Introduction to Population Ecology - Online Tutor, Practice Problems & Exam Prep

Population Ecology

This video, we're going to begin our introduction to population ecology. And so recall from our previous lesson videos that we already defined population ecology, and so we already know that it examines population dynamics or how and why populations change over time. Now also recall that the term population refers to all of the organisms of the same exact species that are living in the same area at the same time. And so in order to define a population, we have to be able to define the boundaries of the area of interest, and this is why population boundaries are important. And population boundaries can either be naturally or arbitrarily defined.

And so notice down below in the image on the left-hand side, we're showing you an example of a natural boundary, which would be the boundaries of these islands that you can see. And on the right-hand side, we're showing you an example of an arbitrary boundary, which would be the boundaries of the states in the USA, such as Florida for example. And so really the ecologist can set the population boundaries to whatever it is that they want so that they can gather the data that they want to focus on. And so that concludes this introduction. We'll be able to learn a lot more moving forward, and I'll see you all in our next video.

Population Size & Density

In this video, we're going to talk about 2 critically important metrics that are used to describe and understand populations, which are population size and population density. Now population size is commonly abbreviated with the variable n and it is simply the total number of individual organisms in the population. So really it's just a simple count. Now as a simple count, it disregards the area that the population occupies and gives no sense of how crowded the population is. And therefore, population size does not provide a standardized measure for comparing different populations in different regions that have different areas.

Now, on the other hand, population density is the population size or n, but within a defined unit of area or volume. And because it does consider the area, population density gives a sense of how crowded the population is. And therefore, it also provides a standardized measure for comparing populations in different regions regardless of whether those different regions vary in their area. And so let's take a look at this image down below where we're looking at populations of these black-tailed deer on these 3 different islands that have different sizes. And so notice that the population size of all three of these islands is identical, and if that was the only thing that we were given, we might assume that these three populations are living under the same conditions.

However, notice that their population densities differ drastically. Where the small island has a high population density indicating that the population is quite crowded and the large island has quite a low population density indicating that the population is not very crowded. And of course, this middle-sized island falls right in between. And so this here concludes our lesson, and we'll be able to get some practice moving forward. So I'll see you all in our next video.

Introduction to Population Ecology Example 1

So here we have an example problem that says forest A is 10 square kilometers and has a wild boar population of 500. Forest B is 50 square kilometers and has a wild boar population of 2,000. How do the population densities of wild boars in the two forests compare? And we've got these four potential answer options down below. Now, of course, recall from our previous lesson videos that population density refers to the population size within a defined unit of area.

And so we can calculate the population densities of these two forests simply by taking the population sizes and dividing them by the area. And so let's go ahead and do that for forest A. And so to calculate the population density of forest A, which I'll abbreviate with PD, all we need to do is take the population size of 500 boar and divide it by the area, which is given to us as 10 square kilometers. And so if you do 50010, what you'll get is an answer of 50 boar per square kilometer. Now, if we do the same but for forest B this time, what we'll get is a population density or PD that's equal to again the population size given to us as 2,000 boar divided by the area given to us as 50 square kilometers.

And so if you do 200050, you'll get an answer of 40 boar per square kilometer. And so notice that forest A has a greater population density than forest B. And so we can indicate that answer option A, forest A has a higher population density, is the correct answer to this example problem. And all of these other options are not correct. So that concludes this problem, and I'll see you all in our next video.

According to the diagram, which of the following countries has the greatest human population density?

• A. Canada.
• B. Russia.
• C. Australia.
• D. India.

Which of the following is an example of population density?

• A. The population of humpback whales in the Atlantic Ocean.
• B. The number of daisies per square meter in your backyard.
• C. The number of chimpanzees in the San Diego Zoo enclosure.
• D. The population of Arizona Bark Scorpions in the Sonoran Desert.

To calculate the population density of people in Florida, you would need to know:

• A. The population of Florida & the population of the whole USA.
• B. The population of Florida & the total area of the whole of the USA.
• C. The total area of Florida & the whole of the USA.
• D. The population of Florida & the total area of Florida.

Factors Affecting Population Size (N)

In this video, we're going to talk about the 4 factors that directly impact population size, and we have them grouped based on whether they increase population size or if they decrease it. Now, of course, births refer to the total number of births in the population over time, and immigration refers to the migration, movement, or influx of individuals into a population. And so, you can think that the 'I' in immigration is for the 'I' in influx or the 'I' in into. Now, of course, deaths refer to the total number of deaths in the population, and emigration refers to the migration, movement, or efflux of individuals exiting a population. And so, you can think that the 'e' in emigration is for the 'e' in efflux or the 'e' in exit.

So let's take a look at this image down below where we can see this population of meerkats here is being impacted by these four factors. Notice these cute little meerkats over here are being born, and those births increase the population size. The migration or influx of individuals into a population also increases the population size. Now, on the other hand, deaths by predators like hyenas will decrease the population size, and emigration or the exiting of individuals outside the population will decrease the population size as well. And so what we're saying here is that these factors in blue are adding to the population and these factors in this reddish color are removing individuals from the population, and that can change the population size.

That concludes this video, and I'll see you all in our next one.

Introduction to Population Ecology Example 2

So here we have an example problem that asks, which of the following equations accurately represents a change in population size over time? And we've got these 4 potential answer options down below that all express the change in population size as the variable delta n. And then they have different combinations of adding and subtracting the four factors we talked about in our last lesson video, births, immigrants, deaths, and emigrants. And so, of course, we know that births are always going to increase the population size and be associated with addition, but deaths are always going to decrease the population size and should be associated with subtraction, not addition. And so for that reason, we can go ahead and eliminate answer option a because it is not correct.

Now we also need to recall from previous lesson videos that the I in immigrants can remind us that this is the migration or movement or influx of individuals into a population size, and so it should increase the population size and be associated with addition, not subtraction. And so for that reason, we can go ahead and eliminate answer option d, because it is not the correct answer. And then, of course, recall from previous lesson videos that the e in emigration can remind us that this is the movement or migration or efflux of individuals exiting the population. So it's going to decrease the population size, should be associated with subtraction, not addition. And so for that reason, we can eliminate answer option b, and of course, this leaves answer option c as the correct answer to this problem.

And so the change in population size is going to be equal to the births plus the immigrants minus the deaths minus the emigrants. And so once again, c here is the correct answer. That concludes this example problem, and I'll see you all in our next video.

Suppose a population of rabbits in a forest has an initial population of 500. Over the course of one year, 200 new rabbits are born, and 150 rabbits die. Additionally, 20 rabbits emigrate out of the forest and 50 rabbits immigrate into the forest. What is the population size at the end of the year?

• A. 500 rabbits.
• B. 530 rabbits.
• C. 580 rabbits.
• D. 600 rabbits.

Which of the following factors directly impacts the population of rabbits in a meadow?

• A. An increase in the amount of food available to the rabbits.
• B. An increase in the number of foxes living in the area.
• C. An increase in the number of rabbits being eaten by foxes.
• D. All of the above.

Which of the following statements is true?

• A. Immigration & emigration only affect local populations, while birth & death affect local & global populations.
• B. Birth & immigration always increase a population’s size.
• C. Death & emigration always decrease a population’s size.
• D. All of the above are true.

What is a Metapopulation?

In this video, we're going to answer the question, what is a metapopulation? A metapopulation is really just a group of spatially separated local populations that are all linked together by immigration and emigration, or they're all linked by the migration of individuals out of one local population and into another local population. These metapopulations occur when organisms inhabit smaller patches of habitat within a mostly uninhabitable area. The cool thing about these metapopulations is that if a local population goes extinct for whatever reason, that area may be recolonized by individuals from the metapopulation. So, let's take a look at this image down below so we can better understand these ideas.

Notice that throughout this image, we're looking at these five spatially separated populations of this specific species of bird called the Eurasian blue tit. What you should notice is that collectively, all five of these spatially separated local populations make up the metapopulation only because all five of these populations are linked by immigration and emigration, as you can see indicated by these arrows throughout this image. Once again, if any one of these local populations undergoes extinction, as you can see happening on these two islands here, individuals from the metapopulation can actually recolonize those areas through immigration and emigration. Down below, we have a very important note, which is that local populations are less stable and therefore more likely to go extinct. However, metapopulations tend to be significantly more stable and therefore make the species significantly less likely to become extinct.

This here concludes our lesson on metapopulations, and I'll see you all in our next one.

Which of the following would NOT be considered a metapopulation?

• A. Newt populations in different ponds within a large national park.
• B. Butterfly populations in different meadows on an island.
• C. The leopard populations in Sub-Saharan Africa and in south Asia.
• D. All of the above are metapopulations.

Population Dispersion Patterns

This video, we're going to talk about how populations often exhibit characteristic patterns of dispersion. Now, dispersion can be defined as the degree of spacing between individuals in a population. Now, of course, recall from previous lesson videos that population density also gives a sense of the degree of spacing and how crowded a population is. However, dispersion provides an even more detailed insight into the degree of spacing than population density alone. Now, populations can really only exhibit 3 general patterns of dispersion, which we have down below in this image.

And the first is a clumped pattern of dispersion, which, as its name implies, is when individuals within a population are aggregated in clumps, patches, or groups, just as you can see in this little diagram in the top left. Now, clumped is actually the most common pattern of dispersion, as resources in nature also tend to be clumped in patches. And also, organisms may clump for several different reasons, including increasing their predation success. As these wolves hunt in packs, they increase their chances of success. But they may also clump for completely different reasons, such as increasing their chances of remaining safe.

Now, the next pattern of dispersion that we have is uniform, and, as its name implies, this is when individuals within a population are uniformly or evenly distanced from one another, just as you can see in this little diagram in the top left. Now, organisms display a uniform pattern of dispersion when they are very territorial about their own personal space and their resources. And so, notice that these birds here are very evenly spaced from one another. Now, last but not least, the final dispersion pattern is a random dispersion pattern. And, as its name implies, this is when there is an unpredictable amount of spacing between individuals and the population.

And often, plants that have wind-dispersed seeds will display this random pattern of dispersion, just as you can see with these dandelions here. Now, one thing that's very important for you to note is that the type of dispersion depends greatly on the scale of observation. And so, for example, if we were to zoom in on just one of these clumped patches, then we might actually say that it has a uniform pattern of dispersion, and so that's why it greatly depends on the scale of observation. Another important note is that a population is not necessarily limited to any one of these dispersion patterns, and the dispersion pattern can actually change for a population over time. So that concludes this video, and I'll see you all in our next one.

Introduction to Population Ecology Example 3

So here we have an example problem that says, listed below are various plant and animal species. Mark each with either a 'c' for clumped, a 'u' for uniform, or an 'r' for random for the type of distribution or dispersion that they are most likely to exhibit. Now one says, red squirrels who actively defend their individual territory. Now recall from our previous lesson videos that territorial organisms often display a uniform type of dispersion where they are evenly spaced. So we'll put a 'u' here for this one.

Now 2 says, elephants who live in herds, which are groups or patches or clumps, and so we'll put a 'c' here for clumped dispersion. Now 3 says dandelions whose seeds are dispersed by the wind, and the wind will randomly disperse these dandelions, so they have a random dispersion and we'll put an 'r' here. Now 4 says humans in the state of Alaska, and Alaska is mostly uninhabitable. So humans live in these groups or patches or clumps within small cities, and so we can say that they are clumped with a 'c'. Now 5 says lions who live in prides for mating, social, and hunting purposes, and these prides are really just groups or patches or clumps.

So once again, they have a clump dispersion. 6 says oyster larvae who can travel hundreds of miles powered only by sea currents, and those sea currents will randomly disperse those oyster larvae, so they will have a random dispersion. We'll put an 'r'. And then last but not least, number 7 says that gray sage plants, which release chemicals that inhibit the growth of plants around them, And so this is going to make them somewhat territorial, and that's going to give them a uniform dispersion pattern. So we'll put a 'u' here.

So that concludes this example problem, and I'll see you all in our next video.

Which of the following species displays a uniform species dispersion pattern?

• A. The human population living in the USA.
• B. Wolves, who hunt & live in packs.
• C. Redwood trees, which compete with each other for light.
• D. Herrings, who swim together in schools to help avoid predation & increase feeding efficiency.