What you'll learn

• The three levels of biodiversity: habitat, species and genetic.
• How field sampling is used to estimate biodiversity in real habitats.
• How to calculate and interpret Simpson’s Index of Diversity and genetic diversity.
• Why biodiversity matters, what threatens it, and how conservation protects it.

What biodiversity means

A habitat is the place where an organism lives, including the physical conditions and other organisms found there. Biodiversity is useful because it gives an indication of how complex, stable and resilient a habitat may be.

Habitat biodiversity

Habitat biodiversity is the variety of different habitats in an area. For OCR, you should be comfortable with examples such as sand dunes, woodland, meadows and streams.

An area containing woodland, meadow and a stream has higher habitat biodiversity than an area containing only a single large field.

Species biodiversity

A species is a group of organisms that can interbreed to produce fertile offspring. Species biodiversity is the variety of species in a habitat.

It has two main components:

• Species richness: the number of different species present.
• Species evenness: how evenly individuals are distributed between those species.

Genetic biodiversity

A gene is a length of DNA that codes for a polypeptide or functional RNA. An allele is a different version of a gene.

Genetic biodiversity is the variety of alleles within a species or population. Different breeds within one species, such as different cattle breeds or dog breeds, show genetic biodiversity.

Species richness and species evenness

Species richness is simple to count: if a meadow contains grasses, daisies, buttercups and clover, its species richness is 4.

Species evenness needs a little more judgement. A habitat where each species has similar abundance has high evenness. A habitat dominated by one species has low evenness, even if several species are present.

Sampling biodiversity in a habitat

Sampling is important because it is usually impossible to count every organism in a habitat. Good sampling reduces bias, allows habitats to be compared and helps you collect data from the full range of organisms and microhabitats present.

Random sampling

In random sampling, every part of the study area has an equal chance of being sampled.

A typical method is:

• Lay out a grid over the study area.
• Use random numbers to generate coordinates.
• Place a quadrat at each coordinate.
• Record the organisms inside the quadrat.
• Repeat many times, then calculate means or percentages.

A quadrat is a square frame of known area, often used for plants or slow-moving organisms. You might record abundance, percentage cover or presence/absence.

Non-random sampling

In non-random sampling, the positions sampled are chosen deliberately. This can be useful, but it may introduce bias if not planned carefully.

• Opportunistic sampling: sampling organisms that are easiest to find. It is quick, but often biased.
• Stratified sampling: dividing a habitat into different zones, then sampling each zone. For example, woodland edge and open meadow could be sampled separately.
• Systematic sampling: sampling at regular intervals, often along a transect, which is a line placed across a habitat.

Systematic sampling is especially useful when there is an environmental gradient, such as moving from dry sand dunes towards wetter ground near a stream.

Fieldwork techniques

Different organisms require different sampling methods.

• Quadrats: plants and slow-moving animals.
• Sweep nets: insects in long grass or vegetation.
• Pitfall traps: ground-dwelling invertebrates; the trap is sunk into the soil so organisms fall in.
• Pooters: small insects or arthropods; suction draws the organism into a container, with a filter to prevent inhalation.

Simpson’s Index of Diversity

Simpson’s Index of Diversity, represented by $D$, uses both species richness and species evenness. OCR will provide the formula when needed, but you must know how to use and interpret it.

$$D=1-\sum {\left(\frac{n}{N}\right)}^2$$

Here, $n$ is the number of individuals of one species, and $N$ is the total number of individuals of all species. $D$ has no units because it is based on proportions.

• A value of $D$ close to 0 indicates low biodiversity.
• A value of $D$ close to 1 indicates high biodiversity.

Genetic biodiversity calculations

Genetic biodiversity can be assessed by looking at how many gene loci are polymorphic.

A locus is the position of a gene on a chromosome. A polymorphic gene locus is a locus where more than one allele is present in the population.

$$proportion\; of\; polymorphic\; gene\; loci=\frac{number\; of\; polymorphic\; gene\; loci}{total\; number\; of\; loci}$$

To convert the proportion to a percentage, multiply by 100.

This is useful for studying isolated populations, such as zoo populations used in captive breeding, rare breeds and pedigree animals. Low genetic biodiversity can increase inbreeding and reduce the population’s ability to adapt to disease or environmental change.

Factors affecting biodiversity

Human population growth

As the human population increases, more land is needed for housing, transport, food production and industry. This can lead to habitat destruction, habitat fragmentation, pollution and overexploitation of resources.

Agriculture and monoculture

A monoculture is the growth of a single crop species over a large area. Monocultures reduce habitat variety and often reduce species richness.

Agriculture may lower biodiversity by:

• Removing hedgerows, woodland and wildflower margins.
• Using pesticides, which kill insect species.
• Using herbicides, which reduce plant diversity.
• Using fertilisers, which can cause nutrient enrichment and affect aquatic habitats.
• Replacing diverse habitats with a single crop species.

Climate change

Climate change can alter temperature, rainfall and seasonal patterns. Species may shift their distribution, breeding times may become mismatched with food availability, and some habitats may be lost or degraded.

Why maintain biodiversity?

Ecological reasons

A keystone species is a species that has a disproportionately large effect on its ecosystem. If it is removed, many other species may be affected because organisms are interdependent through food webs, pollination, seed dispersal and habitat formation.

Maintaining biodiversity also preserves a genetic resource: alleles that may be useful in the future, such as disease resistance in crops or livestock.

Economic reasons

Biodiversity supports ecosystem services such as pollination, soil formation, water purification and fisheries. In agriculture, maintaining soil organisms and plant diversity can reduce soil depletion compared with continuous monoculture.

Biodiverse habitats can also support tourism, recreation and sustainable harvesting.

Aesthetic reasons

Many people value biodiversity because landscapes, species and natural habitats are beautiful, culturally important and enjoyable to experience. Protecting landscapes is therefore also an aesthetic reason for conservation.

Maintaining biodiversity: in situ and ex situ conservation

In situ conservation protects species in their natural habitat. Examples include marine conservation zones and wildlife reserves.

Ex situ conservation protects species outside their natural habitat. Examples include seed banks, botanic gardens and zoos.

Conservation agreements

Conservation often needs cooperation at local, national and international levels.

• CITES is the Convention on International Trade in Endangered Species. It controls or bans international trade in endangered species and products made from them.
• The Rio Convention on Biological Diversity, often called the CBD, is an international agreement aiming to conserve biodiversity, use it sustainably and share benefits from genetic resources fairly.
• The Countryside Stewardship Scheme, or CSS, is a UK scheme that supports land management practices that benefit wildlife, habitats and landscapes, such as maintaining hedgerows, field margins and ponds.

These agreements show that conservation is not only biological. It also involves economics, politics, land use and decisions about how humans interact with ecosystems.