1. Niches and Community Interactions
Read the lesson title aloud.

2. Learning Objectives Define niche.
Describe the role competition plays in shaping communities.
Describe the role predation and herbivory play in shaping communities.
Identify the three types of symbiotic relationships in nature.
Click to show each learning objective.
Read each objective aloud or ask a volunteer to do so.

3. What Is a Niche?
The range of physical and biological conditions in which a species lives and the way the species what it needs to survive and reproduce.
Use the cheetah as a context in which to define niche.
Ask: What kind of habitat do cheetahs live in, what do they eat, and what time of day are they active?
Answer: They live in grassland habitats in Africa, hunt small-to-medium prey such as gazelles, and are active mainly during the day.
The range of African leopards overlaps with cheetahs in some parts of Africa. Explain that while the ranges of these big cats overlap somewhat (that is, they share a habitat), their lifestyles differ. Leopards generally hunt larger prey than cheetahs, and they are excellent climbers, dragging their prey into trees to keep it away from other animals. Leopards also primarily hunt at night.
Point out that although they do share a habitat in some regions, cheetahs and leopards have different niches.
Click to reveal the definition of niche.
Ask a student to read the definition aloud.
Discuss what the different parts of the definition mean. Emphasize that part of a species’ niche involves the physical, or abiotic, external factors to which it is adapted. A species’ niche also involves biological, or biotic, external factors it requires for survival.
If students are struggling with the difference between a niche and a habitat, offer this analogy: A habitat is like the species’ “address” and the niche is like the species’ ecological “occupation”—where and how it “makes a living.” Another analogy to use would be members of a sports team. Tell students that an animal’s niche is like the position an athlete plays in a team sport. Ask a student to name a player position for his or her favorite sport. Then, explain how this position is like a niche. For example, discuss the role a player in a given position fulfills for the team, the physical space the player occupies, and how the player interacts with teammates and competitors.

4. Tolerance
Tolerance: the range of external conditions within which a species can survive and reproduce
Point out that the physical conditions in which a species lives represent a range. For each environmental factor, a particular species has a certain tolerance.
Click to reveal the definition of tolerance.
Explain that this graph shows the response of a hypothetical organism to different values of a single environmental variable, such as sunlight or temperature. At the center of the optimum range, organisms are likely to be most abundant. They become more rare in zones of physiological stress (medium blue), and are absent from zones of intolerance (light blue). When an external factor such as temperature rises above or falls below a species’ optimum range, the individuals of that species experiences stress.
Ask: Why would conditions outside the optimal range cause stress to an organism?
Answer: because it is harder to maintain homeostasis
Guide students to understand that outside the optimal ranges for a given factor, individuals must expend more energy to maintain homeostasis, so they have less energy left for growth and reproduction. Each species has an upper and lower limit of tolerance for every external factor. Beyond those limits, the species cannot survive.
Ask for a volunteer to go to the board to point out or circle where on the spectrum for this particular environmental variable there would be no survival for the hypothetical organism.
Click to reveal the correct answers.
Be sure students understand that there are two extremes for any organism, though these may vary greatly from species to species. No
survival No
survival

5. Competition
When organisms attempt to use the same limited ecological resources in the same place at the same time.
Intraspecific: between members of the same species
Interspecific: between members of different species
Point out that if you look at any community, you will probably find more than one kind of organism attempting to use the same resources. When organisms attempt to use the same limited ecological resource in the same place at the same time, competition occurs.
Ask: What resource do you think these two males are fighting over?
Sample answer: I think they are fighting over food, living space, mates, or a place to raise their young.
Ask: What kind of resources would you expect two plants to compete for?
Sample answer: water, sunlight, nutrients
Point out that competition can occur between members of the same species and between members of different species.
Click to reveal the terms intraspecific and interspecific and their definitions.
Ask students to share examples they can think of that represent both types of competition.
Ask: Are the beetles in the photo here an example of intraspecific or interspecific competition? How do you know?
Answer: It is probably an example of intraspecific competition, because both beetles appear to belong to the same species.
Remind students of the earlier example of leopards and cheetahs, pointing out that leopards are generally nocturnal hunters and cheetahs are generally diurnal hunters.
Ask: Are cheetahs and leopards in competition for food?
Answer: Not strictly speaking; they are generally not hunting at the same time even if they may hunt some of the same prey.

6. Competitive Exclusion Principle
No two species can occupy exactly the same niche in exactly the same habitat at exactly the same time.
Explain that direct competition between species almost always produces a winner and a loser—and the losing species dies out. One series of experiments demonstrated this using two species of single-celled organisms. When each species was grown in a separate culture under the same conditions, each survived, as shown in the graph.
Focus student attention to the graph. Explain that the graph displays the results of the competition experiments. Lead a discussion guiding students to interpret the graph and draw conclusions from the data:
Ask: What happened over time to each species when grown alone?
Answer: Both populations grew to a point and then leveled off.
Ask: What happened to each species when the species were grown together?
Answer: Both populations grew at first, but then the P. aurelia grew much faster and the P. caudatum population stopped growing completely and the population went to zero.
Ask: What resources would members of the two species be competing for?
Sample answer: space, food
Ask: Assume we repeated this experiment over and over and got the same results. What could you conclude about the competition for resources between the two species?
Answer: The species were competing for the same resources at the same time, and one species was better able to compete, pushing out the other species completely.
Ask: How do you think the results would have been different if the two species fed on different food sources?
Sample answer: The populations would not be using exactly the same resources, so both populations would survive together as long as there was the right food type for both.
Emphasize that no two species can occupy exactly the same niche in exactly the same habitat at exactly the same time. If two species attempt to do so, one species will be better at competing for limited resources and will eventually exclude the other species.
Click to reveal the statement of the competitive exclusion principle.

7. Dividing Resources Spruce tree Cape May Warbler Bay-breasted Warbler
Point out that the competitive exclusion principle explains why we rarely find species in natural communities with overlapping niches. Competition creates pressure for each species to specialize the way that it uses resources to survive and reproduce. For instance, the three species of North American warblers shown here all live in the same trees and feed on insects. But one species feeds on high branches, another feeds on low branches, and the third feeds in the middle. The resources utilized by these species are similar yet different.
Ask: Do these species have the same habitat? The same niche?
Answer: They have the same habitat (spruce forest), but they have different niches because they are not feeding in exactly the same place.
Ask: What would happen if two of the warbler species tried to occupy the same niche in the same tree at the same time?
Answer: One species would be better at competing for food in that niche and would eventually exclude the other species.
Emphasize that by causing species to divide resources, competition helps determine the number and kinds of species in a community and the niche each species occupies. Be sure students understand that division of resources is likely a result of past competition.
Yellow-rumped Warbler

8. Predator-Prey Relationships
One predator-prey cycle
Explain that, like competition, predation can also shape communities. Predators can affect the size of prey populations in a community and determine the places prey can live and feed. For example, birds of prey can play an important role in regulating the population sizes of mice, voles, and other small mammals.
Draw student attention to the graph. Ask students to describe the pattern they see between the size of the predator population over time and the size of the prey population over time.
Click to reveal the label for one predator-prey cycle.
Students should understand that this cyclical pattern is a common one, with a slight lag time between the availability of large amounts of prey and an increase in the predator population.
Lead a further discussion in which students interpret the graph and make predictions by asking the following:
Ask: Suppose a bacterial infection kills most of the prey at point B on the graph. How would this affect the predator and prey growth curves at point C? At point D?
Answer: If most of the prey were killed at point B, the predator population would decline between points B and C, allowing the prey population to increase again by point C. Greater numbers of prey would lead to the predator population increasing again, which would be followed by another decline in the prey population. The decline in numbers of prey would lead to another decrease in the predator population, which would allow the prey population to increase yet again at point D.
Ask: Suppose a sudden extended cold spell destroys almost the entire predator population at point F on the graph. How would the next cycle of the prey population appear on the graph?
Answer: The prey population would increase in the next cycle, reaching a peak that is potentially even higher than the previous peaks.
Ask: Suppose a viral infection kills all the prey at point D on the graph. What effect would this have on the predator and prey growth curves at point E? What will happen in future years to the predator population? How could ecologists ensure the continued survival of the predators in this ecosystem?
Sample answer: If a viral infection kills all the prey at point D, at point E, the prey population will be zero and the predator population would be decreased, possibly to zero. In future years, if the predators find another food resource, the predator population may recover. The predator population would not recover in future years without any prey to feed on. To ensure continued survival of the predators in this ecosystem, ecologists could control the viral infection in the prey so that not all of them are killed by the virus, or they could introduce new prey animals to the ecosystem.

9. Herbivory Size Growth Distribution Survival
Explain that the interaction in which an herbivore feeds on producers (such as plants) is called herbivory, an example of which is shown here. Herbivores can affect plant populations in several ways.
Solicit suggestions from the students about what these effects might be.
Click to reveal the bullet points.
Explain that herbivores affect both the size and distribution of plant populations in a community and determine the places that certain plants can survive and grow.
If you ask students to name predators and herbivores, they are likely to mention mammals such as wolves and deer. Widen their perspective by discussing examples of the more prevalent yet often less-familiar predators and herbivores of the insect world. Tell students that insect herbivores, such as beetles and caterpillars, destroy large numbers of crops worldwide, and insect predators, such as ladybeetles and lacewings, eat many of these crop pests.
Ask: How might the use of chemical pesticides to kill insect herbivores affect insect predators?
Sample answer: The pesticides might kill both types of insects. Then, if insect herbivores increase in numbers again, there might not be enough predators left to control them.

10. Keystone Species
A keystone species is a single species that is vital to ecosystem stability.
Point out that sometimes the effect of a single species on community structure is so important that changes in its population can impact the structure and stability of an ecosystem.
Tell students: For example, in the cold waters off the Pacific coast of North America, giant algae called kelp form complex ecosystems called kelp forests. In that ecosystem, sea otters prey on sea urchins. Those urchins are herbivores that graze heavily on the giant kelp. A century ago, otters were nearly eliminated by hunting. Unexpectedly, the kelp forest nearly vanished. Without otters as predators, sea urchin populations skyrocketed. Armies of urchins devoured kelp down to bare rock. Without kelp to provide habitat, many other animals, including seabirds, disappeared. In this community, otters were a keystone species. When otters were protected as an endangered species, their population recovered. Urchin populations dropped, and kelp forests began to thrive again.
Click to reveal the definition of keystone species.
Point out that cheetahs are considered by many scientists to be a keystone species in their ecosystems.
Ask: What might happen to grassland ecosystems in cheetah home ranges if cheetahs become extinct?
Sample answer: Populations of their prey might grow out of control and overgraze the grasslands, disrupting the habitat for other species.

11. Symbiosis: Mutualism
A relationship between two species in which both species benefit
Tell students that biologists recognize three main types of symbiotic relationships in nature: mutualism, parasitism, and commensalism.
Direct students to the example of mutualism on the slide.
Before defining mutualism, describe the relationship between clownfish and sea anemones: Sea anemones use stinging tentacles to protect themselves from predators and to capture prey. Still, certain fishes manage to snack on anemone tentacles. Some species of anemones, however, live in close association with clownfish, which are immune to anemone stings. When a clownfish is threatened by a fish-eating predator, it seeks shelter by snuggling deep into tentacles that would kill or paralyze most other fish. But if an anemone-eating species tries to attack their living home, the clownfish dart out and fiercely chase away fish many times their size.
Ask: What could happen to the sea anemone if the clownfish died?
Answer: The sea anemone might be killed by predators if the clownfish died.
Point out that this relationship is an example of mutualism. Ask student to offer their own definitions of mutualism based on the description of the clownfish-anemone relationship.
After students share a few ideas, click to reveal the definition of mutualism.
Clownfish and sea anemones help each other survive.

12. Symbiosis: Parasitism
A relationship in which one organism lives inside or on another organism and harms it
Tell students that this photo shows a brown leech feeding on the blood of its human host.
Ask: Do both of the species benefit from this interaction? Does only one party benefit?
Answer: Only one party benefits—the leech.
Click to reveal the definition of parasitism.
Explain that the parasite obtains all or part of its nutritional needs from the host organism. Generally, parasites weaken but do not kill their host.

13. Symbiosis: Commensalism
A relationship in which one organism benefits and the other is neither helped nor harmed
Using the photo, explain that small marine animals called barnacles often attach themselves to a whale’s skin. The barnacles benefit from the constant movement of water—which is full of food particles—past the swimming whale. Although the barnacles perform no known service to the whale, they don’t harm it either.
Ask: How is this relationship different from that of the sea anemone and clownfish?
Answer: With the sea anemone and clownfish, both organisms benefit, but here, only one benefits.
Ask: How is this relationship different from that of leeches and their hosts?
Answer: Parasites like leeches benefit by harming their host, but here the barnacles benefit while not harming their host.
Click to reveal the definition of commensalism.

14. Vocabulary Review
No two species can occupy exactly the same niche in exactly the same habitat at exactly the same time
The ability to survive and reproduce under a range of environmental conditions
One species benefits while harming another
Between members of different species
A single species vital to ecosystem stability
An interaction in which one animal feeds on producers
Keystone species
Parasitism
Competitive exclusion principle
Tolerance
Interspecific competition
Herbivory
To review, ask for volunteers to come to the board and draw lines to match the terms with their correct definition or description.
Click to reveal the correct connecting lines.