is sometimes used instead of number of individuals

When organisms are very small (e.g. fungi) or grow tightly packed together (e.g. moss or grass), it is not possible to count individuals accurately, because you can’t tell where one organism ends and the next begins. In these cases, biomass (the dry mass of each species) is used as a measure of abundance instead. To find dry mass, samples are dried in an oven until their mass stops changing, then weighed (you’ll learn more about this in Unit 5).

Farming and Biodiversity

Modern intensive farming has led to significant reductions in biodiversity. Key practices that reduce it:

• Clearing land: removing hedgerows and woodland destroys habitats and removes food and shelter for many species
• Herbicide use: kills weed species, reducing plant diversity and removing food sources for insects and other animals
• Pesticide use: kills pests (e.g. insects), but also kills other species that aren’t pests, such as pollinating insects. This removes food for animals further up the food chain, so the number of species of insects, birds and small mammals falls
• Monoculture: growing a single crop species over a large area provides habitat and food for only a limited range of species

The Conservation–Farming Balance

Ways farmers can conserve biodiversity include:

• Keeping or replanting hedgerows and leaving field margins of wild plants
• Rotating crops rather than growing the same crop every year
• Using fewer pesticides and herbicides, and only where needed
• Keeping ponds, wetlands and patches of woodland on the farm

Maintaining biodiversity has direct benefits for farmers:

• Natural pest control: predators of pests (e.g. hoverflies, ladybirds, birds) are supported by diverse habitats; this can reduce the need for pesticides
• Pollination: diverse habitats support pollinators (bees, butterflies); crops that require pollination give better yields
• Soil health: diverse plant communities support soil organisms that improve soil structure and fertility

However, it’s important to consider that conservation measures also have costs to the farmer:

• Reduced crop area: land used for hedgerows or wildflower margins is not used for crops, reducing yield and income
• Increased management time and cost

Exam Question Practice