1. © SSER Ltd. How can we investigate populations?

2. Ecological Techniques A variety of universal practical approaches exist for investigating the biotic and abiotic components of ecosystems Field ecologists investigate communities by using random sampling techniques that aim to: eliminate bias reduce the effects of chance minimise errors in obtaining measurements/samples Quadrat and transect sampling are commonly used methods for investigating distribution within plant communities and the more sedentary populations of animals such as those found inhabiting seashores

3. Quadrats Frame quadrats are wooden or metal frames of known area (e.g. 0.25 m 2, 0.5 m 2, 1.0 m 2 ) and are generally square-shaped Various approaches are used to determine the representation of species within an area: Species population density (number of individuals per square metre) is determined by counting all individuals within a series of randomly placed quadrat frames and calculating the mean Percentage cover is determined subjectively by assessing the extent to which different species occupy subdivided quadrat frames or by the use of point quadrat frames Frequency of occurrence of a species is determined using subdivided quadrats and point quadrat frames

4. Quadrats Square quadrat frame for determining population densities Subdivided quadrat frame for determining % cover of species Quadrat frames, constructed from wood or metal, are used to investigate the distribution of species

5. Point Quadrat Frame Metal pins are dropped through each hole in turn and ‘hits’ are recorded No. of hits % Cover = ------------------- x 100 Total no. of pins

7. Random Sampling The validity of results obtained from the various sampling methods is dependent upon the adoption of random sampling techniques Strategies for avoiding bias through random sampling utilise a number of approaches – these include random sampling using a grid A grid is created by laying out tapes at right angles to one another to form the axes of the gridded area Pairs of random numbers are used to provide the coordinates for locating quadrats

8. Tapes are laid out at right angles to one another and form the axes for locating quadrats (the size of the grid depends upon the area being investigated) Pairs of random numbers are generated on a calculator or computer and used as coordinates for the location of quadrats

9. Using Random Numbers Pairs of random numbers are selected by reading across or down the table in any direction e.g. X09 Y12, X14 Y07, etc. or X09 Y06, X14 Y03 etc. Many random number pairs are needed for large grids

11. This quadrat has coordinates X09, Y06

12. Transects A transect is a line, created with string or a tape, along which systematic sampling is performed Transects are particularly useful for sampling areas where there is a transition of species from one habitat to another as environmental conditions change Transect studies are used to investigate gradients such as zonation on rocky shores and changes in the species diversity across sand dunes A line transect is one in which all individual organisms touching the tape/string are recorded The most commonly used belt transect involves laying a tape through the area of study and sampling the population with quadrats positioned at regular intervals alongside the tape

13. Belt transect Survey of a Dune System A belt transect was used to investigate the distribution of three species of grass commonly found on sand dunes The transect line stretched from the high water mark to the inland area and 1m x 1m quadrats were used to determine the number of individual plants of each grass species along the profile

14. Present the results as a bar chart

17. A Tale of Two Fields This aerial photograph shows two adjacent fields from a farm in Lancashire taken just prior to the growing season A group of students carried out diversity studies in these fields during the summer months The students found differences in the dandelion populations in the two fields and carried out further studies to clarify these findings using aerial photographs taken during the summer period

18. FIELD 1 - The farmer uses field 1 for his large herd of grazing cows during the summer months

19. FIELD 2 - Field 2 is the farmer’s hay field on the other side of the hedge

20. FIELD 2 - Large populations of tall grasses grow in field 2 during the summer months

21. FIELD 2 - Harvesting in field 2 towards the end of the summer

22. Random Sampling Using A Grid The students used aerial photographs of the two fields to compare the dandelion populations (photographs provided in the workbook) A 15 cm x 15 cm square was drawn around each field photograph and divided up into 1 cm squares The X-axis was drawn along the bottom of the photograph with the Y-axis on the left hand side Random number tables were used to provide coordinates for locating quadrat positions Quadrat positions were identified by treating the crossing coordinate lines as the lower left-hand corner of each quadrat Thirty different coordinates were used to locate quadrats and the number of dandelions in each quadrat was recorded (the north west rule was applied to ensure that no individual was counted twice)

23. Estimate the population size Formula: Mean number of organisms x area of field per quadrat area of quadrat Make sure it’s the same units (m² or cm²)

24. Field 1 – The white dots locate the dandelions in the field

25. Field 2

26. 00 01 02 03 0405 06 07 08 09 10 1112 13 14 15 16 17 18 Field 1 (with an 18 x 18 cm grid)

27. Exercise – A Tale of Two Fields Use the photographs provided in the workbook to create 15 cm x 15 cm grids for the two fields Using the provided random numbers table, count and record the number of dandelions in 30 randomly located quadrats for each field Calculate the mean number of dandelions for each field State a Null and Alternative Hypothesis for the investigation Test the statistical significance of any differences between the two datasets using the Mann-Whitney U Test for non-normal data Provide an explanation for any differences that you find between the grazed field (Field 1) and the hay field (Field 2)

29. The capture-mark-recapture technique can be used to estimate the population size of mobile organisms, such as woodlice Organisms are captured using an appropriate technique (pitfall traps, sticky traps, etc) Captured organisms are marked with non-toxic waterproof paint Marked organisms are released back into the environment After a suitable period of time, the population is sampled again The number of marked and unmarked organisms in the second sample is recorded

30. Estimating The Population Size Data obtained from the capture-mark- recapture technique is used to calculate an estimate of population size using the following equation: number of organisms in first sample x in second sample Population size (P) = ------------------------------------------------------ number of marked individuals captured Example: In an investigation into woodland habitats, 42 woodlice were caught, marked and released. Two days later 40 woodlice were caught of which 8 were found to be marked. Estimate the size of the woodlouse population

31. Estimating The Population Size number of organisms in first sample x in second sample Population size (P) = ------------------------------------------------------ number of marked individuals captured This calculation is only valid if certain assumptions about the sampled population hold 42 x 40 Population size (P) = ------------------------------------ = 210 8

32. Estimating The Population Size Assumptions Marking should not disadvantage individuals in any way, e.g. making organisms more visible to predators Marked individuals are assumed to mix completely back into the original population There should be no immigration or emigration of individuals between sampling periods, i.e. the population is closed It is assumed that there are no births or deaths during the period of the study

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