Investigating Populations Flashcards
How do ecologists sample species?
It would be impossible to count every single member of every single species in a particular habitat.
Taking samples in a carefully controlled way will help provide information about the distribution of species.
There are two main sampling techniques:
Random sampling using quadrats
Systematic sampling using transects
Methods for investigating populations
For plants and animals that don’t move or are slow moving e.g. barnacles , you can use a quadrat
What is a quadrat
A quadrat is a square frame, which is usually divided into a grid of smaller squares by string attached across the frame
Quadrat rule
You count a square if it is more than half covered.
There are two methods of placing random quadrats, both need a grid map with co-ordinates to be produced for the sample area.
The first method involves sampling the area in a regular pattern, for example at every 3rd metre square:
The second method involves assigning each square a number based on its co-ordinates and using a random number table/generator to select the positions of the quadrats.
Which of the following is a description of how quadrats could be placed randomly in a field?
Generate coordinates for each possible quadrat and use a random number generator to decide which quadrats to use.
Which of the following form part of a technique that would be used to determine the mean population density of dandelions in a field:
Sample random quadrats
Sample random quadrats
Count all dandelions in each quadrat and divide by area
To work out population size:
Work out the mean number of each species per quadrat.
Determine the area of a quadrat.
Estimate the total area in which you are sampling (eg Hyde Park) using a map
Divide the total area of Hyde Park by the area of a single quadrat and then multiply by the mean number of each species per quadrat.
Point Quadrat
A point quadrat consists of a horizontal bar supported by two legs. At set intervals along the bar there are holes through which a long pin may be dropped. The species the pin touches is recorded.
The pins are lowered. If 3 out of the 10 plants are dandelion then the frequency of the dandelion plant would be 30%
Random Sampling
Random Sampling avoids BIAS!
This means the data collected is valid
Choose an area to sample
Samples should be random to avoid bias
Lay out 2 tape measures at right angles along two sides of the study area
Obtain a series of co-ordinates using a random number generator or tables. Take your samples from these coordinates
Use an appropriate techniques to take a sample of the population
Repeat a large number of times to ensure that the samples are representative.
Systematic samples along transects
You can use lines called transects to help find out how organisms are distributed across an area
Belt transects- quadrats are placed next to each other along the transect
Line transect – simply the line and any organism over which the line passes is recorded
Abundance
Abundance is the number of individuals of one species in a particular area – i.e. the population size.
But, you have to make sure that these samples are representative of the habitat as a whole. This means that conclusions will be …
valid
Abundance can be measured in different ways e.g.
Frequency – the number of samples a species is recorded in e.g. 70% of samples
Percentage cover – how much of the area you are investigating is covered by a species
Frequency
The likelihood of a particular species occurring in a quadrat
If a species occurs in 15 of 30 quadrats its frequency of occurrence is 50%
It gives an idea of species present and their general distribution
Estimate of Total Population
Exam Question
