Unit 2 · Topic 2.7 Beta

Ecological Succession

Ecological succession is the series of changes in a community after a disruption.

Practice 1: Concept ExplanationPractice 5: Data Analysis

Part 1 · Hook

Why this matters

When a lava flow cools on Hawaii, it leaves black rock with no soil and no life. Within a few years, lichens and ferns appear in the cracks. A farm field abandoned in New England tells a faster story: weeds the first summer, shrubs within a decade, a young forest within a few decades. Both are ecological succession; what differs is whether soil was there to start with.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. How is a disruption that recurs at irregular intervals, such as wildfire in many forests, classified?

  1. Episodic
  2. Periodic
  3. Random
  4. Constant
Show the answer

Recurring, but not at regular intervals, is episodic.

  • Correct: Episodic:
  • Periodic:
  • Random:
  • Constant:

2. What does species richness count?

  1. The number of different species
  2. The number of individuals
  3. How equal the populations are
  4. The number of habitats
Show the answer

Richness is the count of species.

  • Correct: The number of different species:
  • The number of individuals:
  • How equal the populations are:
  • The number of habitats:

Part 4 · See it

See it first

Two rows of stages. Primary succession: bare rock with no soil, lichens and mosses as pioneer species, grasses on thin soil, shrubs, fast-growing trees, mature forest (climax community); it starts after lava cools or a glacier retreats and is slow. Secondary succession: soil remains after a fire or farm, grasses and weeds as pioneer species, shrubs, fast-growing trees, mature forest; it is faster because soil and seeds remain.
Primary succession starts on bare rock with no soil; secondary succession starts where soil remains after a disruption. Both move toward a mature community. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. A disruption leaves bare rock (primary) or bare soil (secondary)pioneer species, which grow fast and spread easily, arrive first
  2. Pioneers change the site: they help make soil, add nutrients and give shadeconditions become suitable for larger plants that could not grow there before
  3. Larger, longer-lived plants arrive and outcompete the pioneersthe community changes in stages, and species richness and biomass increase
  4. Many years pass without a major disruptiona mature, more stable community (climax community) develops

Part 6 · Key ideas

Key ideas

  • Ecological succession is the gradual change in the species of a community after a disruption, in a series of stages.
  • Primary succession starts where there is no soil: new lava rock, rock left by a retreating glacier. Soil must form first, so it is slow (often centuries).
  • Secondary succession starts where soil remains after a disruption such as a fire, a flood, a hurricane or an abandoned farm field. With soil and seeds already present, it is faster (often decades).
  • Pioneer species (lichens and mosses on rock; grasses and weeds on soil) arrive first. Over time biodiversity and biomass increase, and a mature, more stable climax community develops.

Part 7 · Misconception

A common mistake

The wrong idea: Succession after a forest fire is primary succession, because almost everything was burned.

What actually happens: A fire leaves soil, and usually seeds and roots in it, so regrowth after a fire is secondary succession. Primary succession needs a start with no soil at all, such as new rock.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

Data

Plants in abandoned farm fields

Plant species and above-ground plant biomass in fields of different ages since farming stopped, one region (invented data for practice)
Years since farming stoppedPlant speciesPlant biomass (kg per m²)Most common plants
1120.4annual weeds
5251.2grasses, wildflowers
20386.0shrubs, young pines
604518.0pines, young oaks
1504230.0oaks, hickories

1. Which statement best describes how plant biomass changes as the fields age?

  1. It increases throughout, from 0.4 to 30.0 kg per m²
  2. It rises for 60 years, then falls to its starting level
  3. It stays near 1 kg per m² throughout
  4. It falls as larger plants shade out the weeds
Show the answer

Give the direction and the start and end values.

  • Correct: It increases throughout, from 0.4 to 30.0 kg per m²: Right: biomass rises at every step, most of all as trees take over.
  • It rises for 60 years, then falls to its starting level: Biomass is higher at 150 years (30.0) than at 60 (18.0).
  • It stays near 1 kg per m² throughout: It reaches 30.0 kg per m² by 150 years.
  • It falls as larger plants shade out the weeds: Larger plants add biomass; the total rises.

2. Write the setup for the percent change in the number of plant species from the 1-year field to the 20-year field, then give the answer.

Type the setup as on the exam, with brackets and units, then the answer.

Show the answer

Start at 12 species: 38−1212×100\frac{38 - 12}{12} \times 100≈216.7%\approx 216.7\%.

  • Setup: 38−1212⋅100\frac{38 - 12}{12} \cdot 100; answer 216.7 %

3. What kind of succession do these fields show, and why?

  1. Primary, because the fields began with no plants growing
  2. Secondary, because soil remained when farming stopped
  3. Primary, because weeds are pioneer species
  4. Secondary, because the fields began as bare rock
Show the answer

The key question is whether soil was present at the start.

  • Primary, because the fields began with no plants growing: Primary succession needs no soil; these fields had soil.
  • Correct: Secondary, because soil remained when farming stopped: Right: an abandoned field keeps its soil and seeds, so succession is secondary.
  • Primary, because weeds are pioneer species: Pioneer species appear in both kinds; the starting soil decides the kind.
  • Secondary, because the fields began as bare rock: Bare rock would make it primary, and these fields had soil.

Passage

Life returns to a volcano

In May 1980, Mount St. Helens in Washington State erupted. The blast flattened 600 square kilometers of forest, and a fan of hot rock and ash on the volcano’s north side, later named the Pumice Plain, buried the ground under many meters of fresh rock. On the Pumice Plain, nothing survived; a few years later only scattered plants had arrived. Farther out, where the blast knocked trees down but left soil, roots and buried seeds, plants sprouted the first summer, and pocket gophers dug fresh soil up to the surface. Ecologists who have studied both areas since 1980 report that life returned much faster where soil remained than on the rock of the Pumice Plain.

Invented passage for practice, summarizing USGS accounts and published research at Mount St. Helens.

4. Which claim does the passage support?

  1. The eruption killed each plant across 600 square kilometers
  2. Pocket gophers caused the eruption by digging
  3. The Pumice Plain recovered first because it was closest
  4. Life returned faster where soil remained
Show the answer

The passage compares the two areas and states which recovered faster.

  • The eruption killed each plant across 600 square kilometers: Plants sprouted the first summer where soil, roots and seeds survived.
  • Pocket gophers caused the eruption by digging: Gophers dug soil after the eruption; they did not cause it.
  • The Pumice Plain recovered first because it was closest: The passage says the Pumice Plain recovered slowly.
  • Correct: Life returned faster where soil remained: Right: plants sprouted the first summer where soil remained, but slowly on the Pumice Plain.

5. Which detail from the passage best supports calling the Pumice Plain an example of primary succession?

  1. It was buried under fresh rock, with nothing surviving
  2. It lies on the north side of the volcano, facing the blast
  3. It covers part of the 600 square kilometers of flattened forest
  4. It was given its name by scientists after the eruption
Show the answer

Primary succession starts with no soil and no surviving organisms.

  • Correct: It was buried under fresh rock, with nothing surviving: Right: no soil and no surviving life is the starting point of primary succession.
  • It lies on the north side of the volcano, facing the blast: The direction does not decide the kind of succession.
  • It covers part of the 600 square kilometers of flattened forest: The size does not decide the kind of succession.
  • It was given its name by scientists after the eruption: A name does not show anything about soil.

6. Which of these sites would undergo primary succession?

  1. A forest after a wildfire
  2. A cornfield that is no longer farmed
  3. Rock bared by a melting glacier
  4. A floodplain after a spring flood
Show the answer

Primary succession starts on surfaces with no soil.

  • A forest after a wildfire: Soil remains after a fire, so that is secondary succession.
  • A cornfield that is no longer farmed: An old field keeps its soil, so that is secondary succession.
  • Correct: Rock bared by a melting glacier: Right: the glacier scraped the land to bare rock with no soil.
  • A floodplain after a spring flood: Floods leave soil, often adding more, so that is secondary.

7. Explain why secondary succession is faster than primary succession.

Build the answer part by part, then write it in your own words.

Show the answer

Name what secondary succession starts with, what primary succession must do first, and the result.

  • An answer that earns the point: In secondary succession, soil, seeds and roots are already present, while primary succession must first build soil from bare rock, which takes centuries, so plants can grow sooner in secondary succession.

Part 9 · Summary

Summary

Ecological succession is the series of changes in a community after a disruption. Primary succession begins on bare rock with no soil, and pioneer species such as lichens and mosses help form soil; it is slow. Secondary succession begins where soil remains, after a fire, flood or abandoned farm; it is faster. Over the stages, biodiversity and biomass increase until a mature, more stable climax community develops.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections

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