AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/8/8-5)
Unit 8 · Topic 8.5
8.5 Community Ecology
A community is all the populations of different species living and interacting in one place. Communities are described by which species are present and how diverse they are (measured with Simpson's diversity index), and they're shaped by competition, predation and symbioses, which can lead to niche partitioning and trophic cascades.
Key terms
- species diversity
- Simpson's diversity index
- competition
- predation
- symbiosis
- niche partitioning
Describing a community
Community structure is described by species composition (which species live there) and species diversity. Diversity includes species richness (how many species) and evenness (how equally individuals are spread among the species). A forest with 10 species in equal numbers is more diverse than one with 10 species where one species makes up 90% of individuals.
Simpson's diversity index combines both: Diversity Index = 1 − Σ(n/N)², where n is the number of individuals of one species and N is the total number of individuals of all species. It ranges from 0 (one species only, no diversity) toward 1 (many species in even numbers). It's on the AP formula sheet.
Interactions between species
Communities change over time because of how their populations interact. Each interaction can be described by its effect on each species: positive (+), negative (−) or neutral (0).
| Interaction | Effect on species 1 / species 2 | Example |
|---|---|---|
| Competition | − / − | Two bird species eating the same seeds |
| Predation (including herbivory) | + / − | Lynx eating snowshoe hares; deer eating shrubs |
| Mutualism | + / + | Bees and flowers; nitrogen-fixing bacteria in legume roots |
| Commensalism | + / 0 | Cattle egrets eating insects stirred up by grazing cattle |
| Parasitism | + / − | Ticks or tapeworms feeding on a host |
How interactions shape communities
Predator and prey populations often rise and fall in linked cycles: more prey supports more predators, more predators reduce prey, and then predators decline. Long-term records of moose and wolves on Isle Royale in Lake Superior show this kind of linked change.
Two species that compete for exactly the same limited resource can't coexist forever; one will outcompete the other. Coexisting species often avoid this through niche partitioning, dividing a resource by using it in different ways, places or times. Several warbler species in the same spruce trees, for example, feed in different parts of the tree.
A trophic cascade happens when a change at one feeding level ripples down through others. Removing a top predator can let herbivores multiply, which then overeat plants. The return of wolves to Yellowstone in 1995 is often cited: wolves reduced and moved elk, which may have let some streamside trees recover, though scientists still debate how strong that effect was.
Comparing communities with error bars
Field data vary, so ecologists compare means with error bars. A common choice is ±2 standard errors of the mean (SE = s ÷ √n), which marks roughly a 95% confidence interval. If the error bars of two means don't overlap, the difference is likely statistically significant. If they overlap a lot, you can't conclude the means differ.
Worked examples
Try each one yourself first, then open the solution.
- Example 1Calculator allowed
Simpson's diversity index
Two ponds each have 100 animals of 4 species. Pond A: 25, 25, 25, 25. Pond B: 85, 5, 5, 5. Calculate Simpson's diversity index for each and explain the difference.
Show the solutionHide the solution
- Step 1: Pond A: each species is 25/100 = 0.25 of the total; (0.25)² = 0.0625. Σ = 4 × 0.0625 = 0.25. Index = 1 − 0.25 = 0.75.
- Step 2: Pond B: (85/100)² = 0.7225; (5/100)² = 0.0025 for each of the other three, totaling 0.0075. Σ = 0.7225 + 0.0075 = 0.73. Index = 1 − 0.73 = 0.27.
- Step 3: Both ponds have the same richness (4 species), but Pond A is much more even, so its diversity is higher.
Answer: Pond A = 0.75, Pond B = 0.27. Pond A is more diverse because its individuals are spread evenly among species.
- Example 2Calculator allowed
Do the error bars overlap?
Students count beetles in 16 quadrats in each of two meadows. Meadow 1: mean 12 beetles, standard deviation 4. Meadow 2: mean 18 beetles, standard deviation 6. Using ±2 SE, decide whether the means are likely different.
Show the solutionHide the solution
- Step 1: Meadow 1: SE = 4 ÷ √16 = 4 ÷ 4 = 1. ±2 SE gives 12 ± 2, or 10 to 14.
- Step 2: Meadow 2: SE = 6 ÷ √16 = 6 ÷ 4 = 1.5. ±2 SE gives 18 ± 3, or 15 to 21.
- Step 3: The intervals (10–14 and 15–21) don't overlap.
Answer: The ±2 SE intervals don't overlap, so the difference in mean beetle numbers is likely statistically significant; Meadow 2 has more beetles per quadrat.
Common mistakes
- Forgetting the '1 −' in Simpson's index, which gives a number that goes down as diversity goes up.
- Thinking diversity means only the number of species. Evenness matters too.
- Calling an interaction mutualism when only one species benefits. Check the effect on both species.
- Concluding that overlapping error bars prove the means are the same. Overlap means you can't conclude a difference.
On the exam
- Expect to calculate Simpson's index from count data and to compare two communities.
- Graph and data questions often ask whether two means are significantly different based on error bars or confidence intervals; state whether they overlap and what that means.
Connected topics
Videos
Check yourself
4 questions on 8.5 Community Ecology. Pick an answer to see if you got it, and why.
| Species | Community 1 (individuals) | Community 2 (individuals) |
|---|---|---|
| Oak | 10 | 47 |
| Maple | 20 | 1 |
| Birch | 10 | 1 |
| Pine | 10 | 1 |
Field data: counts of four tree species in two forest plots of equal area. Simpson's diversity index: D = 1 − Σ(n/N)².
What is Simpson's diversity index for Community 1?
Community 2 has a Simpson's diversity index of about 0.12. Which statement best explains why it is lower than Community 1's, even though both have four species?
Five similar warbler species live in the same spruce forest and eat insects. Each species feeds mostly in a different part of the trees, such as the tips of high branches or the inner trunk. This pattern most likely reduces
Which of the following correctly matches a relationship with its type?
0 of 4 answered