Biology • Year 11 • Module 4 • Lesson 15
Ecological Succession — Primary, Secondary and Climax Communities
Apply succession concepts to real data, novel scenarios, and diagram interpretation. Practise distinguishing facilitation from inhibition and evaluating the pace of community recovery.
1. Interpret post-fire recovery data
Two adjacent woodland sites in the Blue Mountains were affected by the 2019–20 Black Summer fires. Site A experienced a low-severity ground fire; Site B experienced a high-severity crown fire that incinerated the soil seed bank. A research team surveyed both sites each year for five years after the fire. The table shows the percentage of pre-fire plant species richness that had returned at each site. 9 marks
| Years after fire | Site A — low severity (%) | Site B — high severity (%) |
|---|---|---|
| 0 (immediately after) | 5 | 2 |
| 1 | 38 | 8 |
| 2 | 62 | 14 |
| 3 | 74 | 21 |
| 4 | 83 | 27 |
| 5 | 91 | 33 |
1.1 Describe the trend in species richness recovery for both sites from Year 0 to Year 5. 2 marks
1.2 Using lesson content, explain why Site A recovered significantly faster than Site B, despite both starting from similar fire-affected landscapes. 3 marks
1.3 Identify whether Site A and Site B are undergoing primary or secondary succession. Justify your answer for each site with reference to the data and lesson content. 4 marks
2. Interpret graph — species diversity during primary succession
The stylised graph below models how total species diversity changes during primary succession on a coastal sand dune. 6 marks
2.1 Describe the overall trend in species diversity from the pioneer stage to the climax community. 2 marks
2.2 The graph shows a secondary diversity peak at the ecotone between the late-succession and climax stages. Using the lesson content on inhibition and tolerance, explain why diversity might be highest at this transition zone. 2 marks
2.3 Predict how the graph would differ if the same sand dune were subject to a moderate fire at the “Mid” stage. Draw a dotted line on the graph to show your prediction, and explain your reasoning below. 2 marks
3. Diagram critique — spot the errors
A Year 11 student drew the following description of the difference between primary and secondary succession. There are three biological errors. Identify each error and write the correction. 6 marks (2 per error: 1 identify, 1 correct)
"Primary succession starts after a bushfire when the vegetation has been burned but the soil seed bank is still present. Because soil is available, the first species to return are fast-growing trees that quickly re-establish the canopy. Secondary succession starts on bare lava rock with no soil; it is faster than primary succession because the pioneer lichens and mosses form soil very rapidly within just a few years."
3.1 Error 1: What is wrong?
Correction:
3.2 Error 2: What is wrong?
Correction:
3.3 Error 3: What is wrong?
Correction:
4. Apply to a new scenario — Surtsey Island
Surtsey Island in Iceland emerged from the sea following a volcanic eruption in 1963. It consisted entirely of bare lava and ash with no soil, no seed bank, and no pre-existing organisms. Today, more than 60 years later, it has a developing community of mosses, grasses, seabirds, and some insects, but no trees. 7 marks
4.1 Identify the type of succession occurring on Surtsey and justify your answer using the lesson’s definition. 2 marks
4.2 Explain the role of facilitation in the early stages of Surtsey’s succession. What would need to happen before grasses and herbs could establish on the island? 3 marks
4.3 Surtsey is located at a similar latitude to Norway. Predict what the climax community of Surtsey would be, and explain what factor determines this, not the starting substrate. 2 marks
Q1.1 — Trend description (2 marks)
Both sites show increasing species richness recovery over five years. Site A recovers much faster, reaching 91% of pre-fire diversity by Year 5 compared to only 33% for Site B. Site A shows a steep initial increase (5% to 38% in Year 1) while Site B increases slowly and consistently at roughly 6–8% per year.
Marking criteria: 1 mark — both sites show increases over time. 1 mark — Site A recovers substantially faster than Site B (with reference to at least one figure).
Q1.2 — Why Site A recovers faster (3 marks)
Site A experienced a low-severity fire that left the soil seed bank intact. Within weeks to months, fire ephemerals (annual herbs) germinated from the seed bank, taking advantage of elevated soil nutrients and reduced canopy competition [1]. Many eucalypts and understorey shrubs survived with their root systems and lignotubers intact, allowing rapid resprouting [1]. Site B’s high-severity fire incinerated the seed bank, leaving no stored propagules in the soil, so recovery must depend on wind- or animal-dispersed seeds from surrounding areas, which is a much slower process [1].
Q1.3 — Primary or secondary? (4 marks)
Site A — Secondary succession. The soil structure, nutrients, and (initially) the seed bank remain intact after the low-severity fire. Existing root systems can resprout. The defining condition for secondary succession (previously vegetated land with soil present) is met [1 + 1 for justification].
Site B — Secondary succession (not primary) because soil structure still exists even though the seed bank was incinerated. The site was previously vegetated with soil present; the seed bank destruction slows recovery but does not make it primary succession, which requires bare substrate with no soil at all [1 + 1 for justification]. Accept: some discussion of Site B approaching primary-succession-like conditions due to seed bank loss, provided the student correctly states that true primary succession requires absent soil.
Q2.1 — Overall trend (2 marks)
Species diversity increases from the pioneer stage through early, mid, and late succession as more species can establish in the improving environment. Diversity peaks near the late-succession/climax transition and then slightly declines at the climax community as dominant canopy trees suppress earlier species. [1 mark for increase overall; 1 mark for noting the peak and slight decline at climax.]
Q2.2 — Ecotone diversity peak (2 marks)
The ecotone (transition zone) between late succession and climax contains patches of both open and closed canopy, providing habitat for both sun-loving late-succession species and shade-tolerant climax species simultaneously [1]. Additionally, tolerance allows shade-tolerant species to persist in the understorey while earlier species still hold some space, so both groups co-exist in the same area [1].
Q2.3 — Prediction after fire at mid-stage (2 marks)
The dotted line should show species diversity dropping back toward the early-succession level then recovering again. Justification: Fire at the mid-stage removes the shrub canopy but leaves the soil seed bank intact, triggering secondary succession. Pioneer and early-succession species re-emerge, temporarily reducing late-successional species richness before the community rebuilds [1 for correct direction on graph; 1 for reasoning linking to secondary succession and seed bank].
Q3 — Diagram critique (6 marks)
3.1 Error 1 (“Primary succession starts after a bushfire”): Primary succession does not start after a bushfire. Correction: Secondary succession starts after a bushfire where soil remains. Primary succession starts on bare substrate with no soil (e.g. new lava, bare rock, fresh sand). [1 + 1]
3.2 Error 2 (“first species to return are fast-growing trees” after fire): The first species to return after a bushfire are fire ephemerals (annual herbs from the soil seed bank) and epicormic resprouts from existing trees — not new colonising trees. Correction: The first colonisers of secondary succession are fire ephemerals germinating from the soil seed bank and vegetative resprouts from surviving root systems. [1 + 1]
3.3 Error 3 (“Secondary succession starts on bare lava; it is faster”): Bare lava with no soil describes primary succession, not secondary. Primary succession is the slower process, not faster. Correction: Primary succession starts on bare lava with no soil. Secondary succession starts where soil is already present, making it faster than primary. [1 + 1]
Q4.1 — Type of succession on Surtsey (2 marks)
Primary succession. The lesson defines primary succession as beginning on bare substrate with no soil and no prior biological community [1]. Surtsey emerged from the sea as bare lava and ash with no soil, no seed bank, and no organisms — exactly the conditions that define primary succession [1].
Q4.2 — Facilitation on Surtsey (3 marks)
On bare lava, there is no organic matter, no usable nitrogen, and no moisture retention. Facilitation requires pioneer species — typically nitrogen-fixing cyanobacteria, lichens, and mosses — to colonise first [1]. As they die and decompose, they add organic matter, creating a thin proto-soil that can retain moisture and support further colonisers [1]. Nitrogen-fixing organisms convert atmospheric nitrogen into usable forms, enriching the developing soil. Only once this organic layer has formed can grasses and herbs obtain the nutrients and rooting medium they need to establish [1].
Q4.3 — Climax community and climate determination (2 marks)
Given Surtsey’s subarctic location (similar latitude to Norway), the climax community would likely be a sub-arctic scrub or heath community, possibly with some birch woodland, characteristic of the regional climate [1]. The climax community is determined by the prevailing regional climate (temperature, rainfall, seasonality) — not the starting substrate. The same bare rock in tropical Queensland would climax as tropical rainforest, not scrub. Both started on bare rock but climate sets the endpoint [1].