Measuring Biodiversity

HSC Biology | Free Study Notes

In this lesson

• what abundance means

• what distribution means

• what species richness means

• how field data is collected in basic biodiversity studies

  
  

Why biodiversity is measured

Ecologists do not just describe ecosystems, they measure them.

Measuring biodiversity helps scientists:

• compare one habitat with another

• detect change over time

• monitor the effects of environmental pressures

• make decisions about conservation and management

Abundance

Abundance is the number of individuals of a species in a given area.

What this means

If a survey finds:

• 5 eucalyptus trees in one quadrat

• 40 grass plants in another quadrat

the grass plants have a higher abundance in that sampled area.

Why abundance matters

Abundance helps show:

• how common or rare a species is

• whether a population is increasing or decreasing

• how strongly a species is established in a habitat

Important point

Abundance is about the number of individuals, not just whether the species is present.

Distribution

Distribution is where a species is found within an area.

What this means

A species may be:

• spread evenly through a habitat

• found in clumps

• limited to one small part of the habitat

Why distribution matters

Distribution helps scientists understand:

• habitat preferences

• the effects of abiotic factors such as light, water or soil

• the effects of biotic factors such as competition or predation

Example idea

A plant might only grow in damp, shaded parts of a forest. Its distribution would then be limited rather than widespread.

Species richness

Species richness is the number of different species in a given area.

What this means

If one habitat contains:

• 12 different species

and another contains:

• 5 different species

the first habitat has greater species richness.

Why species richness matters

Species richness gives a simple measure of biodiversity because it focuses on how many different species are present.

An HSC question on species diversity used the number of species in different habitats, which matches this basic idea. 

Important point

Species richness does not show how many individuals of each species are present. It only counts how many different species there are.

Field data basics

Field data is the information collected during biodiversity surveys in the environment.

What field data may include

Basic field data can include:

• the species present

• the number of individuals

• where each species is found

• abiotic factors such as temperature, light or soil moisture

The Module 3 content focus specifically notes that monitoring biodiversity, including abiotic factors, helps ecologists predict future change. 

Basic sampling methods

At this level, students should understand the idea of using sampling techniques rather than trying to count every organism in a whole ecosystem.

Quadrat sampling

A quadrat is a square frame placed on the ground to sample organisms in a small area.

It is useful for:

• plants

• slow-moving organisms

Quadrats can help measure:

• abundance

• species richness

• distribution across a site

Transects

A transect is a line laid across a habitat, with samples taken at intervals along it.

It is useful for:

• studying changes across an environmental gradient

• looking at how distribution changes from one place to another

Why sampling is used

Sampling makes biodiversity studies:

• more practical

• more efficient

• easier to repeat and compare

The NSW Biology syllabus explicitly includes measuring populations using sampling techniques as practical fieldwork content.  

Linking abundance, distribution and species richness

These three ideas measure different parts of biodiversity.

Abundance

How many individuals of one species are present?

Distribution

Where is that species found?

Species richness

How many different species are present?

Together, these give a clearer picture of biodiversity than any one measure alone.

Example of field data use

Imagine two grassland sites.

Site A

• 8 species present

• one species is extremely common

• several others are rare

Site B

• 4 species present

• all species spread widely through the site

From this, you could say:

• Site A has greater species richness

• one species in Site A may have very high abundance

• Site B may show broader distribution for its species

This is why field data needs careful interpretation.

Why these measures matter in Module 3

Module 3 asks students to investigate how environmental pressures affect species diversity and abundance over time. To do that well, students need to understand how biodiversity is measured in the first place. Monitoring abundance, distribution and species richness gives evidence for change in ecosystems. 

Worked example

Exam-style question

Explain the difference between abundance and species richness.

Worked answer

Abundance is the number of individuals of a particular species in an area. Species richness is the number of different species in that area. A habitat can have high abundance of one species but low species richness if only a few species are present.

Why this works

This answer:

• defines both terms clearly

• compares them directly

• shows that they measure different things

Common mistakes

• Mixing up abundance and species richness.

• Thinking species richness means the number of organisms, rather than the number of species.

• Forgetting that distribution is about where organisms are found.

• Assuming a habitat with many individuals automatically has high biodiversity.

• Ignoring field data from abiotic factors when explaining distribution.

Quick quiz

1. What is abundance?

2. What is distribution?

3. What is species richness?

4. Why are quadrats useful in biodiversity studies?

5. Why might two habitats with the same abundance still differ in biodiversity?

   
   

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