What is the difference between species richness and species evenness?

Species richness is the total number of different species present in a community, while species evenness is a measure of the relative abundance of the individuals in each of those species. A community is most diverse when it has both a high number of species and an equal distribution of individuals among them.

How does habitat fragmentation differ from habitat loss in its impact on biodiversity?

Habitat loss refers to the complete removal of an environment, whereas fragmentation breaks a large, continuous habitat into smaller, isolated chunks. Fragmentation is unique because it creates 'islands' that restrict animal movement, leading to smaller gene pools and increased vulnerability to local extinction.

Why is a high Simpson's Index value ($D \approx 1$) indicative of a stable ecosystem?

A high index value indicates that the ecosystem has high richness and evenness, meaning no single species dominates. This diversity provides ecological redundancy, ensuring that if one species declines, others can maintain the ecosystem's functional processes.

What is a common mistake when interpreting a low species evenness in a high-richness area?

The common error is assuming the area has high biodiversity based solely on the number of species. If one species is extremely dominant and others are rare, the ecosystem is actually fragile and has low effective biodiversity, which will be reflected in a low Simpson's Index.

Why is it incorrect to say that genetic diversity only matters for rare or endangered species?

Genetic diversity is vital for all species because it allows for natural selection to occur. Without a variety of alleles, even common species cannot adapt to new diseases, climate shifts, or changing food sources, potentially leading to sudden population collapses.

What error occurs if you forget to subtract the sum of $(n/N)^2$ from 1 in Simpson's formula?

Forgetting to subtract from 1 results in the 'Dominance Index' rather than the 'Diversity Index.' This would mean a high value represents low diversity (dominance by one species), which is the opposite of the standard interpretation of Simpson's Index of Diversity.

Define 'Genetic Diversity' and explain its significance.

Genetic diversity is the variety of different alleles and genes present within a species' population. It is significant because it increases the likelihood that some individuals will possess traits necessary to survive environmental changes, ensuring the species' long-term persistence.

What is 'Ecosystem Diversity'?

Ecosystem diversity is the variety of different habitats, biological communities, and ecological processes within a specific area. High ecosystem diversity usually leads to higher species diversity because it provides a wider range of niches for different organisms to occupy.

State the formula for Simpson's Index of Diversity and define the variables.

The formula is $D = 1 - \sum (n/N)^2$. Here, $n$ is the number of individuals of a particular species, and $N$ is the total number of individuals of all species found in the sample area.

How does human population growth indirectly lead to a decrease in biodiversity?

As the human population grows, the demand for resources like land, food, and energy increases. This leads to habitat destruction for agriculture, overexploitation of species for food, and increased pollution, all of which degrade the environments that support biodiversity.

Biodiversity | OCR AS-Level Biology

4. Biodiversity, Evolution & Disease

Biodiversity

Summary

Biodiversity is the measure of biological variation across all levels of life, from the molecular scale of genes to the global scale of ecosystems. It serves as a critical indicator of environmental health and ecosystem resilience, determining the ability of natural systems to withstand and recover from environmental stressors.

1. The Three Levels of Biodiversity

Genetic Diversity: This refers to the variety of alleles and genes within the genome of a single species. High genetic diversity is essential for a population's survival as it provides the raw material for adaptation to changing environments and resistance to diseases.
Species Diversity: This level considers both species richness (the number of different species in an area) and species evenness (the relative abundance of individuals within each of those species). A balanced ecosystem typically requires both high richness and high evenness.
Ecosystem Diversity: This describes the range of different habitats, communities, and ecological processes within a specific geographical area. Complex ecosystems like coral reefs or tropical rainforests provide numerous niches, supporting higher overall biodiversity.

A nested circle diagram showing the hierarchy of biodiversity: Ecosystem Diversity contains Species Diversity, which contains Genetic Diversity.

2. Quantifying Biodiversity: Simpson's Index

3. Ecosystem Resilience and Importance

Resilience to Change: High biodiversity increases an ecosystem's stability. If an environmental change or disease affects one species, a diverse ecosystem is more likely to have other species that can perform similar ecological roles, preventing total collapse.
Ecological Niche Exploitation: Areas with high habitat diversity, such as coral reefs, offer many specialized roles (niches). This allows a greater variety of species to coexist by reducing direct competition for the same resources.
Human Impact: Biodiversity provides essential services including food security, medicinal resources, and climate regulation. The loss of biodiversity directly threatens the sustainability of these human-dependent systems.

4. Key Distinctions in Biodiversity Measurement

Feature	Species Richness	Species Evenness
Definition	The total count of different species present.	The relative distribution of individuals among species.
Sensitivity	Does not account for population size.	Highly sensitive to dominant or rare species.
Indication	Shows potential biological variety.	Shows how 'balanced' the community is.

Habitat Loss vs. Fragmentation: Habitat loss is the total destruction of an environment, while fragmentation involves breaking a large habitat into smaller, isolated patches. Fragmentation is particularly dangerous as it leads to inbreeding and increased vulnerability to external 'edge effects'.

5. Exam Strategy & Tips