Alpha, Beta & Gamma Diversity – Definitions, Examples, Formula & NEET MCQs

Introduction to Biodiversity Measures and Their Importance

Biodiversity is the sum of the different species. It shows diversity that occurs within a particular area, region, or on Earth. Biodiversity measures the entire array of life forms from the microbes to the plants and animals, and is found at all levels of biological classification. It is estimated by the Convention on Biological Diversity (CBD) that there exist around 8.7 million species on our planet, though not more than 1.5 million have been taxonomically described.

Not only does biodiversity sustain ecosystem productivity and stability, but it also provides ecosystem services such as food, medicine, and clean water. Biodiversity loss due to habitat destruction, pollution, and climate change is disturbing the ecological balance and human health.

Alpha Diversity: Definition, Species Richness, Indices & Examples

Alpha diversity is a description of species richness and evenness. It shows how many species exist in a specific habitat or ecosystem. It quantifies how many species there are and how evenly individuals are distributed across those species. Alpha diversity is typically measured through the use of indices such as the Shannon Index and Simpson's Index.

For example, a coral reef community is able to support more than 1,000 fish species within one reef location, showing high alpha diversity. Greater species richness and evenness make the ecosystem more resilient, as alpha diversity generally increases the types of interactions, productivity, and stability.

Indices for Alpha Diversity

• Species Richness: The most basic measure of biodiversity, showing the overall quantity of various species in a certain region.

• Shannon Index (H): This is responsible for giving an account of the number of species in a given ecosystem and also their distribution.

• Simpson Index (D): Calculates the probability that two individuals randomly selected from a sample belong to the same species, indicating the dominance patterns within the community.

Factors Influencing Alpha Diversity

• Habitat Complexity: More complex structures can also accommodate more species, allowing a greater variety of species to coexist.

• Productivity: The areas of higher primary productivity are characterised by higher species diversity because a large amount of energy is accumulated for various species.

• Disturbances: Moderate disturbance will promote diversity because competitors will not be able to entirely take over and dominate the region. Extremely high or very low levels of disturbance will reduce it.

Beta Diversity: Definition, Indices, Factors & Examples

Beta diversity is the difference in species composition across various ecosystems. It is also referred to as between-community diversity. It indicates the amount of change or turnover in species from one ecosystem or community to another. This also covers in situ and ex-situ conservation of the biodiversity of the area. Beta diversity helps us understand regional patterns of biodiversity and locate areas with characteristic species assemblages.

For Example, research in the Amazon lowlands shows great beta diversity among patches of forests isolated by rivers. In some areas, species turnover has been more than 60%. Measuring beta diversity helps ecologists to evaluate habitat fragmentation, ecosystem integrity, and how environmental conditions shapes distribution of species.

Indices for Beta Diversity

• Jaccard Index: It is a measure of similarity between two communities by comparing shared and unique species.

• Sorensen Index (Dice coefficient): Similar to the Jaccard Index, but gives more weight to shared species. It is more sensitive to common species.

Factors Influencing Beta Diversity

• Environmental Gradients: The changes in environmental conditions (e.g., temperature, moisture, soil type) across a landscape result in variations in species composition between communities.

• Geographical Barriers: Physical barriers such as mountains, rivers, and oceans can limit the movement of species. It results in different communities on either side of the barrier.

Gamma Diversity: Definition, Regional Biodiversity, Indices & Factors

Gamma diversity is the overall species diversity in a wide geographic area. It combines multiple ecosystems and communities into one measure. It is the summation result of alpha diversity (species richness in individual habitats) and beta diversity (species turnover among habitats). Together, these provide an integrated assessment of regional biodiversity.

For example, India's Western Ghats, an identified hotspot of biodiversity, have extremely high gamma diversity. The region has more than 7,400 plant species, with approximately 25% endemic to the area. This highlights that the gamma diversity is very important for identifying unique ecosystems and biodiversity conservation priorities.

Gamma diversity is important for wide-scale planning as well as managing ecosystems. It captures the general richness and patterning of life forms, including biodiversity in plants and animals. High gamma diversity supports biodiversity conservation, ecosystem services, and climate change resilience.

Indices for Gamma Diversity

• Total Species Richness: It is an index of total diversity in a large region or landscape combining richness from all communities.

• Whittaker's Gamma Diversity: It measures the overall species diversity in a landscape or ecosystem, without regard to how species are distributed within local communities.

Factors Influencing Gamma Diversity

• Landscape Heterogeneity: Fragile areas containing different sorts of biotopes and micro-climatic conditions have a level of gamma diversity due to the presence of a much higher number of special and specific species and groups.

• Regional Processes: Some factors like climate patterns, geological history, and evolutionary processes afford a region’s variety on a much bigger scale.

Relationship Between Alpha, Beta, and Gamma Diversity

Alpha, Beta, and Gamma diversity are interconnected measures that jointly help ecologists in understanding biodiversity at different spatial scales.

• Alpha Diversity is used to describe the species richness of a specific location or ecosystem (local diversity). It measures the variety of species within a particular, relatively homogenous location.

• Beta Diversity contrasts the diversity among ecosystems. It quantifies the change or turnover in species from one to another within a region. It connects local diversity (alpha) to regional diversity (gamma).

• Gamma Diversity is the overall species diversity throughout all the ecosystems of a larger region, such as a landscape or a biome.

Relationship: All three levels of biodiversity are mathematically related as:

Gamma Diversity = Alpha Diversity × Beta Diversity

Some basic similarities about Alpha, Beta, and Gamma diversity are discussed below in the table below:

Difference Between Alpha, Beta, and Gamma Diversity

The biodiversity is studied at three levels- alpha, beta and gamma diversity. These measures explain the species richness, species turnover, and regional biodiversity. The table below highlights the difference for quick NEET revision:

Alpha, Beta, and Gamma Diversity NEET MCQs

Q1. In the context of diversity patterns of species, which one of the following statements is incorrect?

1. Alpha diversity is diversity within a single community.

2. Beta diversity is a measure of the change in species composition from one community or habitat to another.

3. Alpha diversity is the regional diversity found among a range of communities in a geographical region.

4. Gamma diversity is the regional diversity found among a range of communities/habitats in a geographical region.

Correct answer: 3) Alpha diversity is the regional diversity found among a range of communities in a geographical region.

Explanation:

The incorrect statement regarding diversity patterns of species is that Alpha diversity is the regional diversity found among a range of communities in a geographical region. However, the correct statement should be Alpha diversity is diversity within a single community. Alpha diversity refers to the diversity of species within a single community or habitat. It is a measure of species richness and/or species evenness within a specific area or ecosystem.

Hence, the correct answer is option 3) Alpha diversity is the regional diversity found among a range of communities in a geographical region.

Q2. What does gamma diversity measure?

1. The overall number of species within a region or continent

2. The richness of species within a single habitat or region

3. The turnover of species between habitats or regions

4. The variety of ecosystems, habitats, and ecological communities on Earth

Correct answer: 1) The overall number of species within a region or continent

Explanation:

Gamma diversity measures the total number of species within a larger region or area and can reflect differences in environmental conditions or historical factors such as biogeographic history. It measures the overall number of species within a region or continent. Hence, option A is the correct answer.

Option(2) The richness of species within a single habitat or region is not measured by gamma diversity. Hence, option B is incorrect.

Option(3) The turnover of species between habitats or regions is not measured by gamma diversity. Hence option C is incorrect.

Option(4) The variety of ecosystems, habitats, and ecological communities on Earth is not measured by gamma diversity. Hence, option D is incorrect.

Hence, the correct answer is Option 1) The overall number of species within a region or continent.

Q3. What does beta diversity measure?

1. The richness of species within a single habitat or region

2. The overall number of species within a region or continent

3. The turnover of species between habitats or regions

4. The variety of ecosystems, habitats, and ecological communities on Earth

Correct answer: 3) The turnover of species between habitats or regions

Explanation:

Beta diversity refers to the turnover of species between habitats or regions, measuring the degree of change in species composition across different environments. It highlights how distinct or similar species communities are in different areas and reflects ecological variation driven by environmental conditions, habitat types, or historical factors like biogeographic barriers. A high beta diversity indicates significant differences in species composition between regions, often due to varying environmental gradients or isolation, while low beta diversity suggests similar species assemblages across habitats.

Hence, the correct answer is option 3) The turnover of species between habitats or regions

Also Read:

• MCQs on Difference between Alpha, Beta, and Gamma Diversity

• Biosphere Reserves

• Types of Pollution

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