- We use sampling to gain insight into organisms in an area. Sampling is taking measurements of a limited number of organisms present within an area.
- Random sampling is selecting individuals by chance.
- You can perform random sampling by:
- marking a grid on the surface
- using random numbers to determine x and y coordinates
- taking a sample at each coordinate pair
- Non-random sampling
- Opportunistic—the weakest option. Uses organisms that are conveniently available
- Stratified—some populations can be divided into strata based on specific characteristics such as gender
- Sampling can never be entirely representative because you never measure the whole population. There is sampling bias, which may be deliberate or accidental. This can be reduced by using random sampling.
- Chance based reliability issues cannot be mitigated, as they arise from chance alone. It is simply possible more snails will be present on one day by chance.
Generally involves a quadrat
Visual survey: generates quantitative data
Estimate abundance using ACFOR scale
Density = mean number of individuals per unit area
Frequency = number or % of sampling units in which a species occurs
% cover: number of quadrat squares occupied by a species
Point frame quadrat: each plant that touches a pin is counted
|Example unit measurement
|m s ⁻¹
|oxygen content in water
|dissolved oxygen probe
- Calculate number of animals per unit area of habitat
- large animals - count all individuals
- small animals - sample and capture-recapture
- A pooter is used to catch insects. By sucking a mouthpiece, insects are drawn into a holding chamber. A mouthpiece filter prevents inhalation.
- Sweep nets are used to catch insects in areas of long grass
- Pitfall traps are used to catch small, crawling vertebrates such as beetles, spiders and slugs.
- Tree beating is used to take samples of the invertebrates living in a tree or bush. A large white cloth is stretched out under the tree, the tree is then shaken or beaten.
- Kick sampling is used to study in organisms in river beds/banks. The river bank is kicked for a period of time and a net is used to collect samples downstream.
- Capture sample and count—this is c₁
- Mark each individual
- Release all individuals and leave traps for a set period of time
- Number captured next time is c₂ Number already marked is c₃ Total population = (c₁xc₂)/c₃