AP Environmental Science: Ecological Succession (Primary vs Secondary)

AP Environmental Science – Ecological Succession (Primary vs Secondary)

AP Environmental Science Study Guide: Ecological Succession (Primary vs. Secondary)

What This Is

Ecological succession is the predictable, gradual change in species composition in an ecosystem over time, following a disturbance or the formation of new land. It matters on the AP exam because it explains biodiversity, ecosystem resilience, and human impacts (e.g., deforestation, wildfires). A real-world example: Mount St. Helens’ 1980 eruption wiped out all life in the blast zone (primary succession), while nearby areas burned but retained soil (secondary succession), leading to different recovery timelines.

Key Terms & Concepts

• Ecological Succession: The process of change in species structure in an ecological community over time.
• Primary Succession: Succession that begins in lifeless areas (no soil), such as bare rock, lava flows, or glacial retreat. Example: Lichens colonizing a newly formed volcanic island.
• Secondary Succession: Succession that occurs in areas where an existing ecosystem was disturbed but soil remains. Example: Forest regrowth after a wildfire or abandoned farmland.
• Pioneer Species: The first organisms to colonize a barren area (e.g., lichens, mosses, grasses). They break down rock into soil in primary succession.
• Climax Community: A stable, self-sustaining community at the end of succession (e.g., old-growth forest). Note: Some ecologists argue climax communities are rare due to constant disturbances.
• Seral Stages: Intermediate stages of succession (e.g., grasses-shrubs-young trees-mature forest).
• Facilitation: Early species modify the environment to make it more suitable for later species (e.g., lichens create soil for plants).
• Inhibition: Early species prevent later species from establishing (e.g., dense grasses blocking tree seedlings).
• Tolerance: Later species outcompete early species for resources (e.g., shade-tolerant trees replacing sun-loving grasses).
• Disturbance: An event (natural or human-caused) that disrupts an ecosystem (e.g., fire, flood, logging).
• Resilience: An ecosystem’s ability to recover after a disturbance. Example: Grasslands recover quickly from fire; rainforests do not.
• Keystone Species: A species that has a disproportionate effect on its ecosystem (e.g., wolves in Yellowstone controlling elk populations, which allows vegetation to recover).

Step-by-Step / Process Flow

How to Compare Primary vs. Secondary Succession on the AP Exam:

1. Identify the starting point:
2. Primary: No soil (e.g., bare rock, sand dunes, volcanic ash).

3. Secondary: Soil present (e.g., after fire, flood, or farming).

4. List the pioneer species:

5. Primary: Lichens, mosses, algae (break down rock into soil).

6. Secondary: Grasses, weeds, fast-growing plants (seeds already in soil).

7. Describe the timeline:

8. Primary: Hundreds to thousands of years (slow soil formation).

9. Secondary: Decades to centuries (soil already exists).

10. Compare biodiversity over time:

11. Both start with low diversity (pioneer species).

12. Both increase in diversity until reaching a climax community (if undisturbed).

13. Apply to a real-world scenario:

14. Primary: Glacier Bay, Alaska (glaciers retreated, exposing bare rock-lichens-mosses-shrubs-spruce forest).
15. Secondary: Yellowstone National Park (after 1988 fires, lodgepole pines regrew from seeds in soil).

Common Mistakes

• Mistake: Confusing primary and secondary succession based on the type of disturbance (e.g., thinking a volcanic eruption is always primary). Correction: Primary succession occurs only if soil is completely destroyed. If soil remains (e.g., after a surface fire), it’s secondary.

• Mistake: Assuming all succession leads to a forest climax community. Correction: Climax communities depend on climate and geography (e.g., grasslands in dry areas, tundra in cold regions).

• Mistake: Forgetting that human activities (e.g., deforestation, agriculture) can reset succession to an earlier stage. Correction: Clearing a forest for farming is a disturbance that triggers secondary succession when abandoned.

• Mistake: Thinking pioneer species are always plants. Correction: In aquatic ecosystems, bacteria and algae are pioneer species (e.g., after a lake forms from glacial melt).

• Mistake: Overlooking keystone species’ role in succession. Correction: Beavers (keystone species) create ponds, which alter succession by flooding forests and creating wetlands.

AP Exam Insights

1. Multiple-Choice Traps:
2. The exam often asks about time scales (primary = slow, secondary = faster).

3. Watch for distractor answers that confuse disturbance type (e.g., "a hurricane causes primary succession"-false, unless it removes all soil).

4. FRQ Themes:

5. Compare primary vs. secondary succession in a given scenario (e.g., "Explain how succession would differ after a volcanic eruption vs. a clear-cut forest").
6. Human impacts on succession (e.g., "How does urbanization affect secondary succession?").

7. Keystone species and succession (e.g., "Describe how the reintroduction of wolves to Yellowstone altered succession").

8. Tricky Distinction:

9. Succession vs. Evolution: Succession is ecological change over time (community-level), while evolution is genetic change over generations (species-level).

10. Graph Interpretation:

11. Expect species richness vs. time graphs—primary succession starts at zero, secondary starts above zero.

Quick Check Questions

1. Multiple Choice

A wildfire burns through a forest, killing most vegetation but leaving the soil intact. Which of the following best describes the type of succession that will occur? (A) Primary succession, because fire is a major disturbance. (B) Secondary succession, because soil remains. (C) Primary succession, because pioneer species will colonize first. (D) Secondary succession, because it will take centuries to recover.

Answer: (B) Secondary succession occurs when soil is present after a disturbance.

2. Short FRQ

After a glacier retreats, exposing bare rock, describe the first three stages of succession and explain the role of pioneer species in this process.

Sample Answer:
1. Pioneer stage: Lichens and mosses colonize bare rock. They secrete acids that break down rock into soil.
2. Early seral stage: Grasses and small herbaceous plants grow in the thin soil.
3. Mid-seral stage: Shrubs and fast-growing trees (e.g., alder) establish, further improving soil quality. Pioneer species (lichens/mosses) facilitate succession by creating soil, allowing later plants to grow.

Last-Minute Cram Sheet

1. Primary succession = no soil (bare rock, lava, sand dunes).
2. Secondary succession = soil remains (fire, flood, farming).
3. Pioneer species in primary: Lichens, mosses.
4. Pioneer species in secondary: Grasses, weeds.
5. Primary succession timeline: Hundreds to thousands of years.
6. Secondary succession timeline: Decades to centuries.
7. Climax community = stable, self-sustaining (e.g., old-growth forest).
8. Human activities (logging, farming) reset succession to secondary.
9. Keystone species (e.g., wolves, beavers) alter succession patterns.
10. Fire can be both a disturbance (secondary succession) and a natural part of some ecosystems (e.g., grasslands).