Biology (9610) | Biodiversity within a community

Welcome to Biodiversity within a Community! (3.1.11)

Hello! This chapter is where we move from studying the building blocks of life (molecules and cells) to looking at the incredible variety of life on Earth. Biodiversity is perhaps the most important concept in ecology, as it measures the health and stability of an ecosystem.

You will learn exactly how scientists measure this variety—both the visible differences between species and the invisible differences hidden in their DNA. Don't worry if the calculation looks intimidating; we'll break it down step-by-step!

3.1.11.1 Genetic Diversity: The Blueprint Variety

Genetic diversity refers to the variety of genes (alleles) present within a species or between different species. This level of diversity is crucial for evolution and survival, as it gives a population the flexibility to adapt to changing environments.

Measuring Genetic Diversity

To determine how closely related organisms are, or how much variation exists within a population, scientists compare their basic biological components. The greater the similarity in these components, the more closely related the organisms are thought to be.

Methods of Comparison:

• Base Sequence of DNA or mRNA:
  

  This is the most direct method. By sequencing sections of DNA (or the mRNA transcribed from it), scientists can directly count the similarities and differences in the order of the A, T, C, and G bases. Organisms with very similar DNA sequences are likely to have separated relatively recently in evolutionary terms.

• Frequency of Specific Base Sequences or Alleles in Populations:
  

  Instead of sequencing the whole genome, we might look at how often a specific version of a gene (an allele) appears in one population compared to another. High frequency differences suggest less genetic mixing or a longer evolutionary separation.

• Amino Acid Sequence of Encoded Proteins:
  

  Since DNA codes for proteins, differences in the DNA base sequence will usually lead to differences in the final protein structure (the sequence of amino acids). By comparing the amino acid sequences of a specific, common protein (like haemoglobin or cytochrome c) across different species, we can determine their relatedness.

3.1.11.2 Species Diversity: The Visible Variety

Species diversity focuses on the range of different species living together in one place—a community.

1. Species Richness

The simplest measure of diversity is Species Richness.

Species richness is the number of different species in a community.

Example: If you survey a forest and find 20 types of plants and 15 types of insects, the species richness is 35.

Limitation of Species Richness:

Species richness is easy to measure, but it doesn't tell the whole story. It ignores the number of individuals of each species.

• Imagine two fields:
• Field A: 5 species, 98% of the individuals belong to one species.
• Field B: 5 species, where each species makes up 20% of the individuals.

Both fields have the same richness (5), but Field B is clearly more diverse and stable because no single species dominates. This is why we need the Index of Diversity.

2. Index of Diversity (\(d\))

The Index of Diversity is a measure that describes the relationship between the number of different species and the number of individuals in each species.

A higher index value (\(d\)) indicates a healthier, more diverse community.

The Index of Diversity Formula

You must recall this formula and be able to use it accurately in calculations:

\(d = \frac{N(N-1)}{\sum n(n-1)}\)

Where:

• \(N\) = Total number of organisms of all species in the community.
• \(n\) = Total number of organisms of each individual species.
• \(\sum\) (Sigma) = The sum of (meaning you must calculate \(n(n-1)\) for every species, and then add those values together).

Step-by-Step Calculation Guide (Don't Panic!)

This calculation is straightforward if you follow the steps systematically.

1. List your data: For every species, record the value of \(n\) (the number of individuals).
2. Calculate \(n(n-1)\): For *each species*, multiply the number of individuals by (number of individuals minus one).
3. Calculate \(\sum n(n-1)\): Add up all the \(n(n-1)\) values calculated in step 2. This is the denominator (bottom part) of the formula.
4. Calculate \(N\): Add up the total number of organisms from all species. This is the overall \(N\).
5. Calculate \(N(N-1)\): Use the overall \(N\) to calculate the numerator (top part) of the formula.
6. Divide: Divide the numerator (Step 5) by the denominator (Step 3) to find \(d\).

Human Activities, Reduced Biodiversity, and Conservation

Human actions often reduce biodiversity, mainly by decreasing species richness (extinction) and reducing genetic diversity (selective breeding or habitat destruction).

How Humans Reduce Biodiversity:

• Habitat Destruction: Clearing forests for agriculture or urban development eliminates entire communities.
• Monoculture: Intensive farming often uses only one or two crop species over vast areas (e.g., fields of wheat). This drastically reduces the species richness of the area, as few other plants or animals can survive there.
• Pollution: Run-off from fertilisers or pesticides can kill non-target species or change the environment, making it less suitable for many organisms.

The Balance: Food Production vs. Conservation

There is a constant conflict between the human need to produce large quantities of cheap food (often requiring intensive, low-diversity farming techniques) and the ethical and ecological necessity of conservation.

Conservation involves protecting habitats and managing species populations to maintain maximum biodiversity.

Did you know? The development of high-yield crops and domesticated animals, while beneficial for food security, relies on selecting specific alleles, which actually reduces the genetic diversity within those particular species, making them potentially more vulnerable to a new disease.