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Counting Dots
Determining Caribou Population
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From the early 1970's when the Porcupine caribou herd was first counted and about every two or three years since then, the Porcupine caribou herd has been counted. The process of counting a population is called 'taking a census.' Over time, since the herd has been counted, their numbers have grown from about 100,000 to about 178,000, and then back down to about 123,000 animals in the2001 census.

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Big group-early July
Courtesy Canadian Wildlife Service


How do you count a whole herd of caribou? Dr. Stephen M. Arthur, a research biologist with the Alaska Department of Fish and Game is involved with census taking. He explains how the census is made:
"In brief, the census method is this: during early summer (usually between July 1-4) caribou from the Porcupine herd are on the coastal plain of northeastern Alaska and northern Yukon. This is the period when warm weather first occurs, with temperatures often in the 80s (F) and 24-hour daylight. Warm weather brings out the insects (mosquitoes, warble flies, and bot flies) that harass the caribou. In response to the insect harassment, caribou tend to form large, dense groups and move either to the Arctic coast or to mountain ridges, both of which provide windy areas with fewer insects. While caribou are in these large groups, it is possible to photograph them using a standard aerial camera that takes 9x9 inch, high-resolution photographs. The caribou on the photographs can be counted to develop an accurate estimate of the total herd size.

Caribou, showing radio collar


"Currently, about 85 caribou from this herd are equipped with radio collars which among other things helps us to find them. On the day of the census, 2 or 3 small airplanes search the coastal plain and eastern Brooks Range mountains to find the radio-collared caribou, as well as any groups without radio-collared caribou. The location of each caribou group is recorded using the aircraft GPS, and this information is relayed by radio to the photography airplane. This is a larger airplane (DeHavilland Beaver) equipped with an aerial camera mounted in an opening in the floor of the plane. The photography airplane will then fly over each group and take as many photographs as are needed to include the entire group. Photographs usually

1000's of dots-summer
Courtesy Canadian Wildlife Service


overlap by 10-20% to be sure that all caribou are photographed. When the film has been developed, the photographs (300-400 of them) must be examined and the areas of overlap identified so that no caribou are counted twice. Then, the caribou visible on each photograph are counted. This a very time-consuming job, so the photographs are usually distributed among 8-10 people for counting."

Declining Numbers
Biologists are not especially concerned about the current decline, because the rate of decline has been slow and this may be part of a natural cycle. However, they will continue to closely monitor the population and attempt to identify potential threats to its recovery. Dr. Arthur continues: "Although the Porcupine herd is still large enough to meet the demands of local villages for food and to provide for the limited sport hunting that occurs, there is some concern that, if the current decline continues, then some action will be needed to stop the decline. Furthermore, the area used by the herd during the calving and post-calving period, and parts of the herd's winter range, are being considered for oil exploration. If human actions occur that reduce the production and/or survival of calves, it will be more difficult to stop the decline and return the herd to previous levels."

Journaling Question
  • How do you think census information can be used to help protect the Porcupine caribou and other wild animal populations?

Try This! Counting "Dots" with a Grid
Biologists use a grid system to make the most accurate counts. You try it!

  1. Click on the image to reveal a large section of a fly-over photograph of a herd. Print one photograph for each group.
  2. Decide how you want to divide the large picture to get the most accurate count. You may want to cut it into rectangular sections, or random shapes following natural lines where the animals lie.
  3. Count the animals found in each section of the photograph and add them together to find the total number of animals in this fly-over sample.
  4. What kinds of problems did you encounter? Did your group have the same count as the other groups? Some of the caribou appear so much smaller, why?

Or This! Density Studies
How crowded are those caribou? You can get an idea of the density of animals (number of animals in certain amount of space) by doing some math while you are counting dots!
The average adult Porcupine caribou is about 1.5 - 2.0 meters (5 - 6.5 feet) long when viewed from above. Of course, the exact length will depend on the posture of the animal: if neck is bent or held straight out, if the animal lying in a curled position, etc. (Calves are much smaller than this, and their length would need to be estimated.)

  • Can you figure out the dimensions (L and W) of the land in the photo using the information we gave you above? Now calculate the area (L x W).
  • What is the total density (number of animals per meters/foot squared) of animals in this photo?
  • Given this high density of animals on this piece of land in the arctic coastal plain, what can you say about the plant life here?