Biodiversity as genetic, species and ecosystem diversity; how species and genetic diversity are measured; and the ecological and economic importance of biodiversity.

What this dot point is asking

The SQA wants you to define biodiversity as having three components - genetic, species and ecosystem diversity - explain how species diversity and genetic diversity are measured, and explain why biodiversity matters both ecologically and economically. This builds directly on how you sample an ecosystem and underpins the later study of human impacts.

The three components of biodiversity

• Genetic diversity is the range of alleles (versions of genes) within a species or population. It is the raw material for adaptation.
• Species diversity is the number of different species in an area (richness) together with their relative abundance (evenness).
• Ecosystem diversity is the variety of different ecosystems or habitats within a region, such as woodland, moorland, river and coast within one landscape.

Measuring species diversity

A simple count of species, called species richness, ignores how individuals are shared out. A wood with 100 trees split evenly across 5 species is more diverse than one where 96 of the 100 belong to a single species. Evenness captures this.

To combine the two, scientists use a diversity index. A common form is Simpson's diversity index:

where $n$ is the number of individuals of each species and $N$ is the total number of individuals of all species. The index runs from $0$ (a single species) towards $1$ (many species, evenly spread): a higher value means greater diversity.

Measuring genetic diversity

Genetic diversity within a population can be assessed by the number of different alleles present and by the proportion of individuals that are heterozygous. Modern methods compare DNA sequences directly. Genetic diversity matters because:

• A population with many alleles is more likely to contain individuals that can survive a new threat (a disease, a parasite or a climate shift), so the population can adapt.
• Populations that pass through a bottleneck (a sharp drop in numbers) or inbreeding lose alleles, becoming more vulnerable to extinction.

Why biodiversity matters

Ecological importance. Diverse ecosystems tend to be more stable and productive, recover better from disturbance, and maintain the food webs and nutrient cycles that keep the system working. Each species can play a role, so losing species can weaken the whole community.

Economic importance. Biodiversity provides ecosystem services of direct economic value: crops and wild foods, medicines derived from wild species, raw materials such as timber and fibres, pollination of crops by insects, water purification, and income from ecotourism. Many of these services would be expensive or impossible to replace artificially.

Examples in context

Example 1. Crop genetic diversity and food security. Seed banks such as the Svalbard Global Seed Vault store thousands of varieties of staple crops. This protects genetic diversity so that breeders can find alleles for disease resistance or drought tolerance, which a single high-yield variety grown across vast areas would lack and which makes monocultures vulnerable to a single pest.

Example 2. Pollinators and economic value. Wild and managed insect pollinators are estimated to underpin a large share of global crop value. A decline in pollinator diversity threatens the yield of fruit, vegetable and oilseed crops, illustrating how biodiversity translates directly into economic worth through an ecosystem service.

Try this

Q1. State the difference between species richness and species evenness. [2 marks]

• Cue. Richness is the number of species; evenness is how equally individuals are spread among them.

Q2. Explain why a high diversity index value is generally good for an ecosystem. [2 marks]

• Cue. It indicates many species spread evenly, which tends to mean a more stable, resilient and productive ecosystem.