Practical: Investigating Biodiversity Using Sampling
Apparatus
 Quadrat
 Random number generator
Method for choosing sample sites
 Mark up a grid on a map or a toscale drawing of the area being studied and label the grid with coordinates
 It is important that the area is big enough to get a representative estimate for the specific habitat/ecosystem
 Use a random number generator to choose a set of coordinates
 This is done to avoid sampling bias which could lead to over or underestimation (either subconsciously choosing areas that are easier to access or that look like they contain individuals)
Getting measurements from quadrats
 The contents within quadrats can be counted using different measurements
 Population density, percentage cover and species frequency are all different ways of counting and recording the number of different species and individuals present within a quadrat
Calculating population density (number of individuals per m_{}^{2}):
 Place a quadrat at the generated coordinate
 Count the number of individuals in each quadrat
 Use a running mean to determine the number of quadrats required to get a representative sample:
 Calculate the mean number of individuals per quadrat for the first two quadrats found, then the mean of the first three, then the mean of the first four and repeat until there is no further significant change in the mean
 To calculate the estimated population size for the whole area or habitat, divide the whole area by the area of one quadrant, then multiply this value by the mean number of individuals per quadrat
Calculating the abundance of a species using percentage cover (%):
 Usually used for plants
 Place a grid quadrat (a square frame split into 100 smaller squares) at this coordinate
 Each square of a grid quadrat is equivalent to 1% cover
 Count the number of squares in each quadrat within which the species occupies over half the square
 If 30 squares contain the species, the percentage cover is 30%
 This method is subjective and therefore the same person should make the estimate for all samples to control this variable
Calculating the abundance of a species using frequency (%):
 Place a frame quadrat at multiple coordinates generated
 Count the number of quadrats that contain the species
 If 3 out of 10 quadrats contain the species the frequency is 30%
Interpreting results from quadrats
 The results from the quadrats can be used to calculate the predicted frequency and density of a species within an area
 Species density indicates how many individuals of that species there are per unit area
 The number of individuals counted across all quadrats is divided by the total area of all the quadrats
 For example, if 107 bluebells were found across 50 quadrats that are 1m^{2 }each the species density would be 107/50 = 2.14 individuals per m^{2}
 It can sometimes be difficult to count individual plants or organisms. When this is the case percentage cover of the species within the quadrat can be estimated instead
 The quadrat is divided into 100 smaller squares. The number of squares the species is found in is equivalent to its percentage cover in that quadrat
 For example, if grass is found in 89 out of 100 squares in the quadrat then it has a percentage cover of 89%
 Species frequency is the probability that the species will be found within any quadrat in the sample area
 The number of quadrats that the species was present in is divided by the total number of quadrats and then multiplied by 100
 For example, if bluebells were found in 18 out of 50 quadrats the species frequency would be (18/50) x 100 = 36%
How to estimate percentage cover of one or more species using a quadrat
Limitations of using quadrats
 Quadrats and transects can only be used for sessile and immobile species (eg. plants and slowmoving animals)
 Some species can be counted to find their abundance but others that are very small or in high numbers require abundance to be calculated using percentage cover or frequency techniques
 The frequency technique shows how common a species is but it does not give information on the estimated number of individuals or the size of the population
 Percentage cover and frequency, when used together, give a good picture of the distribution of a species
 If a species had a high mean percentage cover but a low frequency it would suggest the species lives in groups in preferred areas of the habitat
 This can be used to answer questions such as: does the species prefer light or dark and wet or dry areas etc.
The markreleasecapture method: estimating population sizes of mobile animals
 The two sampling methods described above are only useful for nonmotile (sessile) organisms
 Different methods are required for estimating the number of individuals in a population of motile animals
 The markreleasecapture method is used
 For a single species in the area:
 The first large sample is taken. As many individuals as possible are caught, counted and marked in a way that won’t affect their survival e.g. if studying a species of beetle, a small amount of brightly coloured nontoxic paint can be applied to their carapace (shell)
 The marked individuals are returned to their habitat and allowed to randomly mix with the rest of the population
 When a sufficient amount of time has passed another large sample is captured
 The number of marked and unmarked individuals within the sample are counted
 The proportion of marked to unmarked individuals is used to calculate an estimate of the population size
 The formula for the calculation is:
N = (n_{1} × n_{2}) ÷ m_{2}

 Where:
 N = population estimate
 n_{1 }= number of marked individuals released
 n_{2} = number of individuals in the second sample (marked and unmarked)
 m_{2} = number of marked individuals in the second sample
 Where:
Worked Example
Scientists wanted to investigate the abundance of leafhoppers in a small grassy meadow. They used sweep nets to catch a large sample of leafhoppers from the meadow. Each insect was marked on its underside with nontoxic waterproof paint and then released back into the meadow. The following day another large sample was caught using sweep nets. Use the figures below to estimate the size of the leafhopper population in this meadow.
 No. caught and marked in first sample (n_{1}) = 236
 No. caught in second sample (n_{2}) = 244
 No. of marked individuals in the second sample (m_{2}) = 71
Step One: Write out the equation and substitute in the known values
N = (n_{1} × n_{2}) ÷ m_{2}
N = (236 × 244) ÷ 71
Step Two: Calculate the population size estimate (N)
N = 57,584 ÷ 71
N = 811
Results from markreleasecapture
 When using the markreleasecapture method, there are a few assumptions that have to be made:
 The marked individuals are given sufficient time to disperse and mix back in fully with the main population
 The marking doesn’t affect the survival rates of the marked individuals (e.g. doesn’t make them more visible and therefore more likely to be predated)
 The marking remains visible throughout the sampling and doesn’t rub off
 The population stays the same size during the study period (i.e. there are no significant changes in population size due to births, deaths or migrations into or out of the main population)