Population Size Estimation: Plants & Animals
Introduction:
One of the most important kinds of data collected in ecological studies is the number of organisms existing in the environment. This information forms the basis for understanding of why some species are endangered, yet other species are pests. It helps us understand the habitat requirements of species and their interactions with other species.
Plant populations and stationary animals are relatively easy to count. Quadrants or transects randomly sample a portion of the habitat. From this information we can infer the total number present. Animals that move are much more difficult to count.
The Mark and Recapture Method assesses animal population size by capturing critters, marking them, releasing them back into the population and making a second capture a day to week later. This method can be used on any animal as long as the marks don’t hurt the animals and animals in the population move about freely. Danish biologist C.G.J. Peterson developed the standard mark and recapture method nearly a century ago for studying fish populations. Extensions of the method include ongoing mark and release (more than two captures) and different marks for different capture periods. These methods permit the estimation of recruitment (births and immigration) as well as losses from the population by death and emigration.
Much work has been done on marking techniques: paint or dye markers, radioactive tracers, leg banding, ear tagging, fin or toe clipping. Clearly one assumption of this method is that the marking technique not harm the animal or make it more likely to be eaten by predators once released.
Simulation in the Classroom:
1. First, we will estimate the size of a population of beans found in a cup. Examine the contents of the cup and estimate the number of beans. Record this data in the data table at the end of this lab handout.
2. Reach into the cup and remove a small handful of beans (around 20 beans, the actual number is not critical). Mark each bean removed with the permanent marker. Let the beans dry and return them to the cup.
3. Put your hand over the cup and shake it well so the beans disperse. Remove another handful of beans. Count the number of beans in this sample and count the number of captured (marked) beans.
4. You can estimate the number of beans by using the mathematical equation below:
where:
# of Marked Beans you collected with your second handful = R
Total number of beans you marked = M
Total number of beans you collected on your second handful = S
(marked & unmarked)
Estimated Total population size = N
Written with just the variables identified, we have:
5. Show your work and record this calculated data in the data table at the end of this lab handout.
6. Take another handful of beans from the cup (about 20). Mark more of the beans using your permanent marker making sure to mark only previously unmarked beans. Add the number of marked beans in step 2 with those marked in this step. Return the beans to the cup, shake it well, and remove another handful of beans. Calculate the estimated bean population size again using your new numbers.
7. Show your work and record this calculated data in the data table at the end of this lab handout.
8. Dump all the beans on the table and count them all. Record this data in the data table at the end of this lab handout.
9. Answer questions 1 – 3.
Mark and Recapture with Grasshoppers
Bioretention Flora Counts
Things to take to the Field Site:
Nail Polish
Grasshopper Nets
Grasshopper Collection Bin
100 foot tape measures
Field Guide to Wildflowers
Bioretention Blank Map
Clipboards
Population Size Estimation: Plants & Animals
Data & Questions Sheet
Estimation of Number of Beans in the cup: ______
Calculated Number of Beans in the cup: ______
Display of Calculation:
New Calculated Number of Beans in the cup: ______
Display of New Calculation:
Actual Number of Beans in the Cup: ______
Questions:
1. Which calculation was closer to the actual number of beans? The first or the second?
2. Does this make sense with what you would have predicted? Why or why not?
3. Think about this experiment ‘in the wild’, how is this method of population estimation flawed? In other words, how would an animal population be different from the bean population and how would this affect your estimated population number?
4. Number of Marked Grasshoppers (Day 1): ______
5. Total Number of Grasshoppers Caught (Day 2): ______
6. Number of Marked Grasshoppers Caught (Day 2): ______
7. Calculated Number of Grasshoppers: ______
8. Display of Calculation:
9. Density of Grasshoppers: ______
10. Display of Calculation:
11. Our population size estimate could be inaccurate for a number of reasons. List four reasons that would cause our estimate to be inaccurate.
a)
b)
c)
d)
Flower counts:
Species / Cell 1 / Cell 2 / Cell 3 / Pond