Using Population Graphs to Predict Ecosystem Changes

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

Learn how to predict the changes in the population size of a species using predator-prey relationships, carry capacity, and population density. Don't worry if you're unfamiliar with any of these terms, since this lesson will also explain them and more.

What Is an Ecosystem?

Picture going for a hike in the forest. As you walk deeper into the woods, you see large rocks, streams, and rays of sunlight cutting through the trees. If you're lucky, you might even see animals, like a deer, bear, or fox. All of these things make up the forest ecosystem, or all the non-living and living things in an environment.

An ecosystem is always trying to maintain a delicate balance called homeostasis. If the population of one species changes, the populations of other species will change as well. Scientists have captured these cause and effect relationships in population graphs, or graphs that show changes in a population of organisms over time. Today, we'll be looking at three types of population graphs: predator-prey graphs, carrying capacity graphs, and population density graphs.

Predator-Prey Graph

When you think of an ecosystem, the first relationship that probably comes to mind is predator-prey. In this relationship, the predator stalks the prey, hunting it as a food source. Predator-prey relationships must remain in homeostasis, where there is a particular ratio of the number of predators to the amount of prey in an ecosystem. Let's look at an example in our forest ecosystem.

In the forest, wolves prey on deer. The deer are herbivores and eat the grass. When there is an abundance of grass, the deer will have plenty of food, so their population will increase. The increase in the deer population means more food for the wolves, so their population increases too.

But, as the wolf population increases, they eat more of the deer, so the deer population decreases. Now, there is less food for the wolves, so their population decreases. The deer and wolf populations continue in this cyclic pattern throughout the seasons. This cycle can be represented on a predator-prey graph, which shows how the populations of predators and prey affect each other over time.

Prey and predator populations increase and decrease together in a cyclic pattern.
predator prey graph

Now, what would happen if the wolf population decreased indefinitely? You're probably thinking this would be a bad thing, and you're right. Unfortunately, this scenario is happening right now due to illegal hunting, destruction of their habitat, and global warming. If the wolf population goes down, what will happen to the deer?

The deer population goes up because now nothing is eating them. Still, the deer have a limited food supply of grass. So, when the deer eat all the grass, the deer population declines as well. Now, there is a dramatic decline in the number of wolves and deer - not mention all of the grass that the deer ate.

The ecosystem undergoes a population crash, where populations of species decrease suddenly, and it all began with the decline of the wolf population. The wolf is a keystone species, an important regulator of the rest of the organisms in the ecosystem. If balance is not restored, a population crash can lead to the eventual extinction of some species.

Without predators, the prey population increases and consumes all of their food source.
population crash

Carrying Capacity

As we explained with our deer, an environment can only support so many organisms. Eventually, they will exhaust their food, space and other resources and start to die off. The maximum population of organisms that can be maintained in an ecosystem is called the carrying capacity. When a population goes above the carrying capacity, organisms die and the population declines until homeostasis is reached again. Let's look at a different example in the forest.

Although we usually think of large organisms, like deer and wolves living in the forest, microscopic organisms also play a role. Bacteria are single-celled organisms and live in all areas of the forest, decomposing dead material and helping other organisms obtain nutrients. Bacteria divide rapidly, with some species multiplying every 20 minutes.

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