Biodiversity: Types, Species Richness and Diversity Indices

Flowering meadow with woodland behind it, photographed 22 April 2023. Photo: Nathanieljoyce, Wikimedia Commons, CC BY-SA 4.0; displayed without editing. The photograph illustrates habitat structure; species richness requires a survey.

Conservation and Biodiversity · Undergraduate biology and BS Botany

A field containing thousands of plants may have very little species diversity if nearly all belong to one species. To understand biodiversity, ask what varies, at which level and at which scale. These notes connect clear definitions with original examples, ecological sampling and worked calculations.

In this lesson

• Biodiversity: a precise definition
• The three main levels of biodiversity
• Species richness, abundance and evenness
• Worked example: Simpson diversity without formula confusion
• Alpha, beta and gamma diversity
• Functional, phylogenetic and taxonomic perspectives
• Endemic, native, rare and threatened are different terms
• How to investigate biodiversity reliably
• Why biodiversity matters and how to conserve it
• 20 revision MCQs
• 20-question interactive quiz

Biodiversity: a precise definition

Biodiversity is the variety and variability of living organisms, including variation within species, differences among species and the variety of ecosystems. It includes microorganisms, fungi and invertebrates as well as familiar plants and vertebrates. Counting visible plants is useful, but it does not describe all biodiversity.

Think of a landscape containing several wheat varieties, wild grasses and insects, plus a pond and woodland. Variation among the wheat plants illustrates genetic diversity; the different organisms illustrate species diversity; the pond and woodland illustrate ecosystem diversity. These are related levels, not three competing definitions.

Learning goals: distinguish the three main levels; separate richness, abundance and evenness; interpret alpha, beta and gamma diversity; calculate a defined Simpson variant; and plan a fair ecological comparison.

The three main levels of biodiversity

Levels, examples and suitable evidence

Level	What varies?	Example	Evidence
Genetic diversity	Inherited genetic variants within and among populations of a species	Different alleles in a wild plant population	DNA variation or other suitable genetic measurements
Species diversity	Species identity and relative abundance	A meadow with several grasses, herbs and insects	Species lists and standardized abundance data
Ecosystem diversity	Ecosystems, communities, processes and environmental settings	Woodland, freshwater wetland and grassland in a landscape	Habitat mapping and ecological surveys

Genetic variation: mutation can introduce new variants. Crossing over, independent assortment during meiosis and random fertilization reshuffle inherited variation in sexually reproducing organisms. Recombination does not explain the origin of every new allele, and every visible difference is not necessarily genetic: environment also affects phenotype.

Genetic diversity provides variation on which selection can act. Conserving crop varieties and wild relatives can retain useful traits for breeding, including responses to pests or environmental stress. This potential is valuable, but greater diversity does not guarantee that every population will survive every future change.

Species richness, abundance and evenness

Species richness (S) is the number of different species recorded in a defined community or sample. Abundance is the number of individuals of a species; relative abundance is its share of the total. Evenness describes how evenly individuals are distributed among species.

A sample of 100 plants is not automatically richer than one of 20 plants. The first might contain one species, and the second five. Likewise, two communities with four species can differ greatly if one is dominated by a single species.

Illustrative counts, not field measurements

Species	Community A	Community B
Species 1	25	97
Species 2	25	1
Species 3	25	1
Species 4	25	1
Total individuals (N)	100	100
Richness (S)	4	4

Richness is equal, but Community A is more even. Use comparable area, effort, season and identification methods when interpreting richness differences between real sites. A larger survey often detects more species simply because it samples more individuals and environmental variation.

Worked example: Simpson diversity without formula confusion

Different books use the name “Simpson index” for different expressions. Here we define dominance D = Σp_i², where p_i = n_i/N. We use its complement, the Gini–Simpson diversity = 1 − D. With this variant, a larger value indicates greater diversity.

Community A: p = 0.25 for each of four species.
D = 4 × (0.25)² = 0.25.
1 − D = 0.75.

Community B: p = 0.97, 0.01, 0.01, 0.01.
D = (0.97)² + 3 × (0.01)² = 0.9412.
1 − D = 0.0588.

This confirms the effect of dominance even though both communities contain four species. The complement is the probability that two independent draws from these relative abundances belong to different species. For fixed richness S, its maximum under equal abundances is 1 − 1/S; for four species that is 0.75, not 1.

Exam caution: reciprocal Simpson, 1/D, and finite-sample expressions using n(n − 1) and N(N − 1) also occur. They are related but do not give the same numerical value. Use the exact formula supplied in your course or examination, define symbols and interpret its direction correctly. An empty sample cannot support a meaningful abundance-based calculation.

Alpha, beta and gamma diversity

These terms describe spatial scale and compositional differences; they are not synonyms for genetic, species and ecosystem diversity.

• Alpha: diversity within a specified local unit, such as a plot or habitat.
• Beta: differences in species composition among units; several turnover or dissimilarity measures exist.
• Gamma: diversity of the combined region or landscape.

Species-list example:
Site A: a, b, c, d.
Site B: c, d, e, f.
Local richness is 4 at each site. Regional richness is 6, because c and d are counted once in the combined list.
For the explicitly defined multiplicative measure β = γ/mean α, β = 6/4 = 1.5.

Some texts instead report γ/mean α − 1; for this example it would be 0.5. State the convention rather than treating one number as the only possible beta-diversity value. Comparing species identities is essential: equal alpha richness does not imply identical communities.

Functional, phylogenetic and taxonomic perspectives

Functional diversity concerns differences in organismal traits relevant to ecological roles, such as leaf economics, body size or feeding strategy. It is not restricted to marine ecosystems. Phylogenetic diversity concerns the evolutionary history represented by a set of organisms, often assessed through branches on a phylogeny.

Taxonomic diversity concerns taxonomic composition; some measures also account for taxonomic distinctness. Taxonomy and phylogeny are related, but counting species is not the same as measuring evolutionary branch length.

Two species-rich communities can differ in their traits and evolutionary histories. Recent plant-community research has also found that functional and phylogenetic diversity need not track one another closely. For a BS-level answer, identify the facet and the metric before interpreting “high diversity.”

Endemic, native, rare and threatened are different terms

An endemic species naturally occurs within a particular geographic region and nowhere else. A native species occurs naturally in the area under discussion but may occur in other regions too. Rare describes low abundance or restricted occurrence in a stated context; threatened concerns extinction risk under a stated assessment system.

A species can be both endemic and threatened, but endemism alone does not establish its risk category. Define the geographic scale: endemic to an island is different from endemic to a continent. As a real example, the US National Park Service lists Yellowstone sand verbena among Yellowstone's endemic plants.

Introduced species are not automatically invasive. An invasive non-native species causes harmful effects in the region concerned. A simple richness increase after an introduction therefore need not be a conservation improvement.

How to investigate biodiversity reliably

1. Define the question and target group. A survey of vascular plants answers a narrower question than an inventory of all organisms.
2. Choose a suitable method. Random quadrats can estimate plant abundance in a defined area; systematic sampling along a transect can examine change across an environmental gradient.
3. Standardize effort. Keep quadrat size, number of samples, search duration, season and identification rules comparable.
4. Record identity and abundance. For plants that cannot be separated reliably into individuals, justified cover or frequency measurements may be more suitable.
5. Replicate and document. Retain species names, methods, location context and uncertainty. Take permitted reference photographs rather than damaging protected organisms.
6. Interpret cautiously. Non-detection is not proof of absence, and a single survey is not proof of a long-term trend.

Research inventories use evidence such as observations, specimens and documented surveys. No country-wide claim about the absence of biodiversity institutions is needed to explain sampling. Reliable notes should point students toward identifiable studies and institutions rather than unsupported generalizations.

Why biodiversity matters and how to conserve it

Biodiversity contributes to food resources, pollination, nutrient cycling, cultural benefits and options for future crop improvement. Different organisms contribute differently; no single diversity score fully represents all ecosystem functions or conservation values.

The IPBES 2019 Global Assessment identified major direct pressures including land and sea use change, direct exploitation, climate change, pollution and invasive alien species. Their importance differs across habitats and regions. For a detailed pollution mechanism, continue to air pollution and biodiversity.

In situ conservation protects organisms in their natural settings through habitat protection, restoration and suitable management. Ex situ conservation works outside those settings, for example through seed banks or living collections. These approaches can complement one another; a stored seed collection does not preserve the whole functioning habitat.

Exam answer structure: name the biodiversity level → give a biological example → explain a threat or benefit → identify suitable evidence → propose a matched conservation action. Avoid claiming that richness alone proves a habitat is healthy or that a single global species estimate is a settled fact.

Frequently asked questions

Are the three biodiversity levels independent of one another?

They describe different aspects of living systems but interact. Genetic variation occurs within the species inhabiting ecosystems, and ecological processes influence which populations persist. Measuring one level does not automatically measure the others.

Why can two equally species-rich communities have different diversity scores?

Their relative abundances can differ. A community dominated by one species has lower evenness than an otherwise comparable community with equal abundances. An index that uses abundances can reflect this difference even when richness is identical.

Does a larger Simpson number always mean greater diversity?

Only after identifying the formula. The dominance expression sum pi squared increases with concentration in common species; its complement 1-D and reciprocal 1/D move in the opposite direction. Report the definition alongside the value.

Is beta diversity simply the difference between two richness counts?

No. Communities with equal richness can contain entirely different species. Beta diversity addresses compositional differences, and several metrics are used. A richness-based ratio such as gamma/mean alpha is one explicitly defined convention.

Can a quadrat survey measure all biodiversity in a habitat?

No. It samples selected organisms under particular conditions. A plant quadrat survey does not directly assess microbial diversity, genetic variation or every mobile animal. Match the method to the question and state the limits of inference.

Why conserve both seeds and natural habitats?

Seed collections can preserve plant material and genetic resources, while habitats retain ecological interactions and ongoing population processes. Their strengths differ, so ex situ and in situ conservation can be complementary.

20 revision MCQs with explained answers

Choose one answer before opening the explanation. The interactive quiz below uses a different set of questions.

1. Which observation primarily demonstrates genetic diversity?
   1. Different inherited alleles within one plant species
   2. Different plant species within one quadrat
   3. A wetland and woodland in one landscape
   4. Different daily temperatures in one meadow
   Answer and explanation

   A. Different inherited alleles within one plant species — Genetic diversity concerns inherited variation within and among populations of a species.

2. What is species richness in a defined sample?
   1. The proportion belonging to the most common species
   2. The number of different species recorded
   3. The total count of all individuals
   4. The biomass of the dominant species
   Answer and explanation

   B. The number of different species recorded — Richness counts species, whereas abundance counts individuals.

3. Two samples each contain five species. What additional information is needed to compare evenness?
   1. Only the total survey area
   2. Only the altitude of each site
   3. The distribution of individuals among the species
   4. Only the name of the rarest species
   Answer and explanation

   C. The distribution of individuals among the species — Evenness concerns how relative abundances are distributed.

4. Which process can introduce a new genetic variant?
   1. Sorting existing species names alphabetically
   2. Measuring percentage plant cover
   3. Redistribution of individuals among quadrats
   4. Mutation
   Answer and explanation

   D. Mutation — Mutation can change genetic material; recombination reshuffles existing variation.

5. Which statement about phenotype is sound?
   1. Differences may reflect genetic factors, environmental factors or both.
   2. Every visible difference identifies a new allele.
   3. All individuals with different heights belong to different species.
   4. Environmental effects make inherited variation impossible.
   Answer and explanation

   A. Differences may reflect genetic factors, environmental factors or both. — Phenotypic variation alone does not establish a purely genetic cause.

6. What best illustrates ecosystem diversity?
   1. Equal abundance among three plant species
   2. A landscape containing woodland, grassland and freshwater wetland
   3. Several alleles at a locus in one grass species
   4. More individuals of one species in a field
   Answer and explanation

   B. A landscape containing woodland, grassland and freshwater wetland — Ecosystem diversity concerns different ecological systems and their interacting components.

7. What does relative abundance pi represent here?
   1. The number of species divided by sampling area
   2. The maximum plant height divided by quadrat width
   3. The count for species i divided by the total count
   4. The total count divided by the count for species i
   Answer and explanation

   C. The count for species i divided by the total count — Here pi equals ni/N and the proportions sum to one for a non-empty sample.

8. Which expression is the Gini–Simpson complement used in these notes?
   1. The sum of squared species proportions
   2. The reciprocal of species richness
   3. The total individuals divided by sampling area
   4. 1 minus the sum of squared species proportions
   Answer and explanation

   D. 1 minus the sum of squared species proportions — Define the variant: 1 minus D increases with diversity; D itself measures dominance.

9. For four species with equal relative abundance, what is 1 minus sum pi squared?
   1. 0.75
   2. 0.25
   3. 4
   4. 1
   Answer and explanation

   A. 0.75 — Four proportions of 0.25 give D=0.25 and the complement=0.75.

10. What happens to this complement as one species dominates, with richness held constant?
    1. It must exceed one.
    2. It generally decreases.
    3. It must stay unchanged.
    4. It becomes equal to total abundance.
    Answer and explanation

    B. It generally decreases. — Greater dominance increases the sum of squared proportions, reducing its complement.

11. Which term concerns local within-site diversity?
    1. Gamma diversity
    2. Genetic differentiation alone
    3. Alpha diversity
    4. Beta diversity
    Answer and explanation

    C. Alpha diversity — Alpha refers to the specified local sampling unit or community.

12. Which comparison is central to beta diversity?
    1. Only the number of individuals in one species
    2. Only DNA variation within one individual
    3. Only total biomass of the pooled region
    4. Differences in species composition among sites
    Answer and explanation

    D. Differences in species composition among sites — Beta describes among-site differences; different mathematical conventions exist.

13. What is gamma richness for two sites?
    1. The number of distinct species in their combined list
    2. The sum of counts of every individual
    3. Always the sum of their separate species counts
    4. Always the richness of the larger site only
    Answer and explanation

    A. The number of distinct species in their combined list — Shared species are counted once in the pooled regional list.

14. Which measure explicitly incorporates evolutionary history?
    1. Species frequency alone
    2. Phylogenetic diversity
    3. Total abundance
    4. Plant percentage cover
    Answer and explanation

    B. Phylogenetic diversity — Phylogenetic measures use relationships and, in common approaches, represented branch lengths.

15. Which evidence is most relevant to functional diversity?
    1. Only the alphabetical spread of species names
    2. Only the mass of a soil sample
    3. Variation in ecologically relevant traits among organisms
    4. Only the number of samples taken
    Answer and explanation

    C. Variation in ecologically relevant traits among organisms — Functional diversity concerns trait differences associated with ecological roles.

16. What does endemic mean at a stated geographic scale?
    1. Numerically common in that region
    2. Introduced recently into that region
    3. Automatically endangered in that region
    4. Naturally restricted to that region
    Answer and explanation

    D. Naturally restricted to that region — Endemism describes geographic restriction, not abundance or a risk category by itself.

17. How do native and invasive non-native species differ?
    1. Native status concerns natural occurrence; invasiveness includes harmful effects of a non-native species.
    2. Every native species is endemic.
    3. Every introduced species is invasive.
    4. Native status is determined only by current abundance.
    Answer and explanation

    A. Native status concerns natural occurrence; invasiveness includes harmful effects of a non-native species. — Origin and ecological impact are different aspects of the classification.

18. Why use random locations for a plant quadrat survey?
    1. To make all habitats equally suitable for plants
    2. To reduce deliberate placement bias in the defined survey area
    3. To guarantee discovery of every species
    4. To eliminate the need for replication
    Answer and explanation

    B. To reduce deliberate placement bias in the defined survey area — Random placement helps limit selection bias but cannot guarantee complete detection.

19. What is a suitable purpose of a systematic transect?
    1. Replace species identification with total counts
    2. Prove that all site differences have one cause
    3. Examine distributions along an environmental gradient
    4. Determine every allele in a population
    Answer and explanation

    C. Examine distributions along an environmental gradient — Regular sampling along a gradient can describe changes in distribution.

20. Which is an ex situ conservation example?
    1. Restoring a wild wetland on site
    2. Protecting a woodland population in its habitat
    3. Managing grazing in a native grassland
    4. A documented seed collection maintained outside the natural habitat
    Answer and explanation

    D. A documented seed collection maintained outside the natural habitat — Ex situ conservation occurs outside the natural setting and can complement habitat protection.

Interactive quiz: 20 new application questions

One correct option per question. Select your answers, check your score, then use the explanations to revise. Your answers stay in this browser; this quiz does not send them to a server.

1. A sample contains counts 40, 40 and 20 for three species. What is its richness?

A. 3B. 20C. 40D. 100

2. In the same 100-individual sample, one species has 20 individuals. What is its relative abundance?

A. 20B. 0.20C. 0.02D. 5

3. Two communities have counts 10,10,10 and 28,1,1. Which conclusion is justified?

A. Both have identical evenness because N is equal.B. The first contains more species because all are common.C. They have equal richness but the first is more even.D. The second has greater richness because it has a dominant species.

4. A community has proportions 0.5, 0.3 and 0.2. What is its Gini–Simpson complement?

A. 0.38B. 0.50C. 0.20D. 0.62

5. A researcher reports D=sum pi squared falling from 0.7 to 0.4. What follows for 1-D?

A. It rises from 0.3 to 0.6.B. It falls from 0.7 to 0.4.C. It remains at 0.3.D. It becomes undefined despite both samples being non-empty.

6. An exam gives an index formula different from the one in these notes. What should a student do?

A. Ignore the abundance data and report richness only.B. Use the supplied formula and its interpretation consistently.C. Substitute 1-D without stating the change.D. Assume all Simpson variants give identical numbers.

7. Site A contains a,b,c,d; Site B contains c,d,e. What is combined gamma richness?

A. 4B. 2C. 5D. 7

8. Two sites each contain four species and their union contains six. For beta=gamma/mean alpha, what is beta?

A. 0.5B. 2C. 6D. 1.5

9. Two sites each contain four species but share none. What can be said compared with identical four-species lists?

A. They show greater compositional difference.B. Their beta diversity must be zero because richness is equal.C. They necessarily have different total abundance.D. The regional list still contains only four species.

10. A class finds fewer plant species in two small quadrats than another class finds in twenty large quadrats. What should be addressed first?

A. The conclusion that larger samples never affect detectionB. Unequal sampling area and effortC. Assumed extinction of every undetected speciesD. The need to replace all species names with common names

11. A surveyor places every quadrat beside colourful flowers. What is the main problem?

A. Loss of all genetic variation in the meadowB. Automatic inability to identify any speciesC. Selection bias toward visibly attractive vegetationD. Random sampling error alone, with no placement bias

12. Plant distribution is studied from a shaded forest edge into an open field. Which plan best matches the question?

A. One large quadrat chosen only in full sunB. Pooling all locations without recording positionC. Sampling only the commonest species by eyeD. Replicated transects with standardized quadrats and light measurements

13. One plant species is not found during a short survey. What conclusion is defensible?

A. It was not detected under that survey's conditions.B. It is certainly globally extinct.C. It cannot occur at any other season.D. Its absence proves that the site has no genetic diversity.

14. Plants of one species differ in height between wet and dry plots. Which follow-up helps investigate a genetic explanation?

A. Replace genotype measurements with ecosystem labels.B. Compare inherited variation while accounting for environmental effects.C. Count each height class as a separate species immediately.D. Assume moisture has no influence on growth.

15. A seed bank holds several varieties of one crop species. Which level is directly represented by the varieties?

A. Beta diversity between forests onlyB. A count of different crop species necessarilyC. Genetic diversityD. Regional ecosystem diversity

16. A landscape retains woodland, marsh and grassland, each with distinct ecological communities. What is most directly represented?

A. Only genetic diversity at one locusB. Only evenness within one quadratC. Only total abundance of the dominant grassD. Ecosystem diversity

17. Two meadows have equal richness but different leaf traits among their plants. Which facet may differ?

A. Functional diversityB. Richness itself necessarilyC. Total survey area necessarilyD. The number of recorded species necessarily

18. A plant occurs naturally in several countries but nowhere outside that wider region. How should endemism be described?

A. Invasive solely because it crosses political bordersB. Relative to the wider region, rather than automatically to each countryC. Endemic to every country in which it appearsD. Endangered solely because it is native

19. An introduced species raises local richness while displacing a protected native plant. What is the best interpretation?

A. The introduced species must be native because it now survives.B. Displacement cannot occur if richness rises.C. Richness increased, but the conservation outcome may be adverse.D. Higher richness proves that all conservation objectives improved.

20. A protected habitat is lost, but its rare plant is maintained in cultivation. Which statement is sound?

A. Cultivation restores all original ecological interactions.B. The habitat is conserved in situ merely because the plant survives elsewhere.C. The species must have lost all genetic diversity.D. Ex situ protection preserves some material but does not replace the lost ecosystem.

Continue learning

• Air pollution and biodiversity: mechanisms and control — apply the definitions to a conservation threat.
• Gene linkage and crossing over — follow up the reshuffling of inherited variation.
• Conservation and Biodiversity course — continue studying ecological threats and protection.

Sources and further reading

These independently written notes and teaching examples can be checked against the following curriculum and scientific references (the lesson is independently written and is not endorsed by these organizations).

• Convention on Biological Diversity: Article 2 definitions
• CBD: Biodiversity and sustainable development
• AQA A-level Biology: genetic variation and biodiversity
• OpenStax Biology 2e: community ecology
• Oxford NbS Knowledge Hub: alpha, beta and gamma diversity
• vegan documentation: definitions of diversity indices
• AQA: field investigation practical handbook
• AQA Environmental Science: research methods
• US National Park Service: species inventories and evidence
• US National Park Service: native and invasive species
• US National Park Service: Yellowstone plants and endemism
• FAO: conservation and sustainable use of crop diversity
• IPBES: 2019 Global Assessment, direct drivers of biodiversity loss
• Chao, Chiu and Jost (2014): diversity measures and Hill numbers
• 2024 plant-community research: functional and phylogenetic diversity