IB Environmental systems and societies 3.1C Species Diversity Notes

Species Diversity: Richness and Evenness Combined

1. Species diversity is a key measure of biodiversity, set by species richness and species evenness.
2. Together these determine how balanced and functionally stable a community is.

The Two Components of Species Diversity

Species Richness

1. Species richness is a simple count of how many different species are present.
2. It shows variety but not how evenly individuals are spread among species.
3. Higher richness means more ecological roles and interactions.

Species Evenness

1. Species evenness describes how uniformly individuals are distributed among the species.
2. High evenness, with similar population sizes, suggests a balanced, stable ecosystem.
3. Low evenness, where one or a few species dominate, suggests lower stability.

How Richness and Evenness Combine

1. High richness, low evenness (unstable)
   1. Many species exist, but a few dominate.
   2. The dominant species can outcompete others, causing imbalance.
2. Low richness, high evenness (limited but steadier)
   1. Fewer species, in similar proportions, so none dominates.
   2. More stable than a low-evenness system, but still vulnerable to disturbance.
3. High richness, high evenness (most resilient)
   1. Many species with balanced populations create a strong, resilient ecosystem.
   2. This supports high productivity and resistance to environmental change.

Simpson's Reciprocal Index Quantifies Diversity

1. Biodiversity can be quantified with an index that combines richness and evenness.
2. The most widely used is Simpson's Reciprocal Index (D), which gives a single value for comparison.
3. It lets you:
   1. Compare ecosystems with similar species types.
   2. Monitor how one ecosystem's diversity changes over time.

The Formula

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

1. D is the diversity index; higher values mean greater diversity.
2. N is the total number of individuals in the sample.
3. n is the number of individuals of a single species.
4. Σ means you sum n(n − 1) across all species.
5. A higher D means greater diversity; a value near 1 means one species dominates.

Sampling for Simpson's Index

1. Choose the sampling area
   1. Select two or more comparable sites, for example forest versus grassland.
   2. Make sure the sites are similar in size and conditions.
2. Use a standard method
   1. Quadrats for plants, transects for zonation, and traps or nets for small animals.
3. Record abundance
   1. Count individuals per species, repeating across quadrats for accuracy.

Worked Example

1. Two sites in the same ecosystem are sampled and compared.

1. Step 1: N(N − 1)
   1. Site A = 36 × 35 = 1260.
   2. Site B = 60 × 59 = 3540.
2. Step 2: Σn(n − 1)
   1. Site A = (12 × 11) + (8 × 7) + (10 × 9) + (6 × 5) = 308.
   2. Site B = (20 × 19) + (25 × 24) + (5 × 4) + (10 × 9) = 1090.
3. Step 3: D = N(N − 1) / Σn(n − 1)
   1. Site A: D = 1260 / 308 = 4.09.
   2. Site B: D = 3540 / 1090 = 3.25.

Measuring and Monitoring Biodiversity

1. Conservation depends on collecting biodiversity data at global, regional, and local scales.
2. Global databases such as the IUCN Red List classify species by extinction risk and guide priorities.
3. Citizen science projects, like eBird and the Big Butterfly Count, gather large-scale records from volunteers.
4. Indigenous and local communities add long-term traditional knowledge, sometimes as trained parabiologists.

Case Study: The Kinabalu Birdwing Butterfly

How Biodiversity Data Guides Conservation

1. Identify hotspots to designate protected areas.
2. Track recovery after restoration or pollution control.
3. Inform policy aligned with the UN SDGs, Life Below Water and Life on Land.