APES: Animal Population Survey-Tag and Recapture
Scientists often need to know about the conditions of an area. In order to accomplish this task, they survey and map the land, test the water, investigate the soil and rocks and survey and count the local populations of plants and animals. Because many animals are constantly moving around and hiding from humans, it is difficult to count them.
How do biologists determine the population of a species in a particular area? There are a variety of ways that it can be done, however the most common method involves tagging. With ta and recapture instead of counting every animal, you capture a group of animals and tag them then release them. Then, after some time has passed for the animals to return to their environment and to “redistribute” themselves, you capture another group of the same kind of animals and see how many of them were previously captured and tagged. We would expect that these sample percentages should vary “around” the true population percentage. Using this assumption, you can calculate the approximate population size.
Materials: goldfish crackers markers plastic cups paper lunch bags
Each of the teams in class has a population of “goldfish.” Your goal is to approximate the size of your population using the same tagging and sampling methods that biologists use. The directions are as follows:
1. “Capture a small cup full of fish. This will be your original tagged group. Count and mark these fish using a sharpie. Record the tagged amount in your field notebook as “total number tagged.”
2. Return the tagged group to the bag and shake it up.
3. Now the fishing begins! Capture a small cup full of fish. This is a recapture; try to make it the same size as your original capture. Count the total number of fish you recapture (both tagged and untagged). Record this information in a data table in your field notebook. (Total # captured= # of tagged fish + # untagged fish).
4. Repeat step #3 nine more times, recording the numbers each time.
5. Find the sum of the tagged column and the sum of the captured column.
6. Find the average of the tagged and capture column by dividing the sum by the number of samples.
7. Use the proportion formula BELOW to predict the total number in the population
# originally tagged = average sample percent tagged (X)
Total population (p)
Solving the equation above, one gets the following results:
# tagged = X therefore… p = # tagged
P X
8. Using your information, predict the population size. Now, count your entire population and determine how close your estimate was. Record in your field notebook the predicted population, actual population and how close your estimate was.
Analysis Questions:
1. What could cause your results to be off from the actual population?
2. How would sample size and population size affect these results?
3. How would the number of samples affect these results?
4. If you were predicting a large population (as in a real pond), would the number you were off really have been that bad relatively speaking?
5. What concerns should a biologist have about a species’ habits before using this method to approximate the size of a population? Even with these concerns, does this mean that tagging should not be used by biologists? Are there other uses of tagging?
6. Solve the following problems. SHOW ALL WORK.
a. Gypsy moth populations soar every few years in the Northeastern deciduous forests, causing great damage to the trees their larvae eat. In order to determine the population of gypsy moths in a forest, 200 were trapped, marked and released. The next night, more moths were collected. Of the 150 that were collect, 15 were already marked. What is the size of the population of gypsy moths in the forest?
Hint: 200 marked = 15 recaptured & marked
Total populations 150 recaptured
b. In order to determine snail populations, 340 snails were captured, tagged and released. Later, 420 snails were captured. Of the 420 snails, 16 were already marked. What is the size of the snail population?
c. 150 marlin were captured, tagged and returned to the deep ocean, where they live. Later, when 140 marlin were captured, 7 of the marlin had tags. What is the size of the marlin population?
d. Describe one factor that might interfere with the accuracy of the population estimates for each of the examples above.