How do communities rebuild after disturbance, and what roles do pioneer and keystone species play?
Topic 2.7 Ecological Succession: distinguish primary and secondary succession, describe how communities change over time, and explain the roles of pioneer, keystone and indicator species.
A focused answer to APES Topic 2.7, covering primary and secondary succession, pioneer species, the path to a climax community, keystone and indicator species, and the effects of succession on biomass and biodiversity, with a worked succession-sequencing question.
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What this topic is asking
The College Board (Topic 2.7) wants you to distinguish primary from secondary succession, describe how a community rebuilds over time toward a climax community, and explain the roles of pioneer, keystone and indicator species. You should also describe how succession changes biomass and biodiversity over time.
What succession is
Succession is how ecosystems recover after disturbance, which ties Topic 2.7 directly to the natural disruptions of Topic 2.5.
Primary and secondary succession
The presence or absence of soil at the start is the key difference, and it explains the difference in speed.
The roles of key species
- Pioneer species: the first organisms to colonize a bare or disturbed area; they tolerate harsh conditions and begin building soil (for example lichens breaking down rock and adding organic matter), making the area habitable for later species.
- Keystone species: species whose effect on the community is large relative to their abundance; their presence shapes which other species can persist (linking back to Topic 1.11).
- Indicator species: species whose presence, absence or abundance reflects the condition of the environment; sensitive indicators (such as lichens for air quality or amphibians for water quality) signal ecosystem health.
How succession changes an ecosystem
As succession proceeds from pioneers toward a climax community, the ecosystem generally accumulates more biomass, develops deeper soil, and supports greater biodiversity and more complex food webs. A typical terrestrial sequence runs lichens and mosses, then grasses and small plants, then shrubs, then fast-growing trees, and finally a mature climax forest, with each stage modifying conditions in ways that favor the next. The climax community is not permanently fixed, because a new disturbance can restart succession, which is exactly why moderate periodic disturbance (Topic 2.5) maintains a mosaic of successional stages and so raises overall biodiversity. Succession therefore ties Unit 2 together: disturbance resets it, adaptations equip species to colonize each stage, and the rising biodiversity it produces supports the ecosystem services of Topic 2.2.
Try this
Q1. Identify whether succession on abandoned farmland is primary or secondary. [1 point]
- Cue. Secondary succession (the soil remains).
Q2. Explain why pioneer species are essential for primary succession. [2 points]
- Cue. They are the only organisms that can colonize bare rock, and by breaking down rock and adding organic matter they build the soil that later species need.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2022 (style)4 marksSection II (FRQ). (a) Distinguish between primary and secondary succession. (b) Describe the role of pioneer species in primary succession. (c) Explain why secondary succession usually proceeds faster than primary succession. (d) Define an indicator species and explain how it can reveal the health of an ecosystem.Show worked answer →
A 4-point FRQ on ecological succession.
(a) Distinguish (1 point): primary succession begins on bare substrate with no soil (for example new volcanic rock or land exposed by a retreating glacier); secondary succession begins where a disturbance has removed the community but the soil remains (for example after a fire or abandoned farmland).
(b) Describe (1 point): pioneer species (such as lichens and mosses) are the first colonizers; they tolerate harsh conditions and begin building soil by breaking down rock and adding organic matter, making the area habitable for later species.
(c) Explain (1 point): secondary succession is faster because soil, nutrients and often seeds and roots already remain, so plants re-establish quickly without having to build soil from bare rock.
(d) Define (1 point): an indicator species is one whose presence, absence or abundance reflects the condition of the environment; a decline in a pollution-sensitive indicator signals that the ecosystem is degraded.
Markers reward the soil-versus-no-soil distinction, the soil-building role of pioneers, the faster pace of secondary succession due to remaining soil, and the indicator-species definition with an example use.
AP 2020 (style)1 marksSection I (multiple choice). Which sequence correctly represents primary succession on bare rock? (A) Grasses to lichens to shrubs to trees (B) Lichens to mosses to grasses to shrubs to trees (C) Trees to shrubs to grasses to lichens (D) Shrubs to grasses to lichens to mosses. Justify your choice.Show worked answer →
A 1-point MCQ on the succession sequence. The answer is (B).
Primary succession begins with pioneer species such as lichens and mosses that colonize bare rock and build soil, followed by grasses, then shrubs, then trees as the soil deepens and conditions improve toward a climax community. Option (A) starts with grasses before soil exists; (C) and (D) reverse the order. The trap is forgetting that lichens and mosses, not grasses or trees, are the first colonizers of bare rock.
Related dot points
- Topic 2.5 Natural Disruptions to Ecosystems: describe natural disruptions to ecosystems and explain their short-term and long-term effects on populations and biodiversity.
A focused answer to APES Topic 2.5, covering periodic, episodic and random natural disruptions, fire, drought, storms, volcanism, plate tectonics and climate change, their short- and long-term effects, and ecosystem recovery, with a worked disturbance-analysis question.
- Topic 2.1 Introduction to Biodiversity: describe the three levels of biodiversity and explain how genetic and species diversity contribute to ecosystem resilience.
A focused answer to APES Topic 2.1, covering genetic, species and habitat diversity, species richness and evenness, the value of genetic diversity, bottlenecks and resilience, with a worked diversity-comparison question.
- Topic 1.11 Food Chains and Food Webs: describe how food chains and food webs represent the flow of energy and matter, and predict the effects of changes to a food web.
A focused answer to APES Topic 1.11, covering food chains and food webs, how energy and matter flow through them, keystone species, trophic cascades, and predicting the effects of removing a species, with a worked food-web disruption question.
- Topic 2.6 Adaptations: explain how natural selection produces adaptations and how environmental change shifts which traits are favored over time.
A focused answer to APES Topic 2.6, covering adaptations, natural selection, the role of genetic variation, structural, physiological and behavioral adaptations, specialists and generalists, and how environmental change drives evolution, with a worked selection question.
- Topic 2.2 Ecosystem Services: describe the four categories of ecosystem services and explain how the disruption of ecosystems affects the services they provide.
A focused answer to APES Topic 2.2, covering provisioning, regulating, cultural and supporting ecosystem services, examples of each, their economic value, and how disruption reduces them, with a worked valuation question.
Sources & how we know this
- AP Environmental Science Course and Exam Description — College Board (2020)