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When the Bald Eagle population started to decline in the mid-20th century, scientists began to ask why. Population ecology, the study of organisms of the same species, played a big role in answering that question. In this episode of Crash Course Biology, we’ll take a look at the methods population ecologists use to study a population and the types of data they collect. We’ll also find out how scientists helped bring the Bald Eagle back from the brink.

Introduction to Population Ecology 00:00
Density 2:51
Dispersion 6:02
Demographics 7:13
Population Growth 8:22
Review & Credits 11:26

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CC Kids:
There are a lot of us humans out there.

Around 8 billion now. And yeah, we’ve done some cool stuff.

We’ve made things like the Great Pyramid of Giza, the James Webb Telescope, and Pokemon! And, all of that stuff-making has led us to build some pretty big cities in places that used to be wilderness. A lot of the animals in those once-wild places have had to change things up to live alongside us.

Many animals have had to adapt to urban environments. While some have done really well and seen population explosions, like that adorable trash panda – I mean raccoon – others have suffered from the encroachment of humans. The study of a single species in an area is called population ecology, and, among other things, it helps us see how a specific organism responds to suddenly sharing space with messy, creative humans like us.

Hi, I’m Dr. Sammy, your friendly  neighborhood entomologist, and this is Crash Course Biology. [THEME MUSIC] So far in this series, we’ve looked at ecosystem  ecology and community ecology. But in this episode, we're turning up the magnification, even more—whoops, not that close.

There we go— We're going to focus on populations, groups of individuals  of the same species living in the same place. One excellent example of population ecology in action also happens to be a wonderful success story in the history of biology and conservation: the Bald Eagle. You see, population ecologists are particularly concerned with the features of the group, and whether the size of the population is growing or shrinking.

If the population is changing, the ecologist looks at data to try to understand why the change is happening. And for the Bald Eagle, the change was pretty drastic. The Bald Eagle became the national bird of the United States in 1782.

Unfortunately for the Bald Eagle, the same folks who bestowed upon it this newfound honor of national bird would soon spread across the country, building up massive cities and populating areas that were once wilderness. By the mid-20th century, after years of habitat loss, Bald Eagle populations were in serious decline. And by 1963, the situation had become desperate – only 417 mating pairs had been documented by government biologists in the United States.

With so few pairs to produce offspring, the species was in danger of extinction. After this revelation, the bird became one of the first animals to be placed on the country’s endangered species list. Population ecologists - along with other scientists, conservationists, and activists - began to study the birds closely, hoping to figure out what was causing  this change in population numbers, and how they might reverse it.

To learn more, they needed to collect data, and to collect data, they needed to measure something! Enter the three D’s of measurable population-related features “Diners, Drive-Ins, and Dives” [rock music playing] who let Guy Fieri edit this!? CUT! [long beep] So that would actually be density, dispersion, and demographics.

Broadly speaking, density is an amount of stuff in a given volume of space. When talking about populations, the number of organisms in a specific area tells us how close together or spread out the population is. Like for example, when roosting, Mexican free-tailed bats pack in at a whopping 1,800 adults per square meter.

To put that in perspective, the shirt that you’re probably wearing is likely about one-third that size, and the body of a bat is about the size of a chicken nugget. Now picture trying to carry 600 chicken nuggets around in the front of your shirt —that is DENSE, my friend. But there are a few challenges to consider when measuring the density of a population.

One is where exactly a population begins and ends. Just like ecosystems, populations can exist on vastly different scales. Consider the German cockroach.

Now - say, we’re trying to define the population of cockroaches living in the walls of a single  apartment building. Some of the cockroaches living in the far western walls of the apartment might never mingle with the cockroaches on the eastern end of the building. But, there’s no real barrier preventing their movement, and the western cockroaches could breed with the eastern ones since they’re the same species.

So, an ecologist needs to decide if they’ll consider the cockroaches as a single population or a separate population. Different ecologists might define the populations differently —and that’s ok! As long as each ecologist describes their population, we can understand their study.

Another challenge with density is  actually counting the population. We might be able to tally up large animals, like the Bald Eagle, but counting something like cockroaches living in a wall, or even tulips in a field, is a lot more challenging. For a field of tulips, the ecologist could count the tulips in a defined area, say a square meter.

If the field is a square kilometer, and the distribution of the tulips is pretty even, we can multiply the number of tulips in the square meter by one million, and approximate the population size. But for organisms that don’t stay still, the counting is a little trickier. So our ecologist might count other stationary objects that give clues as to how much life is present, like the number of Bald Eagle nests in an area or the amount of cockroach poop on those walls.

Another way is to trap the organism, mark them in a harmless way, and then release them back into the wild. Like, to study turtle populations  with the mark and recapture method, an ecologist might put a small notch in their shells, or for butterflies, label the wing with a permanent marker. Later, a bunch are recaptured and the amount that are marked can give insights into the size of the total population in that area.

OK, so that’s density, and it’s a measurement that can go a long way in helping us understand changes in populations. But it’s not the only factor. Our next D is dispersion, the spacing pattern between organisms.

In clumped populations, the individuals group together in little clusters. This is common with rural coyotes, which group up in families because of their mating and social behaviors. On the other hand, dandelions show random dispersion patterns, because the seeds are blown in irregular patterns by the wind.

And some plants, like the sage plant, salvia leucophylla, have evolved to secrete toxic chemicals that prevent other plants from getting too close and stealing away nutrients. Not even another sage plant wants to be its neighbor, creating a uniform distribution pattern. And that brings us back to our Bald Eagle.

With the rise of urban areas in the 20th century, eagles were pushed out of traditional nesting areas and sought refuge in non-traditional areas, leading them to a clumped dispersion pattern along lakes and waterways that  brought them into contact with humans. These large, predatory birds were often seen as a threat to small livestock, even though Bald Eagles prefer to hunt fish from lakes or find something that has already died to dine on. But regardless, farmers would often shoot the birds if they spotted them near their animals.

These shooting deaths accounted for some of the Bald Eagle’s original population decline, but not all of it. There’s still one more feature to measure. And that’s demographics, our third and final of the three D’s, which gives us information about who makes up the population, like the age, sex, and offspring potential of individual members of a population.

A shrinking population happens when individuals leave or die. If the decrease is because of a  high death rate or low birth rate, the population may be at risk of extinction, like in the case of  the Bald Eagle. Or, if the population is growing, this could be because of a high birth rate.

The point is, population ecologists are really interested in population growth and shrinkage. To understand this, we’ll need some math. But don’t panic.

We’ll get a little help from the Thought Bubble… To look at population growth, we’ll need to plot the change in a population over time. Ecologists do this all the time with real populations, but we’re going to make ours up, using an imaginary population of rabbits as an example. Why rabbits?

Well, they’re adorable little creatures that — importantly, for our purposes — like to breed really fast. So we’ll plot the changes with two important factors: time and the number of individuals. Say we start with 10 rabbits and follow the population growth over a period of 20 years.

In the graph, 20 years is our time and it sits on the x-axis, and the number of individuals is on the y-axis starting at 10. Let’s look at the growth of our rabbit population with no natural predators and unlimited resources. Welcome to Bunny World, a veritable paradise of salad fixin’s with not a fox to be found.

The curve starts off with a slight increase… and then gets really steep: that's called exponential growth. In fact, if you pick any point on this curve, it tells you how many individuals are in the population at that particular point in time. If you wanted to describe what the whole curve is telling you, it's how fast the population is growing, or the growth rate.

But resources in the real world are limited, so exponential growth tapers off in most real-life situations. Take the fur seals, which were heavily hunted in the United States for many years, until the hunting fell off during World War II. As the population recovered, there was a steep increase in the population size initially.

But limiting factors like resource availability eventually make the curve level off. This is called the logistical growth model. We add in the maximum population size that the environment can sustain, which is this horizontal line at the top of the graph, called the carrying capacity.

There's a limit to the number of bunnies, or seals — or people, for that matter — and that limit might be determined from the bottom-up by natural resources, like edible plants, or from the top-down by predation – like foxes feasting at the bunny buffet. So, as the population size gets close to carrying capacity, growth slows down, and then the population size remains constant as individuals enter and leave the population at about the same rate. Thanks, Thought Bubble!

Because species interact with one another, understanding the population growth of one species lets us know if things are changing in the community — the community being the other plant and animal species in the same environment. This can help us anticipate what will happen in the future, like if a koala population is growing so fast that there won’t be enough of its sole food resource, eucalyptus leaves, to support the population. So even when we’re focused on population ecology — paying attention to a single  species in a single location — the broader community level matters, too, because it can influence the size of that population.

There’s tons of overlap like this in ecology! And the food chain isn’t the only thing that affects population growth. When the population gets too large or crowded in a particular location, factors that depend on the density  of a population come into play.

We call these density-dependent factors, and they include things like competition and disease which may regulate, or in this case slow, the population growth. Picture a population of raccoons where some got a contagious tummy bug from eating your garbage. Because they’re so close together, the disease could spread among  them – a density-dependent factor.

But if a hurricane wipes out half a population of raccoons, that is a density-independent factor. It doesn’t matter how dense the population was, it was the surprise weather event that caused the decrease. When it came to the Bald Eagle, their population was shrinking too fast to just be blamed on illegal hunting, a density-dependent factor.

So ecologists, biologists, and other scientists studied data from the three D’s and eventually came to a consensus: exposure to a pesticide was mostly  to blame for the eagle’s decline. DDT is a chemical pesticide that gained popularity during World War II, and became widely used in the United States as an insecticide after the war. Turns out, the chemical wasn’t killing the adult birds; it was making their eggs less viable.

Exposure to the DDT was causing the birds to lay eggs with much thinner egg shells, leading to fewer eggs surviving long enough to hatch. In 1972, DDT was effectively banned thanks in part to the tireless work of scientists, birders, and government officials who were able to show through their research how harmful the chemical was to wildlife. After the ban, the Bald Eagle populations began to show growth once more.

By 1995, studies showed the eagle population in the country had risen enough to be moved from “endangered” to the less critical label of “threatened.” And in 2007, ecologists declared that the bird had made a stunning comeback, and it was officially removed from the endangered species list altogether. The story of the Bald Eagle is a remarkable testament to the dangers an organism faces when it is forced into contact with humans. But it is also a tale of incredible scientific work and the dedication to uncover and correct the damage we’ve done.

And we couldn’t have done it without population ecology! This series was produced in collaboration with HHMI BioInteractive. If you’re an educator, visit for classroom resources and professional development related to the topics covered in this course.

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