Practical Techniques for Assessing Species Diversity

Field ecologists rely on systematic sampling to estimate species diversity without needing to count every individual. Each technique has specific applications, advantages and limitations that must be understood for both field practice and VCAA examinations.

Grids

Method: The study area is divided into a grid, and sampling units (quadrats, pitfall traps, observation points) are placed at each intersection, or at randomly selected grid squares.

Advantages:
- Systematic coverage ensures representative sampling across the entire area
- Spatial data can be mapped to reveal distribution patterns
- Easy to replicate and standardise

Limitations:
- Assumes uniform habitat — may miss patchily distributed species
- Can be time-consuming and labour-intensive

Applications: Plant surveys, small mammal trapping grids, soil invertebrate studies

Transects

Method: A line (or belt of fixed width) is established across the study area. Observations are made at fixed intervals along the line (point-intercept transect) or all organisms within the belt are recorded.

Types:
| Type | Description |
|—|—|
| Line transect | Organisms on or touching the line recorded |
| Belt transect | Organisms within a fixed width on either side of the line |
| Point intercept | Vegetation touching sampling points along the line |

Advantages:
- Efficient for detecting changes along environmental gradients (e.g. altitude, distance from water)
- Good for habitats with clear zonation (e.g. intertidal, forest-grassland edges)

Limitations:
- Non-random placement may introduce bias
- May miss species that avoid the transect line (observer effect)

Quadrats

Method: A defined-area frame is placed in the study area (randomly or systematically), and all organisms within the frame are identified and counted.

Shapes and their implications:

Edge effects: Organisms at the boundary of a quadrat may be partially inside. Standard protocols specify inclusion rules (e.g. count organisms touching the top and left edges, exclude those touching bottom and right) to ensure consistency and avoid double-counting.

Quadrat size selection:
- Should be large enough to sample representative species assemblages
- Species-area curves are used to identify the minimum adequate quadrat size (the point where adding area no longer significantly increases species count)

Mark-Recapture

Method: Two-stage sampling used for mobile animals:
1. First capture: Sample of animals caught, marked (paint, tags, ear notches, radio collars), and released unharmed
2. Waiting period: Allow marked animals to redistribute in the population
3. Second capture: New sample captured; proportion with marks recorded

Lincoln-Petersen Estimate:
$$N = \frac{M \times C}{R}$$

Where $N$ = population estimate, $M$ = marked in first capture, $C$ = total in second capture, $R$ = recaptured with marks.

Critical assumptions:
- Population is closed (no births, deaths, immigration, emigration between captures)
- Marks do not affect survival or detection probability
- Marks are not lost
- All individuals have equal probability of capture
- Marked individuals mix randomly back into the population

Sources of error if assumptions are violated:
- Mark avoidance → R underestimated → N overestimated
- Mark loss → R underestimated → N overestimated
- Trap-happy behaviour → R overestimated → N underestimated

Choosing the Right Technique

COMMON MISTAKE: Students often forget to state the assumptions of mark-recapture and how violations affect the estimate. VCAA exam questions frequently ask ‘what assumption is violated?’ or ‘how would this affect your estimate?’. Always be able to state the direction of bias (over- or under-estimate) and why.