The person was very angry.
The person was enraged. There are 8 billion of us humans out there and yet we’ve done some pretty cool stuff. We’ve made things like the Great Pyramid of Giza, the James Webb Telescope, and Pokémon. All of that making has led us to build some pretty big cities and places that used to be wilderness. A lot of 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. So far in this series, we’ve looked at ecosystem ecology and community ecology, but in this episode we’re turning up the magnification. Not that close, there we go. We’re gonna 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. 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 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: the three D’s of measurable population-related features: density, dispersion, and demographics. Welcome to Bunny World, a veritable paradise of salad fixins with not a fox to be found. Let’s look at the growth of our rabbit population with no natural predators and unlimited resources: the curve starts off with a slight increase and then gets really steep. This is called exponential growth, and if you pick any point on this curve it tells how many individuals are in that population at that particular point in time.
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, and 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, 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.
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 DDT chemical was effectively banned, thanks to the tireless work of scientists, birders, and government officials who were able to show through their research how harmful it 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 the 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’s 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. 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. Thanks for watching this episode of Crash Course Biology, which was made with the help of all of these nice people. If you want to help keep Crash Course free for everyone forever, you can join our community on Patreon.