2.1.5 [Skills]

Estimating Population Abundance

IB Knowledge Statements

Overview of Sampling Methods

Skill 1: Quadrats - Random Sampling

Step 1

Draw Grids

Step 2

Use randomiser to select a coordinate to measure

Step 3

Count the number of individuals of the species that you want to study

Abundance scales

can be used to estimate the relative abundance of different organisms on, for example, a rocky shore.
These are known as DAFOR scales
- - Each letter indicates a different level of abundance:
  - D = dominant, A = abundant, F = frequent, O = occasional and R = rare.
Quadrats are usually used to define the sample area.
Different types of species are put in different categories, for example seaweeds will be in a different group to periwinkles (periwinkles are a type of mollusc).
These scales allow for general comparison between different sampling sites.
It is a qualitative scale used to judge the abundance of different organisms. Because it is qualitative, it is subjective and so different people may have different judgements of abundance.
Also, there are no distinctions between different species in the same category (e.g. all seaweeds will be treated alike, irrespective of size or other differences).
The lack of quantitative data makes statistical analysis difficult.

Skill 2: Transects

A transect is a straight line that cuts through a natural landscape so that standardised observations and measurements can be made

Transects are used:

along an area with different habitats
to study how far a pollution or human activity affect the population of a species

A transect must be accompanied by quadrats method. The transect must be perpendicular to the environmental gradients

Transects can be analysed by:
- - Continuous
  - Interrupted (equal distances for the intervals)

Environmental gradients are changes in environmental factors through space (e.g. decreasing temperature with increasing altitude up a mountain) or where an ecosystem suddenly ends (e.g. at forest edges).

In these situations, both biotic and abiotic factors vary with distance and form gradients in which trends can be recorded.

The techniques used in sampling such gradients are based on the quadrat method and, as such, are more easily carried out on vegetation and non-motile animals.

Skill 3: Trapping

Skill 4: Capture-Mark-Release-Recapture

Lincoln index

used to estimate the total population size of mobile animals in a study area.
it is unlikely that all the animals in a population are sampled, so a mathematical method is used to calculate the total numbers.

The Lincoln index is an indirect way of estimating the abundance of an animal population

formula is used to calculate abundance rather than counting the total number of organisms directly

The markings must be:

ethically acceptable (non-harmful)
non-conspicuous (so that the animals are not more easily seen by predators)

Skill 5 [HL only]: Indicator Species

Living organisms that are easily monitored and whose status reflects or predicts the condition(s) of the environment where they are found (Landres et al., 1988, Cairns and Pratt, 1993, Bartell, 2006, Burger, 2006)

Indicator species may be sensitive to a number of different environmental conditions:

Lichen, along with mosses, are susceptible to air-borne pollutants dissolved in water (e.g. sulfur dioxide)
Tubifex worms are sensitive to concentrations of heavy metals
Mayfly larva and certain aquatic invertebrates are sensitive to dissolved oxygen levels in water

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