Measuring Biodiversity

Biodiversity can be described qualitatively, but scientists often need to measure it so they can compare ecosystems, monitor changes over time, and evaluate the effects of human activity.

There is no single measurement that captures every aspect of biodiversity. Scientists therefore use several methods, including:

• counting species
• estimating abundance
• calculating diversity indices
• comparing samples from different habitats

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Species Richness

The simplest measure of biodiversity is species richness.

Species richness is: the number of different species present

Suppose two ponds are sampled.

Pond A

Contains:

• 8 plant species
• 5 insect species
• 3 fish species

Total species richness: 8 + 5 + 3 = 16

Pond B

Contains:

• 5 plant species
• 2 insect species
• 1 fish species

Total species richness: 5 + 2 + 1 = 8

Based on species richness alone, Pond A has greater biodiversity.

However, species richness does not tell us how evenly individuals are distributed among the species.

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Species Evenness

Species evenness describes how evenly individuals are distributed among different species.

Consider two ecosystems, each containing four species.

Ecosystem A

 

Ecosystem B

Both ecosystems have: 4 species

But Ecosystem A has much greater evenness.

Ecosystem B is dominated by one species.

Therefore, biodiversity depends on both:

Species richness + Species evenness

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Why Use a Biodiversity Index?

A biodiversity index combines information about the number of species and their abundances into a single numerical value.

This makes it easier to compare:

• different ecosystems
• the same ecosystem at different times
• disturbed and undisturbed habitats
• areas before and after conservation work

Different indices are used in ecology. For introductory work, a simple diversity index is often used.

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A Simple Biodiversity Index

One simple index is: \( D = \frac{S}{N} \)

where:

• D = simple biodiversity index
• S = number of species
• N = total number of organisms counted

A larger value generally indicates greater biodiversity under this simple measure.

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Worked Example 1

A student counts:

• 5 species
• 50 organisms in total

Calculate the biodiversity index.

\( D = \frac{S}{N} = \frac{5}{50} = 0.10 \)

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Worked Example 2

Another habitat contains:

• 8 species
• 40 organisms

\( D = \frac{S}{N} = \frac{8}{40} = 0.20 \)

Using this simple index, the second habitat has greater biodiversity.

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Simpson's Diversity Index

A more informative index is based on both richness and abundance.

One commonly used form is:

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

where:

• n = number of individuals of each species
• N = total number of organisms

Values closer to 1 generally indicate greater diversity.

Values closer to 0 indicate lower diversity.

Different textbooks may use slightly different versions of Simpson's index, so it is important to use the exact formula provided in your course.

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Worked Example: Simpson's Index

Suppose a habitat contains:

First calculate: N = 4 + 3 + 2 + 1 = 10

Now calculate n(n - 1):

So:

Σn(n - 1) = 12 + 6 + 2 + 0 = 20

Then:

$D=1-\frac{20}{10(9)}$$D=1-\frac{20}{90}$$D=1-0.222$$\boxed{D\approx0.778}$

This indicates a fairly high level of diversity.

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Sampling Biodiversity

It is often impossible to count every organism in an ecosystem.

Scientists therefore use sampling methods.

Common methods include:

• quadrats
• transects
• pitfall traps
• sweep nets
• kick sampling
• camera traps

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Quadrat Sampling

A quadrat is a square frame used to sample organisms in a small area.

Quadrats are especially useful for organisms that:

• do not move
• move very slowly

Examples include:

• plants
• mosses
• lichens
• barnacles

A quadrat may be placed randomly in different locations.

Within each quadrat, scientists can record:

• species present
• number of individuals
• percentage cover

Repeating the process provides a more representative sample.

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Random Sampling

Random sampling helps reduce bias.

If students only place quadrats where there appear to be many plants, their results may exaggerate biodiversity.

Instead, locations can be selected using:

• random coordinates
• random number generators
• predetermined grid systems

The goal is to give different parts of the habitat a fair chance of being sampled.

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Transects

A transect is a line placed across a habitat.

Scientists record organisms at intervals along the line.

Transects are useful when environmental conditions change gradually.

For example:

• from the edge of a forest into the centre
• from the top of a beach toward the water
• away from a polluted area
• across a hillside

Transects can reveal how species distribution changes with environmental conditions.

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Sampling Mobile Organisms

Quadrats are not suitable for many fast-moving animals.

Different methods may be used instead.

Pitfall Traps

Used for ground-dwelling invertebrates such as beetles.

Sweep Nets

Used to collect insects from grasses and shrubs.

Kick Sampling

Often used in streams to sample aquatic invertebrates.

Camera Traps

Used to record larger animals without needing to capture them.

Each method is suited to particular organisms and habitats.

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Interpreting Biodiversity Data

Suppose two woodlands are sampled.

Woodland A

Woodland B

Both have:

$4\text{ species}$

But Woodland A has much greater evenness.

Therefore, Woodland A would usually be considered more diverse.

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Comparing Ecosystems

A strong biodiversity comparison should use evidence.

Instead of saying:

Ecosystem A is better.

A stronger conclusion is:

Ecosystem A has greater biodiversity because it contains more species and the individuals are more evenly distributed among those species.

Useful evidence may include:

• species richness
• abundance
• evenness
• diversity-index values
• repeated sample results

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Example Comparison

Suppose:

Grassland A

• 12 species
• Simpson's diversity index = 0.86

Grassland B

• 7 species
• Simpson's diversity index = 0.54

We can conclude that Grassland A has greater biodiversity according to both:

• species richness
• diversity-index value

This provides stronger evidence than relying on just one measurement.

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Why Measurements Can Be Misleading

Biodiversity measurements are estimates.

Several factors can affect the results.

Sample Size

A very small sample may miss species.

In general:

More sampling → More reliable estimate

Time of Day

Some animals are active:

• during the day
• at night
• at dawn or dusk

Sampling at only one time may miss some species.

Season

Species abundance can change throughout the year.

For example:

• flowering plants may only be visible during some seasons
• migratory birds may only be present for part of the year

Weather

Rain, heat, cold, and wind can affect which organisms are observed.

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Sampling Method

The method used can also affect which species are found.

A sweep net may collect insects living in grasses but miss:

• soil organisms
• birds
• large mammals

A pitfall trap may sample ground-dwelling invertebrates but not flying insects.

Therefore, scientists often combine several sampling methods.

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Sampling Effort

Imagine two surveys.

Survey A uses:

• 2 quadrats

Survey B uses:

• 50 quadrats

Survey B will usually provide a more representative estimate because more of the habitat has been sampled.

This is called sampling effort.

Results should only be compared fairly when sampling effort is similar.

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Identification Errors

Biodiversity measurements also depend on correctly identifying species.

Some organisms are difficult to distinguish.

For example:

• closely related insects
• fungi
• microorganisms
• juvenile organisms

Misidentification can change both species richness and diversity-index calculations.

Modern studies sometimes use DNA barcoding to help identify species.

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Habitat Size

Larger habitats often contain more species simply because they contain:

• more space
• more resources
• more microhabitats

Therefore, comparisons should take habitat area into account.

Comparing a 1 m² garden with an entire forest would not be meaningful without controlling for sampling area.

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Human Disturbance

Human activity can strongly influence biodiversity.

For example, scientists might compare biodiversity:

• before and after deforestation
• upstream and downstream from pollution
• inside and outside a protected area
• before and after habitat restoration

Repeated measurements can show whether biodiversity is increasing or decreasing.

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Reliability and Repetition

A single biodiversity measurement may not be reliable.

Scientists therefore repeat samples.

For example:

10 quadrats → calculate average

is usually more reliable than:

1 quadrat → conclusion

Repeated measurements help reduce the influence of unusual local conditions.

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Worked Example: Comparing Two Samples

Two ponds are sampled using the same method.

Pond A

• 10 species
• 100 organisms

Simple index:

$D=\frac{10}{100}$$D=0.10$

Pond B

• 12 species
• 60 organisms

$D=\frac{12}{60}$$D=0.20$

According to this simple index:

$\boxed{\text{Pond B has greater biodiversity}}$

However, a stronger investigation would also examine the distribution of individuals among species.

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Worked Example: Interpreting Richness and Evenness

Consider:

Site A

• 6 species
• each species has about 10 individuals

Site B

• 9 species
• one species has 80 individuals
• the other 8 species have only 1–2 individuals each

Site B has greater species richness.

But Site A has greater evenness.

Therefore, which site is "more biodiverse" depends on the measurement used.

This is why diversity indices can be more useful than species counts alone.

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Designing a Fair Biodiversity Investigation

Suppose students want to compare a school lawn with a natural garden.

A fair investigation could:

1. use the same quadrat size
2. take the same number of samples
3. select locations randomly
4. sample at approximately the same time
5. identify organisms using the same method
6. calculate species richness and a diversity index
7. compare the results

This improves the validity and reliability of the comparison.

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Common Misconceptions

More organisms does not necessarily mean greater biodiversity.

A habitat containing 10,000 individuals of one species has low species diversity.

More species does not always tell the whole story.

Species evenness also matters.

One sample is rarely enough.

Biodiversity varies across space and time.

Different sampling methods cannot always be compared directly.

They may detect different organisms.

A biodiversity index is not an absolute measure of ecosystem quality.

It is one piece of evidence that must be interpreted in context.

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Did You Know?

Scientists can now measure biodiversity using DNA found directly in environmental samples.

This is called environmental DNA, or eDNA.

Organisms leave tiny amounts of DNA behind in:

• water
• soil
• sediment

Researchers can collect a water sample from a lake, for example, and analyse the DNA to detect species that may never have been directly observed.

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Key Terms

Biodiversity – The variety of life in an area.

Species richness – The number of different species present.

Species evenness – How evenly individuals are distributed among species.

Abundance – The number of individuals of a species.

Biodiversity index – A numerical measure used to describe biodiversity.

Quadrat – A frame used to sample organisms within a defined area.

Transect – A line used to sample organisms across an environmental gradient.

Random sampling – Selecting sample locations without deliberate bias.

Sampling effort – The amount of sampling carried out.

Reliability – The consistency of measurements when repeated.

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Key Takeaways

• Biodiversity can be measured using species richness, abundance, evenness, and biodiversity indices.
• Species richness is the number of species present.
• Species evenness describes how evenly individuals are distributed.
• A simple biodiversity index can be calculated using:

$\boxed{D=\frac{S}{N}}$

• Simpson's diversity index incorporates both species abundance and richness.
• Larger diversity-index values usually indicate greater biodiversity for commonly used forms of Simpson's index.
• Scientists use sampling because counting every organism is often impossible.
• Common methods include quadrats, transects, pitfall traps, sweep nets, kick sampling, and camera traps.
• Fair comparisons require similar sampling methods and effort.
• Biodiversity measurements can be affected by sample size, season, weather, time of day, habitat size, and identification accuracy.
• Repeated sampling improves reliability.
• Biodiversity data should be interpreted using evidence and context, not just a single number.
• Modern techniques such as eDNA allow scientists to detect species using genetic material left in the environment.