Measuring Species Diversity

Quantifying biodiversity requires metrics that capture different aspects of community structure. VCE Environmental Science requires proficiency with three key measures: species richness, endemism and Simpson’s Index of Diversity (SID).

Species Richness

Definition: The total number of species present in a defined area or community.

• The simplest measure of diversity
• Easy to count and compare
• Does not account for abundance — a community with 10 species of which 9 are represented by a single individual each looks as ‘rich’ as one where all 10 are equally common

When to use: Comparing diversity between regions; tracking colonisation after disturbance; rapid assessments.

Limitations:
- Sensitive to sampling effort — more sampling almost always finds more species
- Cannot distinguish between communities with very different structures
- Treats a community with 100 common species equally to one with 100 species where 99 are extremely rare

Endemism

Definition: A species is endemic to a region if it naturally occurs there and nowhere else in the world.

Significance:
- Endemic species have restricted ranges → local extinction = global extinction
- Regions with high endemism are conservation priorities
- Endemism reflects evolutionary history: isolation, stable refugia and adaptive radiation

Measuring endemism:
- Number of endemic species in a region
- Percentage endemism = (endemic species ÷ total species) × 100%
- Weighted endemism accounts for geographic range size

Australian endemism examples:
- ~80% of Australia’s flowering plants are endemic
- ~80% of freshwater fish are endemic
- All monotremes (platypus, echidnas) are endemic to the Australia-New Guinea region

Simpson’s Index of Diversity (SID)

SID is a combined measure of species richness and evenness, providing a more complete picture of community diversity.

Formula

$$SID = 1 - \frac{\sum n_i(n_i - 1)}{N(N-1)}$$

Where:
- $n_i$ = number of individuals of species $i$
- $N$ = total number of individuals
- The summation ($\sum$) is over all species present

Range and Interpretation

• SID = 0: No diversity (only one species, or one species utterly dominates)
• SID → 1: Maximum diversity (many species, all equally abundant)
• Higher values = more diverse communities

Step-by-Step Worked Example

Site A has three species:
- Eucalyptus obliqua: 15 individuals
- Acacia melanoxylon: 8 individuals
- Pomaderris aspera: 2 individuals

Step 1: Calculate $n_i(n_i - 1)$ for each species:
- \$15 \times 14 = 210$
- \$8 \times 7 = 56$
- \$2 \times 1 = 2$

Step 2: Sum: $\sum n_i(n_i-1) = 210 + 56 + 2 = 268$

Step 3: $N = 15 + 8 + 2 = 25$; $N(N-1) = 25 \times 24 = 600$

Step 4: $SID = 1 - \frac{268}{600} = 1 - 0.447 = 0.553$

Interpretation: Moderate diversity (0.55); dominated by E. obliqua.

Comparing SID Values

Forest B has the same number of species as Forest A but greater evenness, resulting in a higher SID.

Comparing the Three Measures

EXAM TIP: VCAA frequently provides species count data in a table and asks you to calculate SID, compare values between sites, and draw conclusions. Practice the formula multiple times. A common error is forgetting to subtract 1 (i.e. calculating $\sum n_i^2$ instead of $\sum n_i(n_i-1)$).