# Practical: Investigating Biodiversity Using Sampling ## Use of sampling techniques in fieldwork - Sampling is a method of investigating the **abundance and distribution** of species and populations - It allows researchers to estimate characteristics of a habitat without examining every single organism present - There are two different types of sampling: - **Random sampling** - **Non-random sampling**

In random sampling, the positions of sampling points are completely random or due to chance

In non-random sampling, the positions of sampling points are chosen by the person carrying out the sampling

- Different sampling equipment can be used depending on the habitat and organisms being studied: - **Quadrats** - for plants and slow-moving animals - **Sweeping nets** - for insects in long grass or vegetation - **Pitfall traps** - for ground-dwelling invertebrates - **Pooters** - for collecting small insects - **Tullgren funnel** - for extracting small invertebrates from soil or leaf litter ## The quadrat method of sampling ### Apparatus - Quadrat (typically 0.5m × 0.5m or 1m × 1m) - Random number generator (calculator, computer program, or random number table) - Tape measures or ranging poles (for marking out the study area) - Recording sheets and clipboard - Pencil ### Method for choosing sample sites - Mark up a grid on a map or a to-scale 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 under-estimation - Without random sampling, you might subconsciously choose areas that are easier to access or that look like they contain more individuals

Sampling bias occurs when the sample is not representative of the whole population, leading to inaccurate conclusions

### 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 $\text{m}^2$): - 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 - This tells you when you've sampled enough - The graph below shows an example of a running mean. Notice how the mean fluctuates at first but then **stabilizes after around 8 quadrats**, indicating that sufficient sampling has been done:

- To calculate the **estimated population size** for the whole area or habitat: - Divide the whole area by the area of one quadrat - Then multiply this value by the mean number of individuals per quadrat $$\text{Estimated population} = \frac{\text{Total area}}{\text{Area of one quadrat}} \times \text{Mean individuals per quadrat}$$ #### Calculating the abundance of a species using percentage cover (%): - Usually used for **plants** where individual organisms are difficult to count - Place a **grid quadrat** (a square frame split into 100 smaller squares) at the generated 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

Percentage cover is particularly useful for overlapping plants like grass or moss where counting individuals is impossible

#### 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 $1\text{ m}^2$ each, the species density would be $107/50 = 2.14$ individuals per $\text{m}^2$ - 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 $(18/50) \times 100 = 36\%$ ### Limitations of using quadrats - Quadrats can only be used for **sessile and immobile species** (e.g. plants and slow-moving animals) - Mobile animals will move away from the quadrat before they can be counted - 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 mark-release-capture method of sampling (estimating population sizes of mobile animals) - The sampling method described above is only useful for non-motile (sessile) organisms - Different methods are required for estimating the number of individuals in a population of **motile animals** - The **mark-release-capture method** is used ### Method 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 - For example, if studying a species of beetle, a small amount of brightly coloured non-toxic 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: $$N = \frac{n_1 \times n_2}{m_2}$$ 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

The larger the proportion of marked individuals recaptured, the smaller the total population must be

### 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 non-toxic 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** $$N = \frac{n_1 \times n_2}{m_2}$$ $$N = \frac{236 \times 244}{71}$$ **Step Two: Calculate the population size estimate (N)** $$N = \frac{57584}{71}$$ $$N = 811$$ ### Results from mark-release-capture When using the mark-release-capture 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 - If marked individuals haven't mixed properly, the second sample won't be representative - 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) - Bright marks could attract predators or make individuals less attractive to mates - The marking **remains visible** throughout the sampling and doesn't rub off - If marks disappear, marked individuals will be counted as unmarked in the second sample - 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) - Immigration, emigration, births and deaths would all affect the accuracy of the estimate