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Counting organisms is a very basic and valuable part of understanding nature. Just about every part of understanding nature requires that we know how many individuals there are in a given area. But it is a very hard thing to do. This week we are becoming backyard ecologists! We will walk you through techniques and tools that scientists use to estimate the number of organisms in nature.
To estimate means to roughly count or calculate the quantity of something. To be as accurate, or close to the true population, as possible, scientists use samples. They take a small portion that can be easily counted and use that knowledge to approximate the whole population. A quadrat is a trusty tool used to outline or designate a sample area. It is a frame that is placed on the ground and all individuals are counted that are within the square. Multiple samples are taken at random locations to ensure that the numbers recorded are representative of the entire habitat.
Quadrats & Population Estimation
By: Shannon Stainken
Bubble Snails?
Take a look at all those bubble snails creating slimy trails in the sand! How many are there? We counted about 36 in this photo but it made us think... How many bubble snails live around Sanibel? And how could we figure that out? We could snorkel and try to count every... single... bubble snail. But that would take a lot of time and effort.
Counting organisms is a very basic and valuable part of understanding nature. Just about every part of understanding nature requires that we know how many individuals there are in a given area. But it is a very hard thing to do. Today, we will explore how we try to count, or estimate the numbers of things in nature.
To estimate means to roughly count or calculate the quantity of something. To be as accurate, or close to the true population, as possible, scientists use samples. They take a small portion that can be easily counted and use that knowledge to approximate the whole population. A population is all of the organ-isms of the same species that live in a particular area. There are two ways to measure a population - by population size or by density. The population size is the number of individuals and the density is the number of individuals per unit area or volume.
A quadrat is an area of known size, used by ecologists to estimate population size and density. A quad-rat frame is constructed, then placed on the ground to create a sample. All individuals are counted that are within the square. Multiple samples are taken at random locations to ensure that the numbers recorded are representative of the entire habitat.
Quadrats are useful for sampling plants and still or slow moving creatures. They wouldn't be very helpful if you were trying to study large or fast animals.
Today we’re going to make our own quadrat and learn how to use it. Quadrats are usually made from PVC pipes but since most of us may not have that around, we’ll make a simple version out of household materials. Alternatively, you could also gather four sticks and bind them together with string.
Materials
• Cardboard, construction paper, or poster board
• Measuring tape
• Pencil or pen
• Scissors
Start by measuring the frame. Usually quadrats are 1 square meter but you can make it smaller depending on the size of the paper you have. If we were doing a real study, we would make sure that the size of the quadrat is appropriate for what we’re counting in the study area.
Step 1
Shannon had a piece of foam poster board at her house that she used. It was big enough that she could make her quadrat about 20 inches squared. In science, we always use the metric system but Shannon’s measuring tape was only in inches. If you can, try to measure your frame in centimeters. 20 inches is about 51 centimeters. To make a frame, it’s easiest to first make the outside border. Shannon made her frame 1 inch thick, so the outside square was 22 inches on each side. Next, make the inside square 20 inch-es on each side. If you make your quadrat smaller or larger, that’s perfectly fine, just make sure each side is the same length.
Step 2
Next, cut out the frame using scissors. Ask an adult to help you cut out the center portion, it can be tricky to get started.
Ta da! Your quadrat is complete and ready to use for our next activity.
Population Estimation
*This activity is best suited for 6th grade and up. Please see our modifications at the end for younger students.*
What you’ll need:
Paper
Pen or pencil
Quadrat
Small items (gold fish, puzzle pieces, beads, m&ms, etc)
Step 1
Scatter small items all over a designated area in your home. Shannon scattered puzzle pieces all over her kitchen floor. The puzzle pieces represent a species and her kitchen represents their habitat. Don’t count exactly how many you scatter, just grab five to six handfuls!
Make a table to record your data
Step 2
Place the quadrat randomly onto the floor - try to not place it in an area that you think would be ‘good’ for counting - the goal is to put it in an ordinary place. Count the number of items inside the quadrat and record it under Quadrat 1 in your data table. In this case, there were 9 puzzle pieces. Shannon decided to count the piece that is only partially inside of the quadrat, which is fine, but she must keep that consistent for the rest of her sampling. She could have decided not to count it unless it was completely in the square. As long as she keeps it the same for all 5 quadrats, either method is fine.
Step 3
Repeat, placing the quadrat around the room randomly, 4 more times. To place randomly means to toss it in any direction without thinking or aiming for anything. Record the results in your data table.
Step 4
Find the average number of items counted per quadrat. Shannon counted 10 puzzle pieces on average per quadrat or sample area. To do this, add up the total number of puzzle pieces and divide by 5.
Step 5:
Measure the total area of your habitat. Shannon’s kitchen floor is 132 inches long by 66 inches wide. Again, she really should be measuring in centimeters but she’s making do with what she has in the house…
To get the area of her kitchen, she multiplied length times width.
132 X 66 = 8,712 inches squared
Now we can estimate the total population, or total number of puzzle pieces on the floor, using this formula:
N = (A/a) * n
N = estimated size of the entire population (total number of puzzle pieces)
A = area of entire population (area of kitchen floor)
a = area of sample size (area of the quadrat)
n = number of individuals in sample area (average number of puzzle pieces per quadrat)
First, divide the total area of the habitat by the area of the quadrat:
8,712 ÷ 400 = 21.78 inches squared
Next, multiply that by the number of items counted per quadrat:
21.78 X 10 = 217.8
N = 217.8 or about 218 puzzle pieces total
Finally, count all of the items that you scattered and see how close your result was! Shannon had 210 puzzle pieces on her floor which was only 8 from the estimated total.
Let’s figure out how many blades of grass are in our backyards! Seems like an impossible task doesn’t it? But, using our trusty tool - the quadrat - and some simple algebra, we can estimate it pretty closely.
Follow the same steps we just learned except this time, count the blades of grass inside the quadrat instead of puzzle pieces. You’ll have to determine the area of your entire backyard too.
Remember the formula is:
N = (A/a) * n
N = estimated size of the entire population (total number of blades of grass in your yard)
A = area of entire population (area of your backyard)
a = area of sample size (area of the quadrat)
n = number of individuals in sample area (number of blades of grass inside one quadrat)
Age Modifications:
Grades K - 2: Proceed through steps 1 - 3. Each time you place the quadrat, try to guess how many puzzle pieces there will be in each. Were you close? Do 10 quadrats total. Now look at the entire floor, how many puzzle pieces do you think there are? After 10 quadrats, count to the total number of puzzle pieces up to get a grand total.
Grades 3 - 5: Proceed through steps 1 - 4. Do 10 quadrats instead of 5 and find the average. Make a bar graph of your results for a visual representation. The X axis is the quadrat number and the Y axis is how many items were counted. Shannon graphed her first 5 quadrats below, you can use hers as a template:
additional cool resources
Smithsonian STEM Games