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The user’s input was invalid. There are around 8 billion of us humans now, and yet we have done some pretty amazing things. We have created the Great Pyramid of Giza, the James Webb Telescope and Pokemon. This has led us to build some big cities and places that used to be wilderness. A lot of animals have had to adapt to urban environments, while some have seen population explosions, like the raccoon. Others have suffered from the encroachment of humans.
The study of a single species in an area is called population ecology, and it helps us see how a specific organism responds to suddenly sharing space with us humans. Hi, I’m Dr. Sammy, your friendly neighborhood entomologist, and in this Crash Course Biology series, we’ve looked at ecosystem ecology and community ecology. But in this episode, 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 is the bald eagle. 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 ecologists look at data to try to understand why the change is happening. For the bald eagle, the change was drastic. The bald eagle became the national bird of the United States in 1782, but by the mid-20th century, after years of habitat loss, bald eagle populations were in serious decline. By 1963, 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 - the three D’s of measurable population-related features: 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. For example, Mexican free-tailed bats can pack in at a whopping 1,800 adults per square meter. But there are a few challenges to consider when measuring the density of a population, such as where exactly a population begins and ends. Just like ecosystems, populations can exist on vastly different scales. Consider the German cockroach - some of the cockroaches living in the far western wall of an apartment might never mingle with those on the eastern end, but there’s no real barrier preventing their movement. Welcome to Bunny World, a veritable paradise of salad fixings with not a fox to be found. Here, we can observe the growth of our rabbit population with no natural predators and unlimited resources. This growth starts off with a slight increase and then gets really steep - this is called exponential growth. In fact, if you pick any point on this curve, it tells how many individuals are in that 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.
However, 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 hunting fell off during World War II. As the population recovered, there was a steep increase in 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.
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 soul 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 a 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 3Ds 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 eggshells, leading to fewer eggs surviving long enough to hatch. In 1972, the United States banned the use of DDT. The DDT ban was a result of the hard work of scientists, birders, and government officials who were able to prove the damage the chemical was causing to wildlife. After the ban, the bald eagle population began to grow again. By 1995, studies showed that the eagle population had increased enough to be moved from the endangered to the less critical threatened label. In 2007, ecologists declared that the bird had made a remarkable comeback and was officially removed from the endangered species list. This story of the bald eagle is a testament to the dangers an organism faces when forced into contact with humans, but it is also a tale of incredible scientific work and dedication to uncover and correct the damage we’ve done. We couldn’t have done it without population ecology. This series was produced in collaboration with HHMI BioInteractive. If you’re an educator, visit biointeractive.org for classroom resources and professional development related to the topics covered in this course.
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