Organisms are affected by different abiotic (non-living) factors, such as:
However, these factors vary between different locations, which affects the number of organisms in each location. To estimate the number of organisms in a population, scientists use a method called sampling.
Sampling involves looking at a small section of the population of organisms and drawing conclusions about the rest. The small section of the population that is observed is called a sample. More accurate results can be achieved by taking more samples and using random sampling. These sampling methods are effective at estimating the distribution and abundance of organisms in a sample to work out the size of the population.
The distribution and abundance of organisms in a given area can be determined using fieldwork techniques and methods such as quadrats and transects.
A quadrat is typically a 0.25 m² or 1 m² square frame. It can contain wires inside to divide it into smaller squares. The quadrat is placed on the ground and the plants (or sometimes slow-moving animals (e.g. snails) found inside it can be identified and counted.
The placement of the quadrat is usually random to avoid biased results. This can be achieved by converting the sampling area into a grid format, so that each square on the grid is associated with a number. Then picking random coordinates within the area.
Generally, we sample with a quadrat to estimate the population size or to see if the number of species changes within an area or between areas.
To estimate the number of 3-leaf clovers in a 10 m × 10 m field. We can carry out either of the following methods:
A) Count every single 3-leaf clover in the field
B) Take a sample of 3-leaf clovers from a small area, then use the sample to estimate the entire population of 3-leaf clovers – This is less accurate but less time-consuming
A) This is a very accurate method if done correctly, but it is time-consuming and there is a high likelihood of error.
B) If both methods are done correctly, this method will take significantly less time and there is a lower risk of error. To carry out this estimation of population size, we can:
1. Place the quadrat randomly within the sampling area
2. Count the number of organisms of the same species within the quadrat
3. Repeat this numerous times at different areas within the sampling area, while using the same-sized quadrat
4. Calculate the estimate of the population sizes of the organisms in the sampling using the equation:
You can repeat the process in a different area and compare results to see if population size differs between areas.
Random sampling isn’t always effective if you want to identify patterns in an ecosystem, such as when finding out how physical conditions change across a sampling area. In this case, scientists can use a transect.
A transect is a straight line placed across a habitat, usually made with string or measuring tape. Transects are used to take measurements or make observations. Scientists can place their quadrats at regular intervals along the transect (e.g. every 5 metres). After this, they can record the number and type of each species in each quadrat.