Biodiversity¶

Part of Module 4: Biodiversity, evolution and disease.

Biodiversity is both a scientific measurement topic and a conservation topic. Students need to know how biodiversity is described and quantified, how it is assessed in the field through sampling, how it is currently changing under human pressure, and why it should be maintained — with specific reference to ecological, economic, and aesthetic arguments, and to the international frameworks that support conservation efforts.

Learning Objectives¶

Levels Of Biodiversity¶

Biodiversity refers to the variety of organisms in an area, and it operates at three distinct levels. Habitat diversity measures the number of different habitats present within a region; a landscape containing woodland, meadow, stream, and sand dune has higher habitat diversity than one containing only arable farmland.

Species diversity combines two components. Species richness is simply the total number of different species in a habitat, determined by taking random samples and counting the species present. Species evenness compares the relative abundance of individuals across those species: if 1,000 individuals are distributed equally among 10 species, evenness is high; if 900 of those individuals belong to one species, evenness is low even though richness is unchanged. A habitat can therefore have high richness but low biodiversity if one species dominates. Understanding both components is essential for interpreting diversity data correctly.

Genetic diversity refers to variation in alleles within a population of a species. A high genetic diversity means a large variety of alleles is present in the population's gene pool, which increases the population's capacity to adapt to environmental changes and resist disease. This level of biodiversity is particularly relevant to captive breeding groups, pedigree animals, and rare breeds, where genetic bottlenecks can reduce the allele range.

Sampling Methods¶

It is rarely practical to count every individual organism in a habitat, so representative samples must be taken. Sampling must be designed carefully to avoid bias and to minimise the effects of chance on the results.

Random Sampling¶

In random sampling, coordinates are generated randomly across the study area — for example using a random number generator. Samples are collected at those coordinates, and the process is repeated sufficiently often to give a large sample size. This approach removes human bias in site selection and gives samples that are representative of the whole population. Once random coordinates are established, fieldworkers use various techniques to collect data.

For plants and non-motile organisms, quadrats are the standard tool. A frame quadrat is a square frame divided into a grid; the species present and their abundance within the frame are recorded. The number of individuals per square metre (species density), the frequency with which a species appears in the grid squares (species frequency), and the percentage cover can all be estimated from frame quadrats. A point quadrat uses a horizontal bar through which pins are pushed at set intervals; each species touched by a pin is recorded, and percentage cover is estimated from the proportion of pin-drops that touch each species.

For animals, a range of techniques is available. A pooter collects small invertebrates by sucking air through a tube into a sealed container. A sweep net is swept in a figure-of-eight motion through long grass or low vegetation to sample flying insects and invertebrates. A pitfall trap is sunk flush with the soil surface to catch ground-crawling invertebrates such as beetles and spiders. Tree beating involves striking the branches of a tree or bush so that invertebrates fall onto a white sheet below. Kick sampling is used in rivers: the bank is disturbed by foot and a net held downstream catches dislodged organisms.

Non-Random Sampling¶

Non-random sampling uses selection based on specific criteria rather than chance, which makes it prone to bias but allows more targeted investigation of particular questions.

Opportunistic sampling uses conveniently available organisms — it is the least rigorous method and may not be representative of the whole population. Stratified sampling divides the study population into sub-groups (strata) based on a characteristic; a random sample proportional to stratum size is then taken from each. This is useful when populations naturally contain distinct sub-groups that would be missed or over-represented by purely random sampling.

Systematic sampling uses regular intervals to avoid the patchiness that can arise from pure randomness. It is commonly implemented using transects. A line transect marks a straight line across the study area; observations are recorded at the points where the line passes through species. A belt transect records all species within a defined width on either side of the line using quadrats placed continuously or at set intervals. Transects are particularly effective for studying how species distributions change across an environmental gradient, such as from a woodland edge to an open field, or perpendicular to a shoreline.

Measuring Abiotic Factors¶

Because abiotic factors directly influence the distribution of organisms, measuring them alongside sampling is informative. Light, humidity, and temperature can be measured with sensors; pH and wind speed with probes; dissolved oxygen with specialised oxygen probes. These instruments detect rapid changes, reduce human error, achieve high precision, and allow data to be logged on a computer for statistical analysis.

Simpson's Diversity Index¶

Simpson's Index of Diversity (D) combines species number and relative abundance into a single value, allowing meaningful comparison between habitats:

D = 1 − Σ(n/N)²

where n is the number of individuals of each species and N is the total number of individuals of all species. D ranges from 0 (only one species present, no diversity) to values approaching 1 (very high diversity). The higher the value, the greater the biodiversity of the sample.

To calculate D: divide the number of individuals of each species by the total (n/N), square each result, sum all the squared values, and subtract from 1. A worked example illustrates that two communities with identical species richness can yield different D values if their species evenness differs — the community with a more even distribution always gives the higher index. This is why evenness matters as much as richness.

Genetic Biodiversity¶

Genetic diversity can be assessed by calculating the proportion of polymorphic gene loci in a population. A polymorphic locus is a gene locus at which more than one allele exists in the population; a monomorphic locus has only one allele. The proportion of polymorphic gene loci is calculated as:

proportion of polymorphic loci = number of polymorphic loci / total number of loci

A higher proportion indicates greater genetic diversity. For example, if 18 out of 50 examined loci are polymorphic, the proportion is 0.36 (36%). This value is higher in populations with greater allelic variation.

Genetic diversity is not static. It can increase through gene flow (interbreeding between different populations that introduces new alleles) and through mutation (which creates new alleles). It is reduced by selective breeding, captive breeding programmes (which use only a small founder group), artificial cloning, natural selection (which increases the frequency of advantageous alleles at the expense of others), genetic bottlenecks (sudden population crashes that eliminate most alleles from the gene pool), the founder effect (where a small group splits off and founds a new population, carrying limited genetic variation), and genetic drift (random allele loss over generations).

Human Impact On Biodiversity¶

The human population has grown rapidly, and this growth has had extensive negative impacts on global biodiversity. Deforestation for urban development and agriculture destroys habitats and reduces ecosystem diversity. Overuse of natural resources causes species decline or extinction, directly reducing genetic and species diversity. Urban sprawl fragments and isolates wildlife populations, limiting breeding opportunities and thereby reducing genetic diversity. Pollution kills species directly and degrades habitats in the long term.

Agricultural practices designed to maximise yields pose particular threats. Monocultures — growing single crops over large areas, typically without rotation — directly reduce plant diversity and eliminate the habitats and food sources on which many animal species depend. Monocultures also deplete soil nutrients, which in turn reduces crop resilience and increases dependency on costly inorganic fertilisers. Converting woodland and hedgerows into fields, filling in ponds, draining marshes, and over-grazing destroys habitats relied upon by many species. Herbicides and pesticides can harm non-target species or species that depend on the targeted organisms. Inorganic fertiliser run-off can cause eutrophication in water courses.

Some adjustments can help balance agricultural productivity with conservation: maintaining hedgerows, rotating crops, reducing pesticide and herbicide application, and using organic rather than inorganic fertilisers.

Climate change compounds these pressures. Burning fossil fuels releases greenhouse gases causing global warming. As regional climates shift, suitable habitat ranges for many species expand or contract. Species adapted to specific temperature and rainfall conditions face displacement, and slow-moving species may be unable to shift their range quickly enough, risking extinction. Melting polar ice caps remove habitat for polar species. Rising sea levels increase flooding of low-lying terrestrial habitats. Warmer conditions also allow tropical diseases to spread into previously unsuitable regions.

Why Biodiversity Matters¶

Ecological Reasons¶

Healthy, functioning ecosystems depend on biodiversity because of the complex interdependencies within them. Food webs involve many species, so declines in one species can cascade through the web. Keystone species — those with a disproportionately large effect on their ecosystem — can destabilise entire communities if lost. Nutrient cycles rely on decomposers recycling carbon, nitrogen, sulphur, and phosphorus back through the ecosystem. High biodiversity provides resilience to climate shifts, disease, and other abiotic stresses.

Economic Reasons¶

Diverse ecosystems directly support many industries. They provide raw materials including timber, fabric, latex, and biofuels; wild pollinators are essential to crop reproduction; and many medicines originate from living organisms — aspirin from willow bark, morphine from opium poppies, and penicillin from fungi. Tourism based on natural landscapes and wildlife (ecotourism) generates significant income for many countries. Protecting genetic diversity also safeguards future economic development: wild relatives of cultivated crops carry genetic resources that can be used to breed new varieties with improved traits, and genetic diversity in crops acts as a safeguard against novel pests or diseases.

Monocultures that repeatedly grow single crops deplete soil nutrients and reduce farm productivity, creating economic risk. Maintaining ecological diversity in agricultural settings supports long-term yields and reduces dependence on expensive inputs.

Aesthetic Reasons¶

Biodiversity enriches the human environment and provides inspiration for artists, musicians, writers, and photographers. Contact with nature has been shown to support recovery from stress and injury. Maintaining diverse landscapes and ecosystems preserves the character of valued environments for future generations.

Conservation¶

Conservation is the preservation and careful management of the environment and of natural resources. Two broad strategies exist, each with distinct advantages and limitations.

In Situ Conservation¶

In situ conservation protects species in their natural habitats. Methods include national parks and wildlife reserves, marine conservation zones, controlling invasive or threatening species, protecting food sources and nesting sites, and making hunting of certain species illegal. In situ conservation allows larger populations to be maintained and gives the best chance of long-term recovery because natural ecosystem interactions, evolutionary pressures, and natural selection continue. However, protected species remain at risk from climate change, habitat degradation, and pollution.

Ex Situ Conservation¶

Ex situ conservation relocates or maintains species outside their natural habitats, used especially when species face imminent threat in the wild. Methods include relocating animals to safer regions, captive breeding programmes in zoos, maintaining plant collections in botanic gardens, and storing seeds in seed banks. Seed banks hold seeds under cool and dry conditions to maintain viability; seeds are periodically tested to ensure the collection remains functional. Species can be reintroduced to the wild once populations are stable, but reintroduced animals may struggle because they can lose resistance to local diseases, develop abnormal behaviours in captivity, carry genetic differences from the wild population, or find that suitable habitat is no longer available.

International And National Conservation Agreements¶

International cooperation is essential because many species migrate across national borders. The Rio Convention on Biological Diversity (CBD) aims to develop conservation strategies and the sustainable use of biological resources; it made biodiversity conservation an international legal responsibility and provides governments with guidance on protecting habitats. The Convention on International Trade in Endangered Species (CITES) makes international trading in endangered species illegal, regulates trade in wild plants, animals, and their products through licensing requirements, and raises awareness through education. The International Union for Conservation of Nature (IUCN) secures international conservation agreements and publishes the IUCN Red List of Threatened Species, detailing conservation status and population trends.

At a national level, the UK's Countryside Stewardship Scheme subsidises farmers to manage land sustainably and conserve landscapes. It supports actions such as allowing field margins with natural re-growth, protecting hedgerows, and controlling grazing intensity. This integrates conservation into everyday farming practice rather than treating it as separate from land management.

Common Confusions¶

A habitat can have high species richness but lower overall biodiversity if one species dominates — Simpson's Index captures this because low evenness lowers the D value even when many species are present. Sampling method determines the quality of the data; even a correctly calculated diversity index is only as reliable as the sample it is based on. Conservation arguments are multi-sided: ecological, economic, and aesthetic reasons must all be understood, not treated as purely sentimental or interchangeable.

Key Terms¶

• Biodiversity: the variety of life at habitat, species, and genetic levels.
• Habitat diversity: the range of different habitats in an area.
• Species richness: the total number of different species in a habitat.
• Species evenness: the relative abundance of each species; how evenly individuals are distributed across species.
• Genetic diversity: the variation in alleles within a population of a species.
• Polymorphic gene locus: a gene locus at which more than one allele exists in the population.
• Monomorphic gene locus: a gene locus at which only one allele exists.
• Proportion of polymorphic gene loci: number of polymorphic loci divided by total number of loci; a measure of genetic diversity.
• Gene flow: interbreeding between different populations that introduces new alleles, increasing genetic diversity.
• Genetic bottleneck: a sudden sharp reduction in population size that severely reduces genetic diversity.
• Founder effect: a reduction in genetic diversity when a small group establishes a new population.
• Genetic drift: random changes in allele frequency from generation to generation, which can reduce genetic diversity.
• Random sampling: sampling in which every location or organism has an equal probability of selection, using randomly generated coordinates.
• Systematic sampling: sampling at regular intervals, as in transect surveys.
• Stratified sampling: dividing the population into sub-groups (strata) and sampling each proportionally.
• Opportunistic sampling: sampling using conveniently available organisms; susceptible to bias.
• Quadrat: a square frame used to sample organisms in a defined area.
• Transect: a line along which samples are taken at regular intervals to study distribution changes across a gradient.
• Simpson's Index of Diversity (D): a measure combining species number and relative abundance, calculated as D = 1 − Σ(n/N)²; higher values indicate greater diversity.
• In situ conservation: conservation of species within their natural habitat.
• Ex situ conservation: conservation of species outside their natural habitat.
• CITES: Convention on International Trade in Endangered Species; makes trading endangered species internationally illegal.
• CBD: Convention on Biological Diversity; an international legal framework for biodiversity conservation and sustainable use.
• IUCN: International Union for Conservation of Nature; publishes the Red List of Threatened Species.
• Countryside Stewardship Scheme: a UK government scheme subsidising farmers to manage land sustainably.
• Monoculture: growing a single crop species over a large area, typically without rotation, which reduces biodiversity and depletes soil.
• Keystone species: a species with a disproportionately large effect on its ecosystem relative to its abundance.

Connected Pages¶

• 1.1.3 Analysis
• 3.1.3 Transport in plants
• 4.1.1 Communicable diseases, disease prevention and the immune system
• 4.2.2 Classification and evolution