Week 4 - Sampling, Bio-invasion and Predation, and Succession

LESSON 3.1: Investigating the Distribution of Living Things in the Environment
ACTIVITY A: Sampling the Distribution of Living Things

Required Readings

Science in Action 7
pages 50-54

Science Focus 7
pages 68-78

Watch the movie "CAPTURER

Measuring populations is important work. By studying the habitats, populations, and ranges of different species, we can learn how species interact, where they tend to migrate, and the number of individuals in a species in that area. We can also understand how a species is distributed. Scientists who want to monitor the population trends of different species in the community would find it very difficult to go into a community and try to count or observe every living organism. Scientists have therefore had to develop techniques to help them estimate population sizes and track the range of different organisms. These methods involve counting a small sample that represents the larger population. These accurate counts are then multiplied to give an estimate over the entire area being sampled

There are a number of methods employed by scientists doing population sampling:

Mark-Recapture Method

What about those animals that are not as visible such as the mice in a field or the Bull Trout in Alberta's rivers and streams. How do you measure these populations?

To perform the mark-recapture method, you must first capture the organisms and mark them with some form of identification within a set period of time and a set area. If it is a mouse, you could mark it with a dot of paint. If it is a large mammal, it could be an ear tag. After you have completed the capture and mark step, you will need to record the number caught and marked, and then release your catch back in the area they were captured. After a period of time you will need to return to the sample area and re-capture the organisms.

When you re-capture the organisms you will record how many organisms have the original mark and how many are unmarked. From this you will be able to estimate what the population of this species is in this area.

After you view the video and you are having trouble understanding this method, go to the Internet and the web search engine called "Google". Now type in "mark and recapture method" then type "Enter" to view the results of your search. Now read the possible contents of the various results until you decide which one is most likely to have the information related to this lesson in it. Then you can learn how the process works by checking out a couple of web sites.

  • Mark-Recapture Method
  • Photograph or Video Analysis Method
  • Radio Tracking Method
  • Quadrat Sampling
  • Line Transect
Photograph or Video Analysis Method

Population counting gets a little more interesting when you try to study animals in the wild. Unlike plants, most animals have the tendency to flee or fight at any sign of danger. Neither of these instincts are good when you are a curious scientist. To get accurate estimates of animal populations you must be more creative.

One technique which allows you to keep your distance and gather information about groups of animals is the photograph or video analysis method. This is ideal for birds flying in groups overhead or herds of large animals such as caribou. If a herd can be photographed from an aircraft, it can be analysed later on the ground. If a group is spread over a wide area, numerous photographs may need to be taken.

To simplify the counting task, you can lay a grid over the picture and count the amount of organisms in each grid square. Record each number and add up all the numbers to get a total for the picture. When using video, the ability to analyze the film frame by frame makes counting the specimens a realistic task.

Radio Tracking Method

To better understand a species' range, the radio tracking method is employed to monitor the day to day movements of a species.

In Canada's western Rockies and grassland areas, the grizzly bear population is being carefully studied. Ecologists are worried about the decline in the population of the Grizzly bear in these areas. At one time, the grizzly population was around 6000 bears in Alberta. Today, there are approximately 800 remaining. Management and conservation of the bears needs to be based on a thorough knowledge of the species. To study the bears habitat, a few of the bears have been trapped, fitted with radio collars and released.

Scientists now track the bears with radio receivers from the ground and air. This provides scientists with a clear profile of the complete habitat and movements of the grizzly without disturbing their natural movements.

If human development is proposed for areas that overlap with the grizzlies' range, efforts can then be made to allow the bears' habitat to remain undisturbed.

ACTIVITY B: Quadrant and Line Sampling

Required Readings

Science in Action 7
pages 50-54

Science Focus 7
pages 68-78

Watch the movie "QUADRAT"
If you need to estimate the population for small organisms, quadrat sampling is the best method to use.

Quadrant Sampling

In the spring, when dandelions go through their first flowering cycle, you observe that there may be many flowers all over your lawn. Your parents decide to hire someone to spray the lawn to make the grass grow and to remove the dandelions. You decide to do a study to see how effective the lawn spray will be in removing the dandelions. Before the lawn is sprayed you decide to count the dandelions, but as there appears to be thousands of flowers on the lawn, it would be too time consuming to count them all. So you use quadrat sampling.

You pick a one metre square area of the lawn that appears to represent the general dandelion population. Here you take four short pegs and place one of them in each corner of your sample square meter. You tie a string around the four pegs to distinguish the area you would like to sample. Then you count the dandelion plants that grow in that area. After you have counted the area inside your square meter, you calculate the total area of your lawn to come up with an estimated number of dandelions on the lawn.


1. Dandelions counted in area sampled: 143 dandelions per metre square
2. Size of lawn: 12 m long by 7 m wide
3. Calcuate the area of the lawn: 12m x 7 m= 84 m2
4. Estimate the number of dandelions on the lawn: 143 dandelions x 84 m2 lawn= 12 012 dandelions on my 84m2 lawn.
5. Answer in a complete sentence: It is estimated that there are 12 012 dandelions growing on my lawn that measures 84 m2.
If you were to do this experiment in the fall after the lawn had been sprayed you could then tell if the spraying had an effect on the numbers of dandelions.

Line Transect

When estimating low growing plants, quadrat sampling is the best method to use. But it is not an appropriate method when larger objects need to be measured. The line transect method allows the scientist to estimate which species are most common to an area.

To do a line transect study, you would need a few poles and a string approximately 10 meters in length . Along your string, mark or tie something at even intervals of approximately 50 centimetres. Tie the ends of the string to two poles that can be held in a vertical position by fellow scientists. While they are holding the poles against the ground the string that runs between the two poles should be straight and taut. Now you need to count and record all those things touching or growing directly above or below each interval along the string.

Exercise 3.1: Quadrant Sampling