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How can we investigate populations?

Ecological Techniques

A variety of universal practical approachesexist 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

Quadrats

Frame quadrats are wooden or metal frames of known area (e.g. 0.25 m2, 0.5 m2, 1.0 m2) 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

Quadrats

Square quadrat framefor determining

population densities

Subdivided quadrat framefor determining % cover

of species

Quadrat frames, constructed from wood or metal, are used to investigate the distribution of species

Point QuadratFrame

Metal pins are dropped through each hole in turn and ‘hits’ are recorded

No. of hits% Cover = ------------------- x 100

Total no. of pins

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

Tapes are laid out at right anglesto 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 coordinatesfor the location of quadrats

09 12 14 07 11 11

06 03 01 05 03 10

14 03 07 02 15 00

03 07 00 05 10 14

00 13 04 14 01 14

Using Random Numbers

Pairs of random numbers are selectedby 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 forlarge grids

09 12 14 07 11 11 04 02 11 03

06 03 01 05 03 10 06 02 02 10

14 03 07 02 15 00 09 06 04 11

03 07 00 05 10 14 15 06 11 00

00 13 04 14 01 14 06 03 14 12

12 05 02 03 06 05 08 03 07 01

08 04 09 05 06 04 06 14 12 13

10 01 14 06 09 07 10 11 13 03

14 02 00 10 12 00 13 06 11 10

04 02 11 01 02 11 03 01 07 03

15 15 10 09 05 00 13 08 13 01

14 14 01 01 02 09 05 01 08 15

06 02 15 11 02 11 11 04 05 07

07 05 07 06 09 04 07 00 07 03

14 10 03 06 11 07 13 11 13 12

13 11 13 08 12 07 03 09 11 11

02 01 01 04 12 03 03 02 03 06

14 07 00 00 13 08 14 12 10 11

00 09 01 13 12 11 05 00 00 04

07 03 02 11 15 13 01 05 12 10

10 02 12 05 08 13 06 06 09 11

06 05 07 06 14 04 10 00 03 12

07 12 04 14 10 02 14 04 04 13

01 11 05 11 06 14 06 15 04 01

01 04 05 02 07 13 09 13 08 06

03 09 01 02 02 15 09 09 07 07

06 13 11 05 15 07 11 03 06 07

12 05 03 04 14 06 14 04 04 04

12 06 02 05 04 15 03 09 09 05

05 15 03 10 09 02 03 10 14 05

14 11 14 08 06 03 07 04 13 15

06 03 15 08 11 01 01 13 04 04

05 12 02 11 09 11 11 01 02 00

13 11 15 11 11 09 12 08 12 05

07 13 13 09 00 14 11 13 06 04

09 14 15 07 14 08 05 04 14 12

03 11 12 15 10 15 15 06 13 10

15 08 11 06 05 15 09 05 01 08

This quadrat has coordinates X09, Y06

Transects

A transect is a line, created with string or a tape, along which systematic sampling

is performedTransects 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

Belt transect Surveyof 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

Number of plants/m2 Distance inland from High

Water Mark (m) Sand couch grass

Marram grass Sand Fescue

0-5 3

5-10 8

10-15 27

15-20 6 3

20-25 6

25-30 38

30-35 29

35-40 15

40-45 8 17

45-50 48

50-55 88

55-60 97

Present the results as a bar chart

0

10

20

30

40

50

60

70

80

90

100

0-5 5-10 10-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60

Distance from high water mark (m)

Nu

mb

er o

f p

lan

ts p

er m

2

Sand couch grass

Marram grass

Sand fescue

A Tale of TwoFields

This aerial photograph shows two adjacent fields from a farm in Lancashire taken just prior to the

growing seasonA 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

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

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

FIELD 2 - Large populations of tall grassesgrow in field 2 during the summer months

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

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

quadratwas recorded (the north west rule was applied to ensure

that no individual was counted twice)

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)

Field 1 – The whitedots locate the dandelions

in the field

Field 2

00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18

Field 1(with an

18 x 18 cmgrid)

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 Hypothesisfor 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)

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

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 number of organisms in first sample x in second samplePopulation size (P) = ------------------------------------------------------

number of markedindividuals 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

Estimating The Population Size

number of organisms number of organisms in first sample x in second samplePopulation size (P) = ------------------------------------------------------

number of markedindividuals captured

This calculation is only valid if certain assumptions about the sampled population hold

42 x 40Population size (P) = ------------------------------------ = 210

8

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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