Once upon a time, in some pretty big cities and places that used to be wilderness, a lot of animals had to change things up to live alongside us humans. Many of these animals had to adapt to urban environments, and while some thrived and saw population explosions (like the adorable trash panda, aka raccoon), others suffered due to human encroachment. This fascinating field of study is called population ecology, and it helps us understand how specific organisms respond to suddenly sharing their space with us messy, creative humans.
Meet Dr. Sammy, your friendly neighborhood entomologist, here to give you a crash course in biology.
So far in this series, we've explored ecosystem ecology and community ecology. But in this episode, we're turning up the magnification to focus on populations – groups of individuals of the same species living in the same place. And one of the most compelling examples of population ecology in action is the story of the bald eagle.
Population ecologists are particularly interested in the features of these groups and whether their size is growing or shrinking. In the case of the bald eagle, the change was dramatic. This majestic bird became the national symbol of the United States in 1782. However, the very humans who honored it as their national bird also spread across the land, building massive cities and encroaching upon once-wild areas. By the mid-20th century, after years of habitat loss, bald eagle populations were in serious decline. In 1963, only 417 mating pairs were documented in the United States, and the species was on the brink of extinction.
This dire situation led to the bald eagle becoming one of the first animals to be placed on the country's endangered species list. Population ecologists, along with scientists, conservationists, and activists, began closely studying these birds to understand the reasons behind this decline and how it could be reversed.
To gather data, population ecologists needed to measure certain aspects of these populations. They focused on three critical factors: density, dispersion, and demographics.
Density refers to the number of organisms in a specific area, indicating how closely packed or spread out a population is. For example, Mexican free-tailed bats can pack 1800 adults per square meter, while some creatures, like sage plants, keep their distance from one another.
Measuring density presents its challenges, as determining the boundaries of a population can be tricky. Consider the German cockroach – should all the cockroaches in an apartment building be considered one population, or should we divide them by walls? Ecologists have to decide.
Another challenge is counting these organisms, especially when they don't sit still. Methods such as marking and recapturing individuals can help estimate population sizes.
Next is dispersion, which describes the spacing pattern between organisms within a population. Some populations clump together, like rural coyotes forming family groups, while others disperse randomly, like dandelions carried by the wind.
The bald eagle's story illustrates how urbanization led to changes in their dispersion pattern as they adapted to new habitats.
Lastly, demographics provide insights into the age, sex, and reproductive potential of individuals within a population. A shrinking population can result from high death rates, low birth rates, or both, while a growing population can be attributed to high birth rates.
To understand population growth, ecologists often use mathematical models, like the logistic growth model, which factors in carrying capacity – the maximum population size an environment can support.
Populations don't exist in isolation; they interact with other species in their community. For example, if a koala population grows rapidly and depletes its food source, it can lead to problems not just for the koalas but also for the eucalyptus trees in their habitat.
Density-dependent factors like competition and disease can also influence population growth. For instance, raccoons in close quarters might spread diseases among themselves.
Now, let's revisit the bald eagle's story. The drastic decline in their population couldn't be solely attributed to illegal hunting. Ecologists uncovered that exposure to a pesticide called DDT was a significant factor. While DDT didn't kill the adult birds, it made their eggshells thinner, leading to fewer viable eggs. DDT was eventually banned in 1972, thanks to the efforts of scientists, birders, and officials who highlighted its harmful effects.
Following the ban, bald eagle populations began to recover. By 1995, they were moved from the endangered to the less critical "threatened" status. In 2007, ecologists declared the bald eagle's stunning comeback, officially removing it from the endangered species list.
The story of the bald eagle showcases the challenges and dangers that come with human contact but also highlights the remarkable scientific work and dedication that can help reverse the damage we've done. Population ecology plays a crucial role in understanding and preserving the delicate balance of life on our planet.