Ecologists study populations, or groups of organisms of the same species living in the same area, to better understand their dynamics and relationships.  African elephants are an example of a population.  Ecologists study where and how they live, how their environment affects them, how they interact with each other, and how they grow and reproduce.  They also look at the size of the population or the population density, which is the number of individuals per unit area.  When counting individuals of a substantial population, ecologists count the number of organisms in a small sample.  This sampling method gives an estimation or approximation of a number.  Finally, they look at how they’re spread out within the population: randomly, uniformly, or concentrated.

Some populations do not change over time, while others change more frequently.  For example, if a population has abundant space and food and is protected from predators and disease, the population will grow exponentially.  Exponential growth occurs when individuals of a population reproduce at a constant rate.  The larger the population, the faster it grows.  Logistic growth follows a period of exponential growth and occurs as resources become less available, which causes the population growth rate to slow or stop.  The carrying capacity is the number of organisms of one species that can be supported in an environment.  There must be adequate resources such as food, shelter, and space to support them.

Two types of limiting factors control the growth of a population: density-dependent and density-independent factors.   Density-dependent factors include competition, predation, parasitism, and disease.  Organisms must compete for food, water, space, sunlight, and other essentials to survive.  For example, when a population of bears is small, there is enough food for all.  However, when the population gets bigger, a limited amount of food may lead to bear competition.  As a result of competition, some bears will die or not reproduce, which would then cause a decrease in the population.  Predator-prey relationships can also control populations.  When predators eat prey, they decrease the prey population and subsequently increase their own.  Finally, both disease and the spreading of parasites rob organisms of nourishment and can result in the death of a population.

Density-independent factors include unusual weather, volcanic eruptions, fires, and floods.  These factors can affect the population of plants and animals regardless of the population size.  In addition, human activities such as clear-cutting forests and damming rivers can destroy populations.

Review:

1. When does exponential growth occur?
2. How do researchers study populations?
3. Identify three density-dependent factors that affect populations.