Biology: Mark and Recapture

Purpose

This activity will expose students to the most basic step in population ecology: population size estimation. Students will be exposed to the mark and recapture method through a physical activity and worksheet. As a group, the students will discuss the uses and limitations of the method.

Additional Required Materials

• Trap: an opaque bag or box large enough to hold all tokens
• Tokens: at least 1.5x the number of students and two different types; e.g. plastic animals, poker chips, wooden blocks, large beans, folded pieces of paper, play money
• Markers: for "tagging" tokens, choose an appropriate medium; e.g. markers for plastic tokens, hole punch for paper

Procedure

(The setting for the flavor text can be adjusted for whatever token is used e.g. the ocean for plastic fish, a cave for plastic bats, a Secret Service lab for "counterfeit" dollars. Additionally, larger classes can mark and count both types of tokens for activity variety)

1)Set the stage for the activity:

"We're all field biologists and we've flown to the black forest in Germany to study the local mouse population. Last night each of us set a live capture trap in a different part of the forest and today, we'll see how many we caught."

2)Walk around the room and let each student reach into the "trap" and draw out a single token.

3)Explain the mark and recapture procedure in the context of the tokens:

"Some of you found a mouse in your trap and some of you found a shrew. Because we aren't interested in the shrew population, we can release those back into the forest right now. For those of you that caught a mouse, we are going to mark each mouse we captured with a black marker on the right back leg. This is so when we release the mouse back into the forest, we'll be able to identify the ones we have captured before. This is called the mark and recapture method and is used by biologists around the world to count populations of all sorts of animals like fish, birds, and even snails."

4)Have each student return their token to the container, keeping a class tally of how many mice are released.

"Now, we are going to release all the mice we have captured and marked back into the forest, and in a week, we'll go ahead and set the trap and see how many we can catch. As we release the mice, we need to keep track of how many we marked: can someone volunteer to keep a tally?"

5)Shake the container to mix the tokens. Distribute the worksheets and discuss the field of ecology. A short video on mark and recapture in the field would be appropriate, perhaps in bird banding. Explain how the method of mark and recapture works (bold words indicate an opportunity to pause and check the students' vocabulary and understanding of basic statistics and ecology concepts):

"As I mentioned before, what we are doing today is called mark and recapture. It is a technique that allows biologists to estimate the number of individuals in a local population. The first step in the process is to catch a random subset of the population. Ideally, the traps are set across the entire habitat for a truly representative sample, like we did with the mice in the forest. In our experiment, after we captured the mice, we marked their feet with a marker. In the field, biologists often use bands, collars, or tags to mark the animals. Maybe some of you have seen a bird with a small metal band around their leg: that is an animal that has been captured and marked by biologists. For animals like fish, biologists can often clip one of the bony spines near the tail; the important thing is to make sure we don't harm the animal while making sure the animal cannot remove the marking, or else the next step doesn't work. And that next step is to release all of the animals we captured back into the habitat we caught them from. We want all the animals we caught to intermingle with the population and become randomly distributed in their habitat, just like when we first caught them. Then we wait: how long depends on how fast the animals we caught can move and their life cycles. In our experiment we decided to wait a week, but depending on the animal, you could come back in a month or even a year. What we are going to do next is set our traps again and see how many of the new mice we capture have the markings. By comparing the number of mice we marked with the number of mice we caught the second visit with and without markings, we can estimate the total number of mice running around in the forest."

6)Introduce an alternative example to teach the mathematics portion as a class:

"Before we recapture our mice, let's look at an example problem so you can see how easy the math is. A group of students go down to the lake and set up traps to catch grass carp. On their first visit, they manage to catch 35 fish and mark then by painting a blue dot on the dorsal fin. They come back in a month and catch 40 fish, but only 10 have the blue marking."

"What we have here is a ratio, and we know how to solve ratios."

"In our example, the students would estimate that there are 140 grass carp in total living in the lake. If no one has any questions, let's go check our traps and see how many mice we caught this time."

7)Repeat the trapping procedure and return the shrews to the "forest" while keeping a class tally of the total number of mice and the number of mice with markings on the board. Direct the students to their worksheets and provide guidance where necessary.

8)Once the class has finished the worksheet, go over the calculations as a class to reinforce the learnings. As a class, perform a total count of the tokens and compare the estimate to the actual population.

9)Open a discussion about the reasons to use the mark and recapture method, the assumptions made, and the situations where the method becomes unreliable or inappropriate.

Assumptions

• Closed population: no migration, births, deaths
  
• Marking has no effect on survivability or mixing: changes in mobility or camouflage, acceptance by groups
  
• The chance an individual will be captured are the same for the first and second visits: no change in capturability or behavior
  
• Released individuals mix randomly with the local population: territorial or herd animals are a problem
  
• Markings are permanent during the time period of the study:
  

Discussion Prompts (in the context of the above assumptions)

• Why would the time between visits be different for different types of animals? (speed of mixing, fecundity and mortality rate)
  
• What happens to the estimate if a marking falls off? (population estimate will become positively biased)
  
• What would happen to the estimate if a large number of new animals are born? or Die? (positive bias for births, negative bais for deaths. it would however reflect the current population)
  
• Knowing that wolves live in packs, would mark and recapture be a suitable for estimating the number of wolves in a entire forest? (no, packs and herds are not randomly distributed in their range and local populations)
  
• What would happen if the captured individuals become trap-shy? or trap-happy? (positive bias for shy, negative bais for happy)
• What would happen if the markings made it easier for a predator to catch the individuals? (same as if the marking falls off)
• What way could we try to get closer to the actual population count? (more replicates, larger sample sizes)