Populations In Ecosystems: - Investigating Populations

- Investigating Populations - 

Investigating population:

There are a number of sampling techniques

- random sampling using frame quadrats or point quadrats

- systematic sampling along a belt transect  

Quadrats:

There are two types of quadrats that are frequently used.

Point quadrat consisting of horizontal bar supported by two legs. The intervals are set along the horizontal bar are ten holes each of which are long pins may be dropped. Each species that is touched by the pin is recorded.

Frame quadrat is a square frame divided by string or wire into equally sized subdivisions it's designed to make it more compact, so it's easy to store and transport. It can be placed in the areas being studied.

Point Quadrat Frame Quadrat

There are three factors to consider when using quadrats:

1 )The size of the quadrats to use = this depends on the size of the animal or organism being counted and how they are distributed within the area. Larger species require larger quadrats. In an area which the species are not largely distributed a large number of small quadrats would give a more representative result than a small number of large ones.

2)The number of sample quadrats to record within the study area = larger the number of sample quadrats the more reliable the results will be. As the recording the species within a quadrats

3)The position of each quadrat within the study area = produce a statistically significant results a technique - random sampling must be used

Sampling at random

To obtain a true random sample is to:

> Lay out two long tape measures at right angles, along two sides of the study area.

> Obtain a series of coordinates by using random numbers taken from a table or generated by a computer.

> Place a quadrat at the intersection of each pair of coordinates and record the species within it.

Systematic sampling along belt transects

- Species would need to be measure in a systematic system rather than a random manner. This is important to measure gradual change in the communities of plants and animals to take place.

An example of this would be distribution of organisms along a line of succession. Stages of succession is shown using transects. A belt transect can be made by stretching a string or tape across the ground in a straight line. Frame quadrat is then laid down alongside the line and the species within it recorded. It is then moved along it's own length along a line and the process is then repeated - a record of species in a continuous belt.

Measuring abundance

Random sampling with quadrats and counting along transect used to obtain measures of abundance.

Abundance - number of individuals of a species within a given area for species that don't move around. This is measured in several ways:

• Frequency = likelihood of a particular species occurring in a quadrat.
• Percentage cover = an estimate of the area within a quadrat than a particular plant species covers. This is useful where a species is difficult to count, data could be collected rapidly and individual plants do not need to be covered. Not useful when organisms occur in several overlapping layers.

It's necessary to ensure that the species sample is large, the larger the sample the more representative the data is.

Mark release recapture techniques

Estimated population size  = total no. of individuals in 1st sample X total no. 2nd sample     

                             No. of marked individuals recaptured

The technique requires a no. of assumptions:

1. Proportion of marked to unmarked individual in 2nd sample is the same as the proportion of marked to unmarked individuals in the population as a whole.
2. Marked individuals released from 1st sample distribute themselves evenly amongst the remainder of the population have sufficient time to do so.
3. Population has a boundary = no immigration into or emigration out of the population.
4. Few death and births within the population.
5. Method marking not toxic to the individual nor does it make the individual more conspicuous therefore more liable to predation.
6. Mark or label is not lost or rubbed off during the investigation.