What is the key difference between **species richness** and **species diversity**?

**Species richness** is simply the count of different species in an area. **Species diversity** is a broader concept that includes both species richness and the relative abundance (evenness) of individuals within each species. An area can have high richness but low diversity if one species is overwhelmingly dominant.

How does assessing genetic diversity through **phenotype** differ from using **genotype**?

Assessing genetic diversity via **phenotype** involves observing visible traits, which can be influenced by environment and may not directly reflect underlying genetic variation. Using **genotype** involves direct analysis of DNA to identify specific alleles, providing a more precise and direct measure of genetic variation.

How does the **heterozygosity index** complement **species richness** in assessing biodiversity?

**Species richness** provides a count of different species, giving a broad overview of variety. The **heterozygosity index** delves deeper by measuring genetic variation *within* a single species, indicating its adaptability and long-term viability. Both are needed for a comprehensive understanding of biodiversity.

What is a common mistake when interpreting a high species richness score?

A common mistake is assuming high species richness automatically means high overall biodiversity. High richness can mask low species abundance (uneven distribution) or low genetic diversity within species, both of which can indicate a less stable or resilient ecosystem.

What error might occur if the heterozygosity index is calculated from a non-random or too small sample?

If the sample is non-random or too small, the calculated heterozygosity index may not accurately represent the true genetic diversity of the entire population. This can lead to incorrect conclusions about the population's health and misguided conservation decisions.

What is a common misinterpretation of a low heterozygosity index value?

A low heterozygosity index value is often misinterpreted as simply 'bad.' While it generally indicates reduced genetic diversity and increased vulnerability, it doesn't automatically mean immediate extinction. However, it signals a higher risk for the population to adapt to future environmental changes or disease.

Define **species richness**.

**Species richness** is an ecological measure that quantifies the total number of different species found within a specific geographical area or ecological community. It serves as a basic indicator of biodiversity, reflecting the variety of distinct life forms present.

What is **genetic diversity**?

**Genetic diversity** refers to the total number of genetic characteristics in the genetic makeup of a species, encompassing the variety of alleles and genes within a population or species. It is crucial for a species' long-term survival and ability to adapt to environmental changes.

What does the **heterozygosity index (H)** measure?

The **heterozygosity index (H)** measures the proportion of individuals in a population that are heterozygous at a particular gene locus. It serves as a quantitative estimate of the genetic diversity within that population, reflecting the variability of alleles present.

State the formula for the **heterozygosity index (H)**.

The formula for the heterozygosity index (H) is: $$H = \frac{\text{number of heterozygotes}}{\text{number of individuals in the population}}$$ This ratio quantifies the proportion of individuals carrying two different alleles at a specific gene locus.

Species Richness & Heterozygosity Index | Pearson Edexcel International A-Level Biology

Revision Notes

International A-Level

Pearson Edexcel

Biology

4. Plant Structure & Function, Biodiversity & Conservation

Species Richness & Heterozygosity Index

Summary

This topic explores two key metrics used to assess biodiversity: species richness, which quantifies the number of different species in a community, and the heterozygosity index, which measures genetic diversity within a species. While species richness provides a basic count of species, the heterozygosity index offers insight into the genetic variation crucial for a population's adaptability and long-term survival. Together, these indices provide a more comprehensive understanding of an ecosystem's health and resilience.

1. Definition & Core Concepts

Species Richness refers to the total count of different species present within a defined ecological community or habitat. It provides a straightforward measure of the variety of life forms in an area, indicating how many distinct types of organisms exist there. A higher number of species generally suggests a richer environment.
Species Abundance complements species richness by measuring the relative number of individuals for each species within a given area. An ecosystem might have high species richness but low species abundance if many species are represented by only a few individuals, which can impact overall diversity and stability.
Genetic Diversity quantifies the variety of alleles and genes within the genome of a single species or population. This internal variation is critical for a species' ability to adapt to environmental changes, resist diseases, and maintain long-term viability. It can be assessed by examining phenotypes, genotypes, or specific genetic markers.
Heterozygosity Index (H) is a specific metric used to estimate genetic diversity within a population by calculating the proportion of individuals that are heterozygous at a particular gene locus. A higher heterozygosity index suggests greater genetic variation, which is generally beneficial for population health and resilience.

2. Species Richness: Measurement and Limitations

Measurement: Species richness is determined by identifying and counting all distinct species within a defined area, such as a habitat or community. For small or easily observable areas, this can involve direct observation and cataloging, while larger or more complex ecosystems often rely on sampling methods to estimate the total number of species.
Limitations: While simple to calculate, species richness can be a misleading indicator of overall biodiversity because it does not account for the relative abundance of each species. An area with many species but where one or two species dominate, and others are very rare, may have the same richness score as an area with a more even distribution, yet the latter is considered more diverse and stable. Therefore, it is often used in conjunction with other diversity indices that consider both richness and evenness.

3. Genetic Diversity: Assessment Methods

Phenotypic Assessment: Genetic diversity can be initially inferred by observing the variety of phenotypes, or observable characteristics, within a species. A wider range of physical traits, behaviors, or physiological responses often suggests a broader underlying genetic variation. However, phenotypic expression can be influenced by environmental factors, making it an indirect measure.
Genotypic Assessment: A more direct measure involves analyzing the genotype by sequencing DNA to identify different alleles present at various gene loci. The higher the number of different alleles for a characteristic across individuals, the greater the genetic diversity within that species, providing a precise molecular understanding of variation.
Heterozygosity Index: This index specifically focuses on the proportion of heterozygous individuals in a population, serving as a practical and widely used proxy for genetic diversity. It provides a quantitative measure of how much genetic variation exists at specific gene locations, which can then be averaged across multiple loci to estimate overall genomic diversity.

4. Heterozygosity Index: Calculation and Interpretation

5. Relationship to Overall Biodiversity & Conservation

6. Sampling Techniques for Species Richness Estimation

7. Exam Strategy & Tips