Assessing Changes in Species Diversity

Scientists use rigorous quantitative techniques to measure and monitor biodiversity. Understanding these methods — and their strengths and limitations — is a core skill in VCE Environmental Science.

Why Measure Species Diversity?

• Establish baseline data for future comparisons
• Detect changes caused by threatening processes
• Evaluate effectiveness of conservation interventions
• Inform listing decisions and management priorities
• Meet legal reporting requirements under the EPBC Act 1999 and Flora and Fauna Guarantee Act 1988 (Vic)

Sampling Approaches

Direct counting of every individual in an area is rarely practical. Scientists use sampling — studying a representative subset of the population — and extrapolate to the whole area.

Key Sampling Techniques

Quadrat Shape Considerations

• Different shaped quadrats sample different perimeter-to-area ratios
• Circular quadrats minimise edge effects
• Long narrow quadrats capture habitat variation along a gradient
• Edge effects: organisms near quadrat boundaries may be partially inside — standardised protocols decide inclusion rules

Mark-Recapture (Lincoln-Petersen method)

$$N = \frac{M \times C}{R}$$

Where:
- $N$ = estimated population size
- $M$ = number marked in first capture
- $C$ = total caught in second capture
- $R$ = number recaptured with marks

Assumptions: marks don’t affect survival/behaviour; population is closed (no migration, births or deaths between samples); marks are not lost; sampling is random.

Diversity Metrics

Species Richness

• Simply the total number of species in an area
• Easy to measure but ignores relative abundance
• Two sites can have equal richness but very different community structures

Endemism

• A species is endemic to an area if it is found there and nowhere else
• Measured as the proportion or number of endemic species in a region
• High endemism = high conservation priority (loss of that habitat = global extinction)

Simpson’s Index of Diversity (SID)

SID accounts for both species richness and evenness:

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

Where:
- $n_i$ = number of individuals of species $i$
- $N$ = total number of individuals
- SID ranges from 0 (no diversity) to ~1 (maximum diversity)

A higher SID means a more diverse community. It is less sensitive to rare species than some other indices.

Worked example:
- Species A: 10 individuals; Species B: 5 individuals; Species C: 3 individuals
- $N = 18$
- $\sum n_i(n_i-1) = 10 \times 9 + 5 \times 4 + 3 \times 2 = 90 + 20 + 6 = 116$
- $N(N-1) = 18 \times 17 = 306$
- $SID = 1 - 116/306 = 1 - 0.379 = 0.621$

EXAM TIP: VCAA regularly provides raw species count data and asks you to calculate SID. Practice the formula until it is automatic. Always round to 2 decimal places unless instructed otherwise, and interpret your result (e.g. ‘SID of 0.62 indicates moderate species diversity’).