Biodiversity in Ecosystems

Glossary of Key Biodiversity Terms

Monoculture: Refers to the presence of a single species in a specific area. This practice is most frequently observed in the context of crop farming.
Species Richness: The total number of different species present within a defined area.
Species Diversity: A measure that takes into account the number of species in an area relative to the total number of individuals of those species.
Simpson’s Diversity Index: This represents the probability that two individuals randomly selected from a sample belong to different groups, such as different species or families.

Determining Species Diversity

Scientists employ several quantitative measures to determine the diversity of species within an area:

Species Richness
Species Evenness (also known as relative species abundance)
Percentage cover
Percentage frequency
Simpson’s Diversity Index

The Three Levels of Biodiversity

Biodiversity is defined as the variety of living organisms in a given area and serves as a critical indicator of habitat health. Higher biodiversity generally signifies a healthier habitat. It is analyzed at three distinct levels:

Habitat Diversity: The total number of different habitats present within a specific geographic area. A habitat is defined as the area inhabited by a species, comprised of abiotic factors (physical factors like soil and temperature) and biotic factors (living organisms).
Species Diversity: The measure of both the number of different species and the relative abundance of each species within a biological community. A species is defined as a group of similar organisms capable of reproducing to produce fertile offspring.
Genetic Diversity: The variation of alleles (different versions of the same gene) within a species or a specific population. For example, if the gene is "coat color," the alleles might be black, brown, or white.

Biodiversity Data Collection and Sampling

Data collection involves determining the number of different species or the number of individuals within each species. Because it is often impossible to count every individual, sampling techniques are used to estimate the total population.

Random Sampling

Objective: To ensure the sample is unbiased and that every site has an equal probability of being selected.
Methodology: Grids and random number generators are typically used.
Statistical Validity: Statistical analysis is required to ensure observed variation is not due to chance, allowing for a high degree of confidence that results accurately reflect the whole population.

Non-Random Sampling

Non-random sampling is used when there is significant variety in the distribution of organisms across a habitat, requiring specific samples to ensure all species are represented. Types include:

Systematic Sampling: Samples are taken at fixed intervals, often along a line such as a belt transect.
Opportunistic Sampling: The investigator chooses the sample sites. This is a simple technique but result in biased data.
Stratified Sampling: Different areas within a habitat (strata) are identified and sampled separately. The size of the sample is proportional to the size of that area relative to the habitat as a whole.

Practical Estimation Procedures

Site Selection: Choose a site using random or non-random methods.
Recording: Record the number of species or count individuals of each species.
Equipment: Use appropriate tools like sweep nets, pitfall traps, or quadrats.
Repetition: Repeat the process to ensure reliability.
Extrapolation: Estimate the population for the whole habitat by calculating the mean of the collected data and multiplying it by the total size of the habitat.
Consistency: When comparing different habitats, identical sampling techniques must be used.

Species Richness and Species Evenness

Species Richness: The number of different species in an area. A higher number of species indicates greater richness.
Species Evenness: A measure of the relative abundance of each species. Greater evenness occurs when the population sizes of different species are more similar.
Correlation: Higher levels of both richness and evenness lead to higher overall biodiversity. Conversely, low richness and low evenness result in lower biodiversity.

Habitat Comparison Examples

In a comparison between Habitat A and Habitat B, both contain 2 different species and 30 total organisms:

Habitat A: Contains 28 individuals of Species 1 and 2 individuals of Species 2. Distribution is uneven.
Habitat B: Contains 15 individuals of Species 1 and 15 individuals of Species 2. Distribution is even.
Conclusion: While species richness is identical between the two (2 species each), Habitat B has greater species evenness and therefore higher biodiversity.

In a comparison of two communities:

Community 1: Species distribution is 25%, 25%, 25%, and 25%.
Community 2: Species distribution is 6%, 12%, 70%, and 12%.
Conclusion: Both have the same species richness, but Community 1 has higher species evenness.

Quantitative Measurement Formulas

Percentage Frequency

This measures the evenness of an ecosystem. An ecosystem is considered very even if all species have a similar percentage frequency.

$\text{Percentage Frequency} = \frac{\text{number of individuals in a given species}}{\text{Total number of species in the study}} \times 100$

Percentage Cover

Commonly used with quadrats and along transects to measure ground or foliage cover.

$\text{Percentage Cover} = \frac{\text{number of squares which contain cover}}{\text{Total number of squares}}$

Simpson's Index of Diversity (SDI)

This metric accounts for both species richness and species evenness. It prevents species with very small populations from being treated the same as those with larger populations. Value ranges from 0 to 1:

Values close to 1: Indicate a more diverse habitat better able to cope with changes, such as the introduction of a new predator.
Values close to 0: Indicate a less diverse habitat that is easily damaged by environmental changes.

Diversity Calculation Examples

Flower Field Example

Consider a field with 3 species: Red (3 individuals), White (5 individuals), and Blue (3 individuals). The total number of individuals ( $N$ ) is 11.

Calculation using $D = 1 - \sum (n/N)^2$ :

$D = 1 - ((\frac{3}{11})^2 + (\frac{5}{11})^2 + (\frac{3}{11})^2)$

$D = 1 - (0.07 + 0.21 + 0.07)$

$D = 1 - 0.35$

$D = 0.65$

Since 0.65 is fairly high, the habitat has high species diversity.

Community Comparison Data

Comparison of Community A and Community B using the values for $n(n-1)$ , where $n$ is the number of individuals per species and $N$ is the total number of individuals of all species.

Community A:

Species	Individuals ( $n$ )	$n \times (n-1)$
A	49	2352
B	17	272
C	11	110
D	10	90
E	5	20
F	8	56
Total	$N = 100$	$\sum n(n-1) = 2900$

Community B:

Species	Individuals ( $n$ )	$n \times (n-1)$
A	21	420
B	20	380
C	17	272
D	16	240
E	14	182
F	12	132
Total	$N = 100$	$\sum n(n-1) = 1726$

Simpson's Index ( $D$ ): Refers to the probability that two randomly selected individuals belong to the same species.
Simpson's Diversity Index ( $SDI$ ): Refers to the probability that two randomly selected individuals belong to different species.