Unit 8 · Topic 8.5 · about 30 minutes · 7 parts · 11 scored checks, 2 of them multiple choice, plus 2 from earlier lessons
AP Biology 8.5: Community Ecology
Unit 8: 10 to 15% of the multiple-choice section
What you'll be able to do
- Describe the structure of a community according to its species composition and diversity.
- Explain how interactions within and among populations influence community structure.
Key vocabulary
| Term | What it means | The exam trap |
|---|---|---|
| Species | A group of organisms that can interbreed and produce viable, fertile offspring. | Count each species once when listing what a community contains. |
| Symbiosis | A relationship in which two populations of different species live closely together, such as parasitism, mutualism or commensalism. | Look at the effect on each population separately before naming the relationship. |
| Predator | An organism that kills and eats other organisms, its prey. | Follow both populations over time before describing how a predator affects its prey. |
| Niche | The way a population lives in its community: what it uses and how, where and when it uses it. | Name the resource two populations share before comparing their niches. |
Tap to open the picture full sizePart 1 Think first Two forests, four kinds of tree
Commit to a guess first. Being wrong now makes the right idea stick better.
Predict first
A teaching example. Two forests each hold 100 trees of the same four species. In forest X there are 25 trees of each species. In forest Y there are 97 oaks and 1 tree of each of the other three species. Which forest is more diverse?
In short. A community is a group of interacting populations of different species, and its structure is described by its species composition and species diversity.
- Simpson's Diversity Index = 1 − Σ(n/N)², where n is the number of organisms of one species and N the number of organisms of all species.
- Interactions among populations determine how they access energy and matter, and they can be characterized by positive and negative effects.
- Competition, predation and symbioses, including parasitism, mutualism and commensalism, can drive population dynamics.
Part 2 Learn it Describing and shaping a community
2a. Composition and diversity
The structure of a community is measured and described in terms of species composition, which species it contains, and species diversity, which also takes in how its individuals are shared among those species.
One measure is Simpson's Diversity Index:
Diversity Index = 1 − Σ(n/N)²
Here n is the total number of organisms of a particular species and N the total number of organisms of all species. For each species, find n/N, square it, add up the squares, and subtract the sum from 1.
For 10 organisms, 5 of one species and 5 of another: (0.5)² + (0.5)² = 0.5, so the index is 1 − 0.5 = 0.5. The sum of squares is smaller when individuals are spread evenly over more species, so the index runs from 0 toward 1, and a higher value means a more diverse community.
Quick check · Calculate
A teaching example: a pond holds 40 animals: 20 frogs, 12 newts and 8 turtles. Using Simpson's Diversity Index = 1 − Σ(n/N)², what is the pond's index? Give your answer to two decimal places.
2b. Communities are interacting populations
Communities are groups of interacting populations of different species that change over time based on the interactions between those populations.
Interactions among populations determine how they access energy and matter within a community. Two populations that eat the same food each get less of it when the other is common. A plant whose flowers feed an insect may have its pollen carried by that insect. Who eats what, and who uses which resources, flows from these interactions.
Quick check · Multiple choice
In a meadow, bees and hummingbirds both drink nectar from the same flowers. After the bee population doubles, each hummingbird gets less nectar. What does this best show?
2c. Positive and negative effects
Relationships among interacting populations can be characterized by positive and negative effects, written + and −, with 0 for no effect.
- Competition (− / −): two populations use the same limited resource.
- Predation (+ / −): a predator kills and eats its prey.
- Parasitism (+ / −): a parasite lives on or in a host and feeds on it.
- Mutualism (+ / +): each population benefits from the other.
- Commensalism (+ / 0): one population benefits and the other is unaffected.
Parasitism, mutualism and commensalism are kinds of symbiosis. Competition, predation and symbioses can drive population dynamics: they change how populations grow and shrink.
Quick check · Match
Match each interaction to the effects it has on the two populations.
2d. Modeling interactions
These relationships can be modeled. Examples include:
- Predator and prey: more prey feeds more predators; more predators eat more prey, so prey numbers fall, and then predator numbers fall too.
- Cooperation: individuals or populations working together, each gaining.
- Trophic cascades: a change at one feeding level passes down to the levels below. More hawks can mean fewer seed-eating mice, and so more seedlings.
- Niche partitioning: populations that use the same resource use different parts of it, such as two birds feeding at different heights in the same trees, which lessens competition between them.
Quick check · Fill the statement
Complete the description of two modeled interactions in a teaching example.
Two warbler species feed in the same spruce trees, one near the treetops and one near the trunk. This use of different parts of one resource is , and it lessens between them. When owls increase and eat more of the mice that eat grass seeds, and the grass spreads, the chain of effects is a .
Part 3 Watch it done Two tide pools
Predict first, then watch each step, then finish the reasoning yourself.
A teaching example. Two tide pools each hold 40 animals of the same four species. Pool A holds 10 snails, 10 crabs, 10 sea stars and 10 anemones. Pool B holds 31 snails, 3 crabs, 3 sea stars and 3 anemones. What is Simpson's Diversity Index for each pool?
Predict first
Before you open the first step, predict: how will the two pools' index values compare?
Step 1 · Find each share in pool A
Each species is 10 of 40, so n/N = 0.25 for all four.
Step 2 · Square, add and subtract
0.25² = 0.0625, and four of them add to 0.25. Index = 1 − 0.25 = 0.75.
Why this step?
Why square each share before adding?
Unsquared shares always add to 1, so they cannot tell communities apart. Squaring weights a large share heavily, so a community dominated by one species gets a large sum and a low index.
Step 3 · Find each share in pool B
Snails: 31/40 = 0.775. Crabs, sea stars and anemones: 3/40 = 0.075 each.
Your step · Calculate
A teaching example: a tide pool holds 31 snails, 3 crabs, 3 sea stars and 3 anemones, 40 animals in all. Using Simpson's Diversity Index = 1 − Σ(n/N)², what is the pool's index? Give your answer to two decimal places.
About 0.38 for pool B against 0.75 for pool A. Both pools have the same four species, but pool B is dominated by snails, so it is less diverse.
Part 4 Try it Predators, prey and competitors
Two new situations, both teaching examples. First, put the events of a predator and prey model in order. Second, predict what happens to one beetle species when a competing species is removed from a field.
Try it · Put it in order
In a model of a predator and its prey, the prey population starts small and its food is plentiful. Put the events in order, from first to last.
Try it · Decide and justify
Beetle species A and beetle species B both eat the same seeds in a field, and the seeds are in short supply, and neither eats the other. Researchers remove species A from half of the field. Predict what happens to species B in that half.
Which statement supports your decision?
Part 5 Explain it Two warblers in one tree
On the exam this is handwritten, so answer on paper first. Nothing you type here leaves this page. Then check your answer against the model and each scoring point.
Free response is 50% of the exam score. This one is shaped like Question 4, Conceptual Analysis.
Free-response practice · Question 4 style
A teaching example. Two warbler species live in the same forest and both eat insects, which are in limited supply. Species P feeds mainly near the tops of the trees, and species Q feeds mainly on the lower branches.
(a) Describe the effect on each species, positive or negative, if both fed on the same insects in the same part of the trees.
(b) Explain how feeding in different parts of the trees affects the interaction between the two species.
(c) Predict how the number of species Q would change if species P were removed from the forest, and justify your prediction.
(d) Explain how this interaction affects the two species' access to energy within the community.
Model answer
(a) They would compete for the same limited food. Competition harms both: each would get fewer insects than if the other were absent (− / −).
(b) This is niche partitioning. Each species uses a different part of the trees, so they take less of each other's food, which lessens competition between them.
(c) Species Q would likely increase. Without P, any insects the two species shared are left to Q, so it would have more food.
(d) Interactions among populations determine how they access energy within a community. Because P and Q divide the trees, each gets energy from insects in its own part, and the amount each gets depends on how much the other takes.
Score your own answer. This is a self-check, not a grade. Tick each point your answer makes.
Part 6 Check yourself Two meadows
Both case questions use the table.
Students count the flowering plants in a plot in each of two meadows. Each plot holds 100 plants of the same four species. These numbers are a teaching example.
| Species | Meadow 1 | Meadow 2 |
|---|---|---|
| Aster | 40 | 85 |
| Clover | 30 | 5 |
| Daisy | 20 | 5 |
| Yarrow | 10 | 5 |
Case question 1 · Calculate
Using the table and Simpson's Diversity Index = 1 − Σ(n/N)²: what is the index for Meadow 1? Give your answer to two decimal places.
Case question 2 · Multiple choice
Using the table: Meadow 2's Simpson's index is 0.27. Which statement compares the two meadows correctly?
The rest stand on their own and do not use the case.
Check yourself · Spot the error
A pond holds 6 frogs and 4 newts, 10 animals in all. A student works out its Simpson's Diversity Index, 1 − Σ(n/N)². Tap the line with the mistake.
Check yourself · Sort it
Each card describes an interaction between two populations. Tap a card, then the effects the interaction has on the two populations.
Both gain
One gains, one is harmed
One gains, one is unaffected
Both are harmed
Part 7 Keep it Bring back earlier ideas
A community is many populations, each limited by its resources, each a separate species. Pulling back carrying capacity and what makes two groups separate species sets community structure beside them.
From 8.4 Effect of Density on Populations · Multiple choice
A grassland's total available food and water can sustain about 300 bison over many years. In some years the herd briefly reaches 340 before falling back. Which statement about the grassland's carrying capacity for bison is best?
From 7.10 Speciation · Multiple choice
Two populations of a sexually reproducing fish live in separate lakes. Under the biological species concept, which finding would show that they are separate species?
In three lines
- A community's structure is described by its species composition and species diversity, measured by Simpson's index, 1 − Σ(n/N)².
- Interactions among populations determine how they access energy and matter, with positive and negative effects that can be modeled.
- Competition, predation, parasitism, mutualism and commensalism can drive population dynamics.
Exam strategy for this topic
- Section I, Multiple Choice: 60 questions in 90 minutes.
- Section II, Free Response: 6 questions in 90 minutes.
- This lesson's Explain it is shaped like free-response question 4, Conceptual Analysis.
- A four-function, scientific, or graphing calculator is allowed on both sections of the exam.
For a diversity question, compute Simpson's index for each community from n/N, and remember that a higher value means more diversity. For an interaction, give each population its sign, +, − or 0, before naming the relationship.
Questions students ask
What is a community in ecology?
Communities are groups of interacting populations of different species that change over time based on the interactions between those populations.
How do you calculate Simpson's Diversity Index?
Diversity Index = 1 − Σ(n/N)², where n is the total number of organisms of a particular species and N is the total number of organisms of all species. Square each species' share, add the squares and subtract the sum from 1.
What does a higher Simpson's Diversity Index mean?
A higher value means a more diverse community: when organisms are shared more evenly among more species, the sum of the squared shares is smaller, so 1 minus that sum is larger.
What are the main kinds of interactions between populations?
Competition, predation and symbioses, including parasitism, mutualism and commensalism. They can be characterized by positive and negative effects and can drive population dynamics.
What is niche partitioning?
Populations that use the same resource use different parts of it, such as two birds feeding at different heights in the same trees, which lessens competition between them.
Course alignment, for teachers
AP Biology topic 8.5, Unit 8: Ecology.
- Skill 5.B: Use confidence intervals and error bars to estimate whether sample means are statistically different.
This lesson maps all 5 essential knowledge statements of the topic to the part that teaches each one: 5 are checked by a scored question. The map is in the lesson file.