What is the fundamental difference between species richness and species evenness?

Species richness is a simple count of the number of different species present, while species evenness describes how close in numbers each species in an environment is. A community can have high richness but low evenness if one species is much more abundant than the others.

How does an Index of Diversity differ from a simple Species Richness count?

An Index of Diversity accounts for both the number of species (richness) and the relative abundance of each (evenness). This provides a more robust measure of biodiversity because it reflects the probability that two individuals chosen at random belong to different species.

Why might a community with 20 species be considered less diverse than one with 15 species?

If the community with 20 species is dominated by a single species (e.g., 99% of individuals), it has very low evenness. The community with 15 species might have a more balanced distribution of individuals, making it more ecologically diverse despite the lower richness.

What is the danger of using a very small sample size to determine species richness?

Small sample sizes lead to 'under-sampling,' where rare species are likely to be missed. This results in an artificially low richness count that does not reflect the true biological variety of the ecosystem.

Why is it incorrect to say that a farm with 50 different weed species is 'healthier' than a forest with 30 species?

Richness is only one aspect of health; the forest may contain specialized, stable niches and native species, while the farm's richness might consist of opportunistic or invasive species that do not support a complex food web.

What error occurs if you compare the richness of a 1-hectare plot to a 100-hectare plot directly?

This is a scale error; larger areas naturally contain more habitats and individuals, leading to higher richness. Comparisons are only valid if the sampling area or effort is standardized.

Define 'Species Richness' in the context of community ecology.

Species richness is the number of different species represented in an ecological community, landscape, or region. It is a count of species, and it does not take into the account the abundances of the species.

What is a 'Quadrat' and how is it used to measure richness?

A quadrat is a square frame of a known area used to isolate a subset of a habitat. By identifying all unique species within multiple randomly placed quadrats, ecologists can estimate the total species richness of the larger area.

What does the symbol $S$ represent in biodiversity formulas?

In ecological formulas, $S$ typically represents the total number of species recorded in a sample or community, which is the numerical value of species richness.

How does the practice of monoculture in farming affect species richness?

Monoculture involves growing only one species of crop over a large area, which eliminates the variety of niches and food sources available. This leads to a drastic reduction in the species richness of plants, insects, and birds in the agricultural landscape.

Species Richness | AQA AS-Level Biology

Genetics, Variation & Interdependence

Species Richness

Summary

Species richness is a fundamental ecological metric representing the total number of different species present within a specific community or habitat. While it serves as the simplest measure of biodiversity, it provides a limited view because it does not account for the relative abundance or distribution of individuals among those species.

1. Definition & Core Concepts

Species Richness is defined as the total count of distinct species inhabiting a particular geographical area or ecological community.

It is represented mathematically by the symbol $S$ , where each unique species contributes exactly one unit to the total, regardless of how many individuals of that species exist.

As the most basic component of biodiversity, it provides an immediate snapshot of biological variety but lacks the complexity of more advanced diversity indices.

2. Underlying Principles

The principle of species richness operates on the assumption that a higher count of species generally correlates with a more complex and potentially resilient ecosystem.

However, richness is an unweighted metric, meaning a rare species with only one individual is treated with the same statistical importance as a dominant species with thousands of individuals.

Ecologists use richness to identify 'biodiversity hotspots'—areas where evolutionary and environmental factors have allowed a high number of lineages to coexist.

A species-accumulation curve showing how species richness increases with sampling effort until it reaches a plateau representing the true richness of the area.

3. Methods & Techniques

Because it is often impossible to count every organism in a large ecosystem, ecologists use sampling techniques to estimate richness.

Random Sampling involves placing sample plots (quadrats) at coordinates generated by a random number generator to ensure every part of the habitat has an equal chance of being studied, which minimizes human bias.

Systematic Sampling uses fixed intervals, such as placing quadrats every 5 meters along a transect line, which is particularly useful for observing how richness changes across environmental gradients like moisture or altitude.

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions

Species Richness

Summary

1. Definition & Core Concepts

Species Richness is defined as the total count of distinct species inhabiting a particular geographical area or ecological community.

It is represented mathematically by the symbol $S$ , where each unique species contributes exactly one unit to the total, regardless of how many individuals of that species exist.

As the most basic component of biodiversity, it provides an immediate snapshot of biological variety but lacks the complexity of more advanced diversity indices.

2. Underlying Principles

The principle of species richness operates on the assumption that a higher count of species generally correlates with a more complex and potentially resilient ecosystem.

However, richness is an unweighted metric, meaning a rare species with only one individual is treated with the same statistical importance as a dominant species with thousands of individuals.

Ecologists use richness to identify 'biodiversity hotspots'—areas where evolutionary and environmental factors have allowed a high number of lineages to coexist.

A species-accumulation curve showing how species richness increases with sampling effort until it reaches a plateau representing the true richness of the area.

3. Methods & Techniques

Because it is often impossible to count every organism in a large ecosystem, ecologists use sampling techniques to estimate richness.

4. Key Distinctions

It is critical to distinguish between Species Richness (the count of types) and Species Evenness (how equal the populations of those types are).

Feature	Species Richness	Species Evenness
Focus	Number of different species	Relative abundance of each species
Sensitivity	High for rare species	High for dominant species
Complexity	Low (Simple count)	High (Requires population data)

An Index of Diversity (like Simpson's or Shannon's) combines both richness and evenness into a single value to provide a more accurate measure of ecosystem health.

5. Exam Strategy & Tips

When comparing two habitats in an exam, always check if the total number of individuals is provided; if one habitat has more species but is dominated by just one of them, it may be less 'diverse' despite having higher 'richness'.

Remember that species richness is a qualitative count, whereas an index of diversity is a quantitative calculation; never use the terms interchangeably in long-answer questions.

If a question asks why richness might be low in a managed environment (like a farm), focus on factors like monoculture, the removal of hedgerows, and the use of pesticides which eliminate niches.

6. Common Pitfalls & Misconceptions

A common mistake is assuming that high species richness always indicates a 'healthy' ecosystem; for example, a site heavily invaded by many non-native species might have high richness but low ecological integrity.

Students often forget that richness is highly dependent on sampling effort; a larger sample size will almost always yield a higher richness count simply because rare species are more likely to be encountered.

Do not confuse richness with abundance; abundance refers to the total number of individuals of a single species, while richness refers to the number of different species types.