Population Sampling Methods - Online Tutor, Practice Problems & Exam Prep

Sampling stationary organisms can be effectively conducted using transects and quadrats, allowing ecologists to estimate population sizes without counting every individual. For mobile organisms, the mark and recapture method is utilized, involving capturing, marking, and releasing individuals, followed by a second capture to estimate total population size using the equation $m/n=r/c$. Molecular tools can also assess population size through genetic analysis, indicating population density based on genetic diversity.

Sampling Stationary Organisms: Transects & Quadrats

In this video, we're going to talk about sampling stationary organisms using transects and quadrats. And so what's important to know is that populations are often way too large for ecologists to study them in their entirety. And so instead, ecologists tend to examine these populations by strategically sampling smaller parts of the larger population, and then they can use those smaller samples to represent the entire population. Now stationary organisms or organisms that don't move at all and very slow moving organisms can be sampled using what are known as transects or quadrats. Now transects are simply lines of known position and length extending through a habitat, and so this makes it very easy for an ecologist to either count the number of organisms that fall on the line or to count the number of organisms that fall within some distance of that line.

And so notice that the image on the left-hand side over here is showing you an example of one of these line transects, which you can see right there. Now quadrats on the other hand are rectangular plots, and often they are just squares that can be set up either at random locations within the habitat or they could be set up at regular intervals along a transect. And so notice that in this image on the left-hand side, you can see a quadrat is being used in combination with the transect. But again, they can be used in random locations as well. And so over here on the right, you can see a zoom-in of one of these quadrats, and, again, it's really just this rectangular plot or a square-shaped plot, if you will, and all you got to do is count the number of organisms that fall within it.

Now organisms along a transect or within a quadrat, again, can be very easily counted up. And then that data can be extrapolated or used to estimate the total population size without actually having to go out and count every single organism in the entire population. Now again, these methods only work for stationary or slow moving organisms, as mobile organisms will tend to scatter away from the line transects and scatter out of the quadrats. Now this concludes this video, and we'll get some practice moving forward, and then talk about mobile organism sampling. So see you all in our next video.

Population Sampling Methods Example 1

So here we have an example problem that says, a group of ecology students is studying the population size of dandelions, a species of flowering plants, in a meadow with a total area of 1,000 square meters. Using quadrats measuring 1 meter by 1 meter, they randomly place 10 quadrats in different areas in the meadow and count the number of dandelions in each quadrat, which yields the following data that you can see right here. And then it asks, using the average number of dandelions per quadrat, what is the estimated total population size of dandelions in the meadow? And we've got these 4 potential answer options down below. And so to solve this problem, we're first going to need to calculate the average number of dandelions per quadrat, and we can do that by adding up all of these numbers here and then dividing it by 10.

So let's go ahead and do that first, and when we do that, what we get is that the average number of dandelions per quadrat can be calculated with this calculation that you see here. And when you type this in your calculator, you end up getting an answer of 12.2. And so that's 12.2 dandelions per quadrat on average. Now we need to realize that the quadrats are measuring 1 meter by 1 meter, meaning that they're 1 square meter, and the total area of the meadow is 1,000 times greater at 1,000 square meters. So what we need to do is take this number here and multiply it by 1,000 to get the, estimated number of dandelions in the entire meadow.

So 12.2×1000 comes out to 12,200, and notice that this answer here matches with answer option b. And so we can indicate that option b is the correct answer to this example problem, and that concludes this problem. So I'll see you all in our next video.

Sampling Mobile Organisms: Mark & Recapture

This video, we're going to talk about sampling mobile organisms using the mark and recapture method. For highly mobile organisms that move around a lot, those transects and quadrats that we talked about in our previous lesson videos are not going to work at all. Instead, ecologists use the mark and recapture method to sample these mobile organisms. This mark and recapture method can be broken up into 3 steps that we have numbered in the top half of our image right here. Let's imagine a scenario where we're trying to determine the total population size of these highly mobile fish in this lake.

It would be really hard for us to go underwater and count up all of these moving fish, and good luck not counting Bill multiple times as he looks almost identical to Sally over here. In situations like this, we use the mark and recapture method. In step number 1, all we have to do is go out and capture a small subset of the population, and in this situation, we've gone and captured 5 fish. Then all we need to do is mark or tag these fish in some way that does not interfere with their normal activity, and all of the marked fish have this reddish background behind them.

In step number 1, all we gotta do is capture and mark. Now, in step number 2, all we do is release all of those fish that we captured and marked in step number 1, and then we just wait a long enough period of time, maybe a few days or weeks, for those marked individuals that we released to mix back into the population evenly, as you can see here in this image. Then, in step number 3, we go back and we capture again, and hopefully, we recapture some of the fish that we captured the first time around. Notice that in this image we captured a total of 5 fish, but only one of the fish is marked, so that is the only fish that we captured the first time around. Once we've done this, we can go on and calculate the unknown total population size that we were trying to determine all along, and we can do that by using this formula that you can see down below.

This formula or equation says that the ratio of mn is equal to the ratio of rc. Here, m refers to the number of marked individuals in the first capture, which in this situation was 5 total fish. n is the unknown total population size that we were trying to determine all along. r is the number of marked individuals in the second capture, and c is the total size of the second capture. There were 5 fish caught in the second capture, but only one of them was marked, and so that one would be the r.

If we have any 3 of these 4 variables, we can calculate the unknown missing variable using our algebra skills, and we're going to do that in our next video. So, I'll see you all there.

Mark & Recapture: Calculations & Assumptions

In this video, we're going to talk about the mark and recapture calculations and assumptions. Recall that we ended the last lesson video introducing this equation that you can see right here, which allows us to estimate the total population size after the mark and recapture method. Recall from that last lesson video that m is equal to the number of marked individuals in the first capture, which was 5, and n is equal to the unknown total population size, which we're trying to calculate. This is going to be equal to r, which is the number of marked individuals in the second capture, which was just 1, over c, the total size of the second capture, which was 5. All we need to do is use our algebra skills to solve for the variable n, which we can do in several different ways, but what we're going to do here is take the reciprocal of both sides of the equation.

Literally just flip these ratios around so that we have n5 is equal to 51, and of course, 51 is just 5. To isolate the variable n, all we need to do is multiply both sides of the equation by 5, and what we get when we do that is n=25. That is the total population size in the scenario from our last lesson video. Now, over here on the right, what we have are mark and recapture method assumptions, which must be maintained in order for this mark and recapture method to work appropriately. There are three assumptions.

The first assumption is that the marked individuals must fully mix between captures. The second assumption is that marked and unmarked individuals are equally as likely to be captured. This means that the behavior of those marked individuals did not change and they didn't devise a way to avoid being captured a second time. Third, the population size must be stable during the sampling, which means that there's no significant births, deaths, immigration, or emigration, or if they do happen that they balance each other out. This concludes this video, and I'll see you all in our next one where we'll be able to practice these concepts.

Population Sampling Methods Example 2

So here we have an example problem that says, you and a group of population ecologists have been tasked with finding the population size of prairie dogs in a plot of land in Texas. You and your team captured 31 prairie dogs and marked them before releasing them back where you found them. 1 week later, you return to the same area and capture 47 prairie dogs, and 12 of them were already marked by your team the previous week. Estimate the total population of prairie dogs in the area. And we've got these 4 potential answer options down below.

Now, of course, this problem is related to the mark and recapture method. So recall from our last lesson video, the equation for that method is right here where mn is equal to rc. And so, recall that m is equal to the number of marked individuals in the first capture, which we're told there are 31 of them, so m is equal to 31. And that's going to be over n, which is the unknown total population size, so we'll leave that as the variable n. And this is going to be equal to r, which is the number of marked individuals in the second capture, and we're told in the problem that there are 12 of them, so r is equal to 12, and this is going to be over c, which is the total size of the second capture, which the problem tells us that there are 47 of them.

And so all we need to do is use our algebra skills to solve for the missing variable n here, and we can do that in many ways, but what we're going to do is take the reciprocal of both sides of the equation. So what we'll get is n31 is equal to 4712. And then all we need to do is multiply both sides of the equation by 31, and what we end up getting is n is equal to 4712 times 31. And so if you take your calculator and do 47 divided by 12 and then multiply that answer by 31, what you'll get is an answer for n that is about 121.42, about. And so notice that this answer right here matches closest with answer option c, which says 121.

So the correct answer to this example problem is option c, and that concludes this problem. So I'll see you all in our next video.

A group of population ecologists capture & mark 8 starlings in a grassland, then a few days later they capture 15 starlings but none of them are marked. What is the estimated starling population size in the grassland?

• A. 120
• B. 60
• C. Impossible to estimate – a smaller sample is required.
• D. Impossible to estimate – a larger sample is required.

You & your team of population ecologists capture 16 turtles in Lake Springfield, mark each of them with a dot of waterproof paint on their shell & release them back into the lake. One week later you go back to Lake Springfield & capture 4 turtles that have been marked with paint, & 22 that are unmarked. Estimate the total population size of turtles in Lake Springfield.

• A. 124
• B. 88
• C. 62
• D. 104

When calculating population size with the mark and recapture method, which of these assumptions must be made?

• A. Marked individuals are more likely to migrate out of the habitat than unmarked individuals.
• B. Marked individuals have the same probability of being captured as unmarked individuals.
• C. Some individuals will immigrate into the population between the first & second capture.
• D. All of the above.

Which of the following is an appropriate method for sampling an ostrich population?

• A. Transects.
• B. Quadrats.
• C. Mark & recapture.
• D. All of these methods are appropriate.

Molecular Tools to Determine Population Size

This video, we're going to briefly discuss molecular tools used to determine population size. And so, population sampling strategies involving molecular genetics can be used to estimate population size. And so the main idea here is that DNA samples can be acquired and sequenced from many different individuals in the population, and after analyzing those sequences, if the sampled individuals are deemed to be closely related, then the population size is likely small. Now, on the other hand, if after analyzing the sequences, the sampled individuals are deemed to be distantly related, then the population size is likely large. And so, notice down below in the image, we're looking at these two populations of this downy woodpecker.

On the left-hand side, what you'll notice is that the sample showed that the individuals are closely related, and that means that they have relatively low genetic diversity, and that implies a relatively small population size. Now, on the other hand, on the right-hand side over here, what you'll notice is that the sample shows quite high genetic diversity, indicating that these individuals are more distantly related, and that implies a large population size. And really that's the main idea of this technique. So we'll be able to apply this in our next video, and I'll see you there.

Ecologists are using molecular tools to analyze DNA samples from a population of endangered wolves to estimate their population size. They collected DNA samples from 20 individual wolves and found that most of the sampled wolves were closely related to each other. What can be inferred about the population size of these wolves based on this data?

• A. The population size is likely large since closely related individuals indicate a high level of genetic diversity.
• B. The population size is likely small since closely related individuals indicate a lack of genetic diversity.
• C. Nothing about the population size can be inferred from the genetic relatedness of individuals.
• D. The population size is likely large since closely related individuals indicate high reproductive success.