Classification and Biodiversity
5. Measuring Biodiversity
Learning outcomes
- I can define biodiversity.
- I can identify factors that affect biodiversity.
- I can explain why biodiversity is important.
- I can interpret biodiversity data.
- I can compare biodiversity between ecosystems.
What Is Biodiversity?
Biodiversity means the variety of living organisms in an area.
An ecosystem with many different organisms and a balanced range of species generally has greater biodiversity than an ecosystem containing only a few species.
Biodiversity can be considered at several levels:
- Genetic diversity – variation in genes within a species.
- Species diversity – variety of species within an area.
- Ecosystem diversity – variety of habitats and ecosystems within a region.
When ecologists measure biodiversity within a particular ecosystem, they often focus on species diversity.
Why Measure Biodiversity?
Scientists measure biodiversity to understand the condition of ecosystems.
Biodiversity measurements can help scientists:
- Compare different ecosystems.
- Detect environmental change.
- Monitor habitat destruction.
- Study the effects of pollution.
- Identify areas requiring conservation.
- Monitor recovery after restoration.
- Investigate the effects of climate change.
- Determine whether species are disappearing or recovering.
A biodiversity survey can therefore provide evidence about how an ecosystem changes over time.
Species Richness
One of the simplest ways to measure biodiversity is species richness.
Species richness is the number of different species present in an area.
For example:
Ecosystem A contains 5 species.
Ecosystem B contains 12 species.
Ecosystem B has greater species richness.
However, species richness does not tell us how many individuals of each species are present.
Worked Example: Species Richness
Scientists investigate two ponds.
Pond A
- 4 species of aquatic plants.
- 6 species of insects.
- 3 species of fish.
- 2 species of amphibians.
Total species richness:
4 + 6 + 3 + 2 = 15 species
Pond B
- 3 species of aquatic plants.
- 4 species of insects.
- 2 species of fish.
- 1 species of amphibian.
Total species richness:
3 + 4 + 2 + 1 = 10 species
Based only on species richness:
Pond A has greater biodiversity.
However, we would need information about the abundance of each species for a more complete comparison.
Species Abundance
Species abundance refers to the number of individuals of a particular species in an area.
Imagine a woodland containing:
- 100 oak trees.
- 40 pine trees.
- 25 birch trees.
- 10 maple trees.
The species richness is:
4 species
The abundance of oak trees is:
100 individuals
Richness and abundance therefore describe different aspects of a biological community.
Species Evenness
Species evenness describes how evenly individuals are distributed among the species in a community.
Consider two ecosystems.
Ecosystem A
- Species 1 = 25 individuals
- Species 2 = 25
- Species 3 = 25
- Species 4 = 25
Ecosystem B
- Species 1 = 97 individuals
- Species 2 = 1
- Species 3 = 1
- Species 4 = 1
Both ecosystems contain four species.
Therefore, they have the same species richness.
However, Ecosystem A has much greater evenness.
Why Evenness Matters
Imagine an ecosystem containing ten species.
If approximately equal numbers of each species are present, the community has high evenness.
Now imagine another ecosystem containing the same ten species, but 95% of all organisms belong to one species.
The two ecosystems have the same species richness but very different community structures.
This is why scientists often consider both:
Richness + Evenness
when assessing biodiversity.
Species Diversity
Species diversity considers both:
- The number of species present.
- Their relative abundances.
Therefore:
Species richness alone is not always enough to describe biodiversity.
Two ecosystems can contain the same number of species but have very different species diversity.
Worked Example: Comparing Communities
Consider two grasslands.
Grassland A
- Species A = 20
- Species B = 20
- Species C = 20
- Species D = 20
Grassland B
- Species A = 74
- Species B = 2
- Species C = 2
- Species D = 2
Both have:
Species richness = 4
However, Grassland A has much greater evenness.
Therefore, Grassland A would have a higher diversity value using an index that considers both richness and evenness.
Biodiversity Indices
Scientists sometimes combine richness and abundance information into a biodiversity index or diversity index.
Several different indices are used in ecology.
One commonly used measure is Simpson's Diversity Index.
Different textbooks use slightly different forms of the equation, so it is important to check which version is being used.
One common form is:
D = 1 - Σ[n(n - 1) / N(N - 1)]
where:
n = number of individuals of one species
N = total number of individuals of all species
A value closer to 1 generally represents greater diversity when this version of the index is used.
Why Use a Diversity Index?
Suppose two forests each contain 20 species.
Simply counting species suggests they have equal biodiversity.
However:
Forest A contains reasonably similar numbers of each species.
Forest B is dominated by one species.
A diversity index can detect this difference because it considers relative abundance, not simply the number of species.
Worked Example: A Simple Biodiversity Dataset
Scientists sample two ecosystems.
Ecosystem A
- Beetle = 30
- Ant = 25
- Spider = 20
- Grasshopper = 25
Total organisms = 100
Ecosystem B
- Beetle = 85
- Ant = 5
- Spider = 5
- Grasshopper = 5
Total organisms = 100
Both ecosystems have:
Species richness = 4
But Ecosystem A has much greater evenness.
Therefore, Ecosystem A has greater species diversity.
Interpreting Biodiversity Tables
Ecologists often record biodiversity data in tables.
For example:
| Species | Forest A | Forest B |
|---|---|---|
| Oak | 25 | 80 |
| Pine | 30 | 5 |
| Birch | 20 | 10 |
| Maple | 25 | 5 |
Both forests contain four species.
But Forest A has a much more even distribution.
Forest B is dominated by oak trees.
Therefore, Forest A has greater species diversity even though both have the same species richness.
Interpreting Biodiversity Graphs
Biodiversity data may also be displayed using:
- Bar graphs.
- Pie charts.
- Line graphs.
- Species accumulation curves.
- Maps.
When interpreting biodiversity data, ask:
- How many species are present?
- Which species is most abundant?
- Which species is least abundant?
- Are the species evenly distributed?
- Is one species dominant?
- How does one ecosystem compare with another?
- Has biodiversity changed over time?
Sampling Biodiversity
Scientists cannot usually count every organism in an ecosystem.
Instead, they take samples.
A sample is a smaller part of the ecosystem used to estimate characteristics of the whole area.
Common sampling techniques include:
- Quadrats.
- Transects.
- Pitfall traps.
- Sweep nets.
- Kick sampling.
- Camera traps.
Different techniques are appropriate for different organisms and habitats.
Quadrat Sampling
A quadrat is a frame placed over a known area.
Quadrats are particularly useful for organisms that:
- Do not move.
- Move very slowly.
- Are distributed across the ground.
Examples include:
- Plants.
- Mosses.
- Lichens.
- Barnacles.
Scientists identify and count organisms inside several quadrats.
The samples can then be used to estimate abundance and distribution across a larger area.
Worked Example: Quadrat Sampling
A student places five quadrats in a grassland.
The numbers of daisies found are:
12, 8, 10, 15, 5
Total daisies:
12 + 8 + 10 + 15 + 5 = 50
Mean number per quadrat:
50 ÷ 5 = 10 daisies per quadrat
If each quadrat represents 1 m², the estimated density is:
10 daisies per m²
Random Sampling
Sampling should often be random to reduce bias.
If a student deliberately places every quadrat where the most flowers are visible, the results will overestimate flower abundance.
Random sampling gives different locations a fair chance of being selected.
Scientists may use:
- Random coordinates.
- Random-number generators.
- Grid systems.
This helps produce more representative data.
Sample Size
The number of samples collected affects reliability.
Imagine estimating the biodiversity of an entire forest using only one quadrat.
That quadrat might not represent the forest accurately.
Using more samples generally:
- Reduces the influence of unusual samples.
- Provides a better representation of the habitat.
- Improves confidence in the estimate.
Therefore, ecologists usually collect multiple samples.
Transects
A transect is a line placed across an ecosystem.
Scientists record organisms at points along the line or within quadrats placed along it.
Transects are particularly useful when investigating how organisms change across an environmental gradient.
For example, scientists might investigate how plant species change:
- Moving away from a river.
- Moving up a mountain.
- Across a rocky shore.
- Moving away from a road.
- From sunlight into shade.
Biotic and Abiotic Factors
Biodiversity is influenced by both biotic and abiotic factors.
Biotic factors involve living organisms.
Examples include:
- Competition.
- Predation.
- Disease.
- Food availability.
- Invasive species.
Abiotic factors are non-living environmental conditions.
Examples include:
- Temperature.
- Water availability.
- Light.
- Soil pH.
- Salinity.
- Oxygen concentration.
- Nutrient availability.
Climate and Biodiversity
Climate strongly affects biodiversity.
Important climatic factors include:
- Temperature.
- Rainfall.
- Seasonal variation.
- Sunlight.
Warm, wet environments can support very high biological productivity and many ecological niches.
This helps explain why tropical rainforests contain extremely high species diversity.
Habitat Diversity
A region containing many different habitats can often support more species than a very uniform environment.
For example, an area containing:
- Forest.
- Streams.
- Grassland.
- Wetlands.
- Rocky areas.
provides many different ecological conditions.
Different organisms can occupy different niches.
Greater habitat variety can therefore contribute to greater biodiversity.
Habitat Size
Larger habitats often support more species than smaller habitats.
A larger habitat may contain:
- More resources.
- More individuals.
- More microhabitats.
- Greater environmental variation.
Small isolated habitats may support smaller populations, which can increase the risk of local extinction.
Disturbance
Environmental disturbances can alter biodiversity.
Examples include:
- Fires.
- Floods.
- Storms.
- Droughts.
- Volcanic eruptions.
- Human land clearing.
The effects depend on factors such as:
- Intensity.
- Frequency.
- Duration.
- Ecosystem type.
Some ecosystems are naturally adapted to periodic disturbances such as fire.
Very severe or frequent disturbances can greatly reduce biodiversity.
Human Activities and Biodiversity
Human activities can strongly affect biodiversity.
Major pressures include:
- Habitat destruction.
- Habitat fragmentation.
- Pollution.
- Overharvesting.
- Introduction of invasive species.
- Climate change.
These pressures can reduce population sizes, eliminate habitats, and increase extinction risk.
Habitat Destruction
Habitat destruction occurs when natural habitats are removed or dramatically altered.
Examples include:
- Deforestation.
- Wetland drainage.
- Urban development.
- Mining.
- Conversion of natural ecosystems into farmland.
When habitat disappears, organisms may lose:
- Food.
- Shelter.
- Breeding sites.
- Territory.
Species unable to move or adapt may decline.
Habitat Fragmentation
Habitat fragmentation occurs when a large continuous habitat is divided into smaller isolated areas.
For example, roads and agricultural land may divide a forest into separate patches.
Fragmentation can:
- Isolate populations.
- Reduce movement between populations.
- Reduce access to resources.
- Reduce gene flow.
- Increase local extinction risk.
Even if some habitat remains, fragmentation can therefore reduce biodiversity.
Pollution
Pollution can change environmental conditions and make habitats unsuitable for some organisms.
Examples include:
- Oil pollution.
- Plastic pollution.
- Pesticides.
- Fertilizer runoff.
- Heavy metals.
- Air pollution.
Sensitive species may disappear while pollution-tolerant species survive.
A community may therefore become less diverse.
Invasive Species
An invasive species is a non-native species that spreads and causes ecological, economic, or other significant harm.
Invasive species may:
- Compete with native species.
- Prey on native organisms.
- Spread disease.
- Alter habitats.
- Disrupt food webs.
Native species that have not evolved alongside the introduced species may be particularly vulnerable.
Why Biodiversity Is Important
Biodiversity is important for several reasons.
It contributes to:
- Ecosystem functioning.
- Food production.
- Pollination.
- Nutrient cycling.
- Soil formation.
- Water purification.
- Medicines and useful biological materials.
- Genetic resources.
- Cultural and recreational value.
Biodiversity also increases the range of biological responses available when environmental conditions change.
Biodiversity and Ecosystem Stability
More diverse ecosystems can sometimes be more resilient to environmental change because several species may perform similar ecological roles.
For example, if one pollinator species declines, another species may continue some pollination.
However, the relationship between biodiversity and ecosystem stability is complex.
High biodiversity does not make an ecosystem immune to disturbance.
Instead, biodiversity can contribute to ecosystem functioning and resilience.
Biodiversity and Food
Humans depend on biodiversity for food.
Agriculture uses:
- Crop species.
- Livestock species.
- Pollinators.
- Soil organisms.
- Natural predators of pests.
Wild relatives of crop plants can also contain useful genes for characteristics such as:
- Disease resistance.
- Drought tolerance.
- Heat tolerance.
Maintaining genetic diversity can therefore be important for future food security.
Biodiversity and Medicine
Living organisms produce enormous numbers of chemical substances.
Some have been used in developing medicines.
Plants, fungi, bacteria, and animals can therefore provide valuable biological compounds.
If species become extinct before they are studied, potentially useful biological information may also be lost.
Genetic Diversity
Biodiversity exists within species as well as between species.
Genetic diversity refers to genetic variation among individuals within a population or species.
High genetic diversity can increase the chance that some individuals possess characteristics that allow them to survive environmental changes or diseases.
Low genetic diversity can make populations more vulnerable.
Ecosystem Diversity
Ecosystem diversity refers to the variety of ecosystems or habitats within a region.
For example, a region containing:
- Forests.
- Rivers.
- Wetlands.
- Grasslands.
- Coastal habitats.
has greater ecosystem diversity than an area containing only one habitat type.
Each ecosystem can support different communities of organisms.
Comparing Ecosystems Fairly
When comparing biodiversity between ecosystems, sampling methods should be as consistent as possible.
Scientists should consider:
- Sampling area.
- Number of samples.
- Time spent sampling.
- Sampling method.
- Season.
- Weather.
- Time of day.
- Ability to detect different species.
Otherwise, differences in the data might result from the sampling method rather than real differences in biodiversity.
Worked Example: Comparing Two Forests
Scientists investigate Forest A and Forest B.
They use:
- The same quadrat size.
- 30 randomly located quadrats in each forest.
- The same identification method.
- The same sampling period.
Forest A contains 42 plant species.
Forest B contains 19 plant species.
Forest A has greater plant species richness in the samples.
However, before concluding that Forest A has greater overall biodiversity, scientists should also consider:
- Species abundance.
- Evenness.
- Other groups of organisms.
- Whether sampling was representative.
Comparing Biodiversity Over Time
Scientists may repeatedly survey the same ecosystem.
For example:
Year 1: 36 species
Year 5: 31 species
Year 10: 23 species
This suggests declining species richness.
However, scientists should investigate possible causes and consider whether differences in:
- Sampling effort.
- Weather.
- Season.
- Detection.
could have influenced the measurements.
Good ecological conclusions require more than simply observing that two numbers are different.
Indicator Species
Some organisms are particularly sensitive to environmental conditions.
These organisms can sometimes be used as indicator species.
Their presence, absence, or abundance may provide information about environmental quality.
For example, certain aquatic invertebrates are sensitive to water pollution.
Finding many pollution-sensitive species can provide evidence about water quality.
However, scientists normally combine indicator-species evidence with other environmental measurements.
Species Accumulation
As scientists take more samples, they often discover additional species.
Initially, new samples may reveal many new species.
Later, fewer new species are discovered because most common species have already been recorded.
This relationship can be shown using a species accumulation curve.
If the curve begins to level off, this suggests that additional sampling is finding relatively few new species.
Sampling Error
Ecological sampling always contains some uncertainty.
Possible sources of error include:
- Misidentifying species.
- Missing hidden organisms.
- Organisms moving during sampling.
- Using too few samples.
- Sampling only convenient locations.
- Changing weather conditions.
- Different observers using different methods.
Scientists reduce these problems through careful experimental design and repeated sampling.
Worked Data Interpretation
Scientists record insects in two habitats.
Habitat A
- Species 1 = 20
- Species 2 = 18
- Species 3 = 22
- Species 4 = 20
- Species 5 = 20
Habitat B
- Species 1 = 92
- Species 2 = 2
- Species 3 = 2
- Species 4 = 2
- Species 5 = 2
Both habitats contain five species.
Both contain 100 insects.
However:
Habitat A has high evenness.
Habitat B is strongly dominated by Species 1.
Therefore, Habitat A has greater species diversity despite having the same species richness.
Worked Data Interpretation: Richness
Scientists sample three ponds.
Pond A: 18 species
Pond B: 11 species
Pond C: 23 species
Based only on species richness:
Pond C has the greatest species richness.
However, this does not automatically prove that Pond C has the greatest biodiversity in every sense.
Scientists would also want information about:
- Abundance.
- Evenness.
- Genetic diversity.
- Sampling effort.
Biodiversity Hotspots
Some regions contain exceptionally high biodiversity and large numbers of species found nowhere else.
These areas can be particularly important for conservation.
Protecting a relatively small area in such a region can sometimes help conserve many species.
Biodiversity Is More Than Counting Species
It is tempting to think:
More species = biodiversity completely measured.
But biodiversity is more complex.
Scientists may investigate:
Genetic diversity
Species richness
Number of species.
Species evenness
Distribution of individuals among species.
Ecosystem diversity
Variety of ecosystems.
A good biodiversity assessment therefore considers what exactly has been measured.
Common Mistakes
Thinking Biodiversity Means Only the Number of Species
Species richness is one measure of biodiversity, but biodiversity also includes genetic and ecosystem diversity.
Confusing Richness With Abundance
Richness = number of species.
Abundance = number of individuals.
Ignoring Evenness
Two ecosystems can have identical species richness but very different diversity because their abundances differ.
Assuming One Quadrat Represents an Entire Ecosystem
A single sample may not be representative.
Choosing Sampling Locations Deliberately
This can introduce sampling bias.
Assuming High Biodiversity Makes an Ecosystem Indestructible
Biodiversity can contribute to resilience, but even highly diverse ecosystems can be severely damaged.
Comparing Samples Collected Differently
Different sampling methods or effort can create misleading comparisons.
Assuming Every Non-Native Species Is Invasive
A non-native species is considered invasive when it spreads and causes significant harm.
Drawing Conclusions From Species Richness Alone
Scientists should consider abundance, evenness, sampling effort, and the purpose of the investigation.
Check Your Understanding
1. Define biodiversity.
2. Name the three major levels at which biodiversity can be considered.
3. What is species richness?
4. What is species abundance?
5. What is species evenness?
6. Why can two ecosystems with the same species richness have different biodiversity?
7. What information does a biodiversity index attempt to combine?
8. Why do ecologists use sampling?
9. What is a quadrat?
10. Why should quadrat locations often be selected randomly?
11. How does increasing sample size generally improve an investigation?
12. When would a transect be particularly useful?
13. Give three abiotic factors that can affect biodiversity.
14. Give three biotic factors that can affect biodiversity.
15. Explain how habitat destruction can reduce biodiversity.
16. Explain how an invasive species can affect native biodiversity.
17. Why is biodiversity important to humans?
18. Habitat A and Habitat B contain the same six species, but Habitat A has much greater evenness. Which habitat would generally have greater species diversity?
19. Why must scientists use similar sampling methods when comparing ecosystems?
20. Explain why measuring biodiversity involves more than simply counting the number of species.
Key Terms
- Biodiversity – variety of life at genetic, species, and ecosystem levels.
- Genetic diversity – genetic variation within a species or population.
- Species diversity – diversity of species considering richness and relative abundance.
- Ecosystem diversity – variety of ecosystems or habitats within an area.
- Species richness – number of different species present.
- Species abundance – number of individuals of a species.
- Species evenness – how evenly individuals are distributed among species.
- Diversity index – numerical measure used to describe species diversity.
- Sampling – studying part of a population or ecosystem to estimate characteristics of the whole.
- Quadrat – frame defining a known area for ecological sampling.
- Transect – line along which organisms or environmental conditions are sampled.
- Random sampling – selecting samples without deliberately favoring particular locations.
- Habitat fragmentation – division of continuous habitat into smaller isolated areas.
- Invasive species – non-native species that spreads and causes significant harm.
- Indicator species – species whose presence, absence, or abundance can provide information about environmental conditions.
- Species accumulation curve – graph showing the number of species detected as sampling effort increases.
- Resilience – ability of an ecological system to respond to disturbance while maintaining or recovering important functions.
Key Takeaways
- Biodiversity is the variety of living organisms and biological systems.
- Biodiversity can be considered at genetic, species, and ecosystem levels.
- Species richness measures the number of species present.
- Species abundance measures the number of individuals.
- Species evenness describes how evenly individuals are distributed among species.
- Two ecosystems can have equal species richness but different species diversity.
- Diversity indices can combine information about richness and relative abundance.
- Scientists usually estimate biodiversity by sampling rather than counting every organism.
- Quadrats are useful for sampling stationary or slow-moving organisms.
- Transects are useful for investigating changes across environmental gradients.
- Random sampling helps reduce bias.
- Larger sample sizes generally produce more reliable estimates.
- Climate, habitat diversity, resources, competition, predation, and disturbance can influence biodiversity.
- Human activities such as habitat destruction, pollution, invasive species introductions, overharvesting, and climate change can reduce biodiversity.
- Biodiversity contributes to ecosystem functioning and resilience.
- Humans depend on biodiversity for food, medicines, pollination, nutrient cycling, and many other ecosystem services.
- Biodiversity data must be interpreted carefully.
- Fair comparisons require similar sampling methods and sampling effort.
- Measuring biodiversity allows scientists to compare ecosystems, monitor environmental change, and make evidence-based conservation decisions.