September Public Teen Library Event: Mark-Recapture

The EcoReach volunteers were so busy working with the kids, we forgot to take pictures. We’ll have more for our next event!

This month, the kids brainstormed some ways that we might estimate the organisms in different environments such as the number of trees in a forest, or the number of cows in a herd. Some ideas that came up were quadrats (a frame used in ecology and geography to estimate the size of a population or species over a large area), aerial photos, and making physical markings on organisms.


Then we talked about how techniques such as quadrats or aerial photos don’t work for organisms like fish or mice. Finally, we introduced the kids to mark-recapture, which is exactly what it sounds like. When scientists want to estimate the size of a population, they will “mark” an animal, either with a photo or a physical marking, and then they will return and “recapture” the animals to see how many individuals were previously caught and how unmarked animals are there. With this, you can use various math formulas to calculate the number of animals in a population.

Examples of mark-recapture

We focused on the Lincoln-Peterson index to calculate the size of a population. In short, this index is a way to estimate the number of animals in a population with only two visits to the study site. This estimation assumes no animals leave the area, die, or are born, which means the second visit must be very soon after the first visit. And because this is a type of mark-recapture, during the initial visit, the animals are marked in some way


Lastly, we then did an activity so that the teens could practice this technique. They were given a bag of beans, and they used the mark-recapture technique to estimate the size of the ‘population’ of beans. We’re be back at the library in a few weeks! Stay tuned!

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