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Keystone Species: AP Environmental Science Crash Course Review | 3

Keystone Species: AP Environmental Science Crash Course Review

If you have been reviewing ecosystems for the AP Environmental Science exam, you may have run into the term keystone species. In this AP Environmental Science review and study guide, we’ll break down what keystone species are, why they are important, and everything you need to know to answer questions about on the AP Environmental Science exam.

Keystone Species

First: what is a keystone species? It is one that maintains the entire balance of a particular community. Named after the last stone placed in an arch, which keeps the arch standing, the keystone species similarly maintains the balance of the ecosystem and maintains the biodiversity an ecosystem through the role it plays. When a keystone species is removed from its ecosystem, the whole ecosystem is changed. The extinction of these species leads significant changes in the ecosystem.

Keystone Species Characteristics

A keystone species is typically a predator, though it can also be an engineer, and does not need to have a large population to control the ecosystem. A mountain lion, for example, can cover an expansive area, controlling the population of other smaller animals by eating them. Non-keystone species will often adapt their behavior around the keystone species.

The Effects of Keystone Species Elimination

The main effect of the disappearance of a keystone species is a significant change in the ecosystem. For a keystone predator species, this significant change will be an increase in the populations of the predator’s food sources. With the mountain lion example above, the population of small mammals would increase if the mountain lion disappeared. These uncontrolled populations might overgraze on the area’s plant life, resulting in a decrease in biodiversity and a scarcity of resources for other species. For an engineer keystone species, the environment will change because the keystone species is no longer engineering their environment.

The elimination of a keystone species can also leave an ecosystem open to an invasive species. An invasive species that would have previously been controlled by the keystone species would not experience this same resistance if the keystone species were eliminated. 

Herbivore Keystone Species

These species do not always need to be predators; they can also be engineers. The herbivore elephant is an example of a keystone species in African savannas. Elephants eat small trees and other plants, which keeps the savanna a savanna instead of allowing it to become a forest. Even if a tree gets to be sizable, the elephants’ large size gives it the ability to knock it over and eat it. Without the elephants to control the tree population, grazing animals like antelopes and zebras would experience more competition for food since they eat grasses. Mice and small rodents depend on the savannah being a grassland to have spaces to burrow into the soil. Lions and other carnivorous animals depend on the grazing animals for prey. Without elephants in this ecosystem, trees would grow up and push out grazing animals, making significant changes in the savanna ecosystem. 

Wolves in Yellowstone National Park

A prominent example of a keystone species in the United States that you may encounter on the AP Environmental Science exam is the grey wolf. Until the early 1900’s, grey wolves were relatively unthreatened and were a keystone species. Grey wolves would kill elk and deer, leaving their carcasses and providing food for scavengers. As people moved west and set up farms, wolves became unpopular. The grey wolves would kill livestock, leading farmers to shoot them. By the early 1900’s, wolves were eliminated from Yellowstone National Park. Since the wolves are a keystone species, the ecosystem underwent significant changes which led to a large increase in the number of herbivores in the park. These herbivores consume more plants as they increase in number, which leads to less plant diversity and periodic starvation for herbivorous species.

In 1995, several dozen grey wolves were captured in Canada and reintroduced to the park. Because grey wolves are native species to the area, the wolves did well, reproducing and increasing their number. Combined with bans on hunting, the wolves have since grown back into the keystone species they once were and are effectively controlling the ecosystem at Yellowstone National Park. 

Keystone Species on the AP Environmental Science Exam

On the APES exam, you will likely see multiple choice questions that directly ask about keystone species. Familiarity with the definition, as well as some examples of keystone species, will help you to answer these questions correctly.

In the free response section of the test, you will also see questions that deal with keystone species, but they may not directly ask about them. You will need to combine your knowledge with other information about ecosystems to adequately answer these questions. Below, we will break down a free response question that involves keystone species with how you might go about answering it on the APES exam.

Humans have spent over 100 years and billions of dollars trying to drive various species of mosquito to extinction. Despite the inventions of pesticides and the ensuing mass spraying of these chemicals, mosquitoes remain a human health threat. In contrast, we are experiencing the near extinction some species of elephants, despite our efforts to conserve them. The primary reason for this contradiction is that the mosquito is an r-selected species, and the elephant is a K-selected species.

a. How do an r-selected species and K-selected species population growth rates differ?

b. What are the patterns of variation in population size for both creatures?

c. What are their frequencies of reproduction?

d. How many offspring do they both have?

e. How does this relate to their present risk of extinction?

As you can see, this question does not directly ask about keystone species, but the elephants referenced in the question are keystone species.

For part (a) of this question, you need to understand r selected and k selected species. R-selected species grow at exponential rates very quickly, have short lifespans, and have a large number of offspring. Among K-selected species, populations take longer to grow, have longer lifespans and have few offspring. The keystone elephants and many keystone species are k selected while the mosquitoes are r selected.

Part (b) also deals with r and k selected species. R-selected species have populations that vary more widely because they typically have larger numbers of individuals to begin with. K-selected species have fewer variations and are more susceptible to decreases in population because they have fewer offspring.

Part (c) addresses reproduction of each species. The keystone elephants in the question have a long gestational period, leading to a slow frequency of reproduction. In contrast, the mosquitoes have a short gestational period, resulting in rapid reproduction.

Part (d) addresses how many offspring each species has. Since the elephants have a long gestational period, they have relatively few offspring. The mosquitoes, on the other hand, have many offspring enabled by their short gestational period.

Part (e) asks you to combine this information and reflect on the risk of extinction for each species. Due to the short gestation period and a large number of offspring, mosquitoes are unlikely to become extinct. In contrast, the keystone elephants are vulnerable. Their long gestational period and limited offspring leave them more susceptible to extinction.

In this APES crash course review, we have covered what keystone species are, how they function in their ecosystems, some examples of keystone species, and how to answer questions involving keystone species on the AP Environmental Science exam. A solid understanding of this information will set you up for success on the AP Environmental Science exam.

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