19.5 - Investigating Populations

Question 1

What is abundance

Answer 1

The number of individuals of a species within a given area

Question 2

Is it possible to identify and count every organism in a habitat

Answer 2

• no
• would be time-consuming and would almost certainly cause damage to the habitat being studied.
• For this reason only small samples of the habitat are usually studied in detail.
  -As long as these samples are representative of the habitat as a whole, any conclusion drawn from the findings will be reliable

Question 3

What are the 3 sampling techniques used when sampling habitats

Answer 3

• Mark-release-recapture
• systematic sampling (transect)
• random sampling

Question 4

What are the 2 types of quadrats used in sampling

Answer 4

1) Frame Quadrat
2) Point Quadrat

Question 5

How does a point Quadrat work

Answer 5

• Consists of a horizontal bar supported by two legs.
• At set intervals along the horizontal bar are ten holes, through each of which a long pin may be dropped
• Each species that the pin touches is then recorded.

Question 6

How does a frame Quadrat work

Answer 6

• a square frame divided by string or wire into equally sized subdivisions
• It is often designed so that it can be folded to make it more compact for storage and transport.
• The quadrat is placed in different locations within the area being studied.
• The abundance of each species within the quadrat is then recorded

Question 7

What are the three factors to consider when using quadrats:

Answer 7

• the size of Quadrat to use
• The number of sample quadrats to record within the study area
• The position of each quadrat within the study area

Question 8

Why do we need to consider the size of quadrat to use.

Answer 8

• It will depend on the size of the plants or animals being counted and how they are distributed within the area.
• Larger species require larger quadrats.
• Where a population of species is not evenly distributed throughout the area, a large number of small quadrats will give more representative results than a small number of large ones.

Question 9

Why do we need to consider the number of sample quadrats to record within the study area.

Answer 9

• The larger the number of sample quadrats the more reliable the results will be.
• As the recording of species within a quadrat is a time-consuming task a balance needs to be struck between the reliability of the results and the time available.
• The greater the number of different species present in the area being studied, the greater the number of quadrats required to produce reliable results for a valid conclusion.

Question 10

Why do we need to consider the position of each quadrat within the study area.

Answer 10

To produce statistically significant results a technique known as random sampling must be used.

Question 11

sampling at random is important…

Answer 11

• …to avoid any bias in collecting data.
• Avoiding bias ensures that the data obtained are reliable.

Question 12

What is random sampling used for

Answer 12

• to estimate population size in a given area
• organisms must be relatively evenly distributed in an area

Question 13

Describe how you would do random sampling

Answer 13

1) Divide the area into a grid and assign each square coordinates
2) Use a random number generator (e.g. a calculator or random number table) to randomly pick numbers — avoids bias
3) Use the numbers as coordinates to place the quadrats, then record the species within them

Question 14

What is transect sampling used for

Answer 14

• measuring abundance and distribution of a species
• it is a systematic technique that is more informative than random sampling
• used to sample when organism distribution has spatial variation
• usually looking at the effect of an abiotic factor on distribution of organisms

Question 15

Describe who you would do transect sampling

Answer 15

1) A belt transect can be made by stretching a string or tape across the ground in a straight line.
2) A frame quadrat is laid down alongside the line and the species within it recorded at each measured interval
3) count or calculate the % cover of organisms in the quadrat
4) It is then moved its own length along the line and the process repeated.
5) This gives a record of species at each distance

Question 16

Random sampling with quadrats and counting along transects are used to obtain

Answer 16

measures of abundance

Question 17

For species that don’t move around, it can be measured in several ways, depending on the size of the species being counted and the habitat. Examples include:

Answer 17

• frequency
• percentage cover

Question 18

Describe how you measure abundance for species that don’t move around using frequency

Answer 18

• it is the likelihood of a particular species occurring in a quadrat.
• e.g. if a species occurs in 15 out of 30 quadrats, the frequency of its occurrence is 50%.
• This method is useful where a species, such as grass, is hard to count.
• It gives a quick idea of the species present and their general distribution within an area.
• However, it does not provide information on the density and detailed distribution of a species.

Question 19

Describe how you measure abundance for species that don’t move around using percentage cover

Answer 19

• it is an estimate of the area within a quadrat that a particular plant species covers.
• It is useful where a species is particularly abundant or is difficult to count.
• The advantages in these situations are that data can be collected rapidly and individual plants do not need to be counted.
• It is less useful where organisms occur in several overlapping layers (more probably plants)

Question 20

To obtain reliable results, it is necessary to ensure that the sample size is…

Answer 20

• large
• many quadrats are used and the mean of all the samples is obtained.
• The larger the number of samples, the more representative of the community as a whole will be the results.

Question 21

What is Mark-release-recapture techniques used for

Answer 21

• motile organisms.
• Motile animals move away when approached.
• They are often hidden and are therefore difficult to find and identify.
• To estimate the abundance of these organisms, we need to use this technique

Question 22

How does the Mark-release-recapture technique work

Answer 22

• A known number of animals are caught, marked in some way, and then released back into the community.
• Some time later, a given number of individuals is collected randomly and the number of marked individuals is recorded.
• The size of the population is then calculated as follows:

Question 23

What assumptions is the Mark-release-recapture techniques reliant upon

Answer 23

• The proportion of marked to unmarked individuals in the second sample is the same as the proportion of marked to unmarked individuals in the population as a whole.
• The marked individuals released from the first sample distribute themselves evenly amongst the remainder of the population and have sufficient time to do so.
• The population has a definite boundary so that there is no immigration into or emigration out of the population.
• There are few, if any, deaths and births within the population.
• The method of marking is not toxic to the individual nor does it make the individual more conspicuous and therefore more liable to predation,
• The mark or label is not lost or rubbed off during the investigation

Question 24

Anecologist was estimating the population of sandhoppers on a beach. One hundred sandhoppers were collected, marked and released again, A week later 80 sandhoppers were collected, of which five were marked. Calculate the estimated size of the sandhopper population on the beach. Show your working.

Answer 24

100 x (80 divided by 5) = 1600

Question 25

When using the mark-releaserecapture technique, explain how each of the following might affect the final estimate of a population.
A) The marks put on the individuals captured in the first sample make them more easily seen by predators and so proportionately more are eaten than unmarked individuals.

Answer 25

Population over-estimated (appears larger) as there will be proportionally fewer marked individuals in the second sample.

Question 26

When using the mark-releaserecapture technique, explain how each of the following might affect the final estimate of a population.
B) Between the release of marked individuals and the collection of a second sample an increased birth rate leads to a very large increase in the population

Answer 26

Population over-estimated / appears larger as there will be proportionally fewer marked individuals in the second sample because all the ‘new’ individuals will be unmarked.

Question 27

When using the mark-releaserecapture technique, explain how each of the following might affect the final estimate of a population.
C) Between the release of marked individuals and the collection of a second sample, disease kills large numbers of all types of individual.

Answer 27

No difference because the proportion of marked and unmarked individuals killed should be the same.

Question 28

Ina mark-releaserecapture exercise, a sample of 120 woodlice were marked. After five days a second sample of 120 woodlice were collected. The population size was found to be 960. Calculate the number of marked woodlice that there were in the second sample.

Answer 28

(120 x 120) divided by 960 = 15