1. Habitats, Niches, and Species Interactions
Read the lesson title aloud.

2. Learning Objectives
Identify the factors that determine and describe habitats and niches.
Explain how competition shapes communities.
Explain how herbivory shapes communities.
Explain how keystone species shape communities.
Identify the three primary ways organisms depend on each other.
Click to show each learning objective.
Read each objective aloud or ask a volunteer to do so.

3. Habitat
A habitat is an area with a particular combination of physical and biological environmental factors that affect which organisms can live within it.
Click to reveal the definition of a habitat
Use the moray eel as a context in which to define habitat.
Ask: What kind of habitat do moray eels live in?
Answer: They live in marine habitats with rocky crevices or coral reefs.
Ask: Does the moray eel share its habitat with any other organisms? How do you know?
Answer: Yes. There are lots of marine animals and plants that live in rocky crevices and coral reefs. In the photo you can see some coral that share this moray eel’s habitat.

4. Microhabitats and Microbiomes
A microhabitat is a tiny part of a much larger habitat. It has its own set of environmental conditions called its microclimate. A microbiome is a microbial community.
Read the definition of a microhabitat.
Have students point out as many microhabitats as they can see in the figure.
Answer: Student responses can include, but are not limited to, the tree’s leaves, the bark on the tree, the wing of the bird, the rotting log, the bark of the rotting log, the forest floor, the wing of the beetle, beetle burrows in the tree, termite tunnels in the log,
As a class, compare and contrast two microhabitats that they have identified. Have them point out similarities and differences between the two microhabitats.
Ask: What is the microhabitat of the bird louse?
Answer: A bird’s feathers.
Ask: Why is it important to study larger scale habitats such as an entire forest as well as the microhabitats inside that forest?
Answer: When ecologists study a larger habitat, they can understand what kinds of species these habitats support and the diversity of life in them. By studying microhabitats, ecologists can learn about very specific species that live in these much smaller scale habitats.
Then, discuss the concept of a microbiome. Point out that microbiomes play important roles in their habitats. For example, the microbiome in your gut greatly influences your health. For example, imbalances in a person’s gut microbiome have been linked to irritable bowel syndrome and inflammatory bowel disease, among other conditions.

5. 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

6. The Niche
A species niche includes the range of physical and biological conditions in which it can survive and reproduce, as well as the way it obtains the resources it needs.
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.
Use the figure of the warblers to further differentiate between habitat and niche.
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.

7. 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 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.

8. 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, causing the other species to die off.
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 and adequate space and other resources.
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.

9. Dividing Resources
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.
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.

10. 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 might 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 might happen in future years to the predator population? 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.

11. Herbivory Size Growth Distribution Survival
Explain that the interaction in which an herbivore feeds on producers (such as plants) is called herbivory. 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.

12. Keystone Species
A keystone species plays a vital and unique role in maintaining structure, stability, and diversity in an ecosystem
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.

13. Symbioses
Commensalism A relationship in which one organism benefits and the other is neither helped nor harmed.
Mutualism A relationship between two species in which both species benefit.
Parasitism A relationship in which one organism lives inside or on another organism and harms it.
Tell students that biologists recognize three main types of symbiotic relationships in nature: commensalism, mutualism, and parasitism.
Click to reveal the definition of commensalism.
Give students an example of a commensalistic relationship. 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.
Click to reveal the definition of mutualism.
Use the photo to explain an example of a mutualistic relationship: 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.
Click to reveal the definition of parasitism.
Ask students: Have you ever seen a brown leech feeding on the blood of human host? Tell students that this is an example of a parasitic relationship. Explain that a parasite obtains all or part of its nutritional needs from the host organism. Generally, parasites weaken but do not kill their host.
Clownfish and sea anemones help each other survive.