QCE Biology - Unit 3 - Functioning ecosystems and succession
Ecological Succession | QCE Biology
Learn primary and secondary succession, pioneer species and changes in abiotic factors, biodiversity and biomass.
Updated 2026-05-18 - 6 min read
QCAA official coverage - Biology 2025 v1.3
Exact syllabus points covered
- Describe the process of ecological succession.
- Distinguish between primary and secondary succession.
- Identify the features of pioneer species that make them effective colonisers.
- Explain successional changes, with reference to species interactions, abiotic factors, K- and r-selected species, biodiversity and biomass.
- Appreciate that the fossil record and sedimentary rock characteristics provide evidence of past ecosystems.
Ecological succession is part of the way QCE Biology turns living systems into evidence students can describe, analyse and evaluate. The safest way to study it is to connect each term to a data pattern, a biological mechanism and a limitation.
Original Sylligence diagram for biology succession.
Core explanation
Succession
Succession is directional change in community composition after a new surface appears or a disturbance changes an existing ecosystem.
Primary succession
Primary succession begins where there is no soil, such as bare rock after lava flow or glacial retreat. Soil must form before larger plants can establish.
Secondary succession
Secondary succession begins after disturbance where soil remains, such as after fire, flood or clearing. It usually proceeds faster because seeds, roots, nutrients and microbes may remain.
Pioneer species
Pioneer species tolerate harsh conditions, disperse well, grow quickly and modify the environment. They can trap soil, add organic matter and make the site suitable for later species.
Succession patterns and evidence
Succession changes both biotic and abiotic conditions. Early communities often have low biomass, simple food webs, high light exposure and strong abiotic stress. Later communities usually have more soil organic matter, greater biomass, more shade, more niches and stronger competition.
| Stage | Typical features | Example organisms or evidence | | --- | --- | --- | | Bare or disturbed surface | Low biomass, few nutrients, harsh abiotic conditions | Bare rock, ash, exposed sand or cleared soil | | Pioneer stage | Rapid colonisers tolerate stress and disperse well | Lichens, mosses, grasses, herbs or algae | | Intermediate stage | Soil, shade and organic matter increase | Shrubs, young trees, increasing decomposer activity | | Later stage | More complex structure and stronger competition | Mature trees, layered vegetation, larger food webs |
Primary succession is slow because soil formation must occur first. Secondary succession is faster because soil, seeds, roots, spores, microbes and nutrients may remain after the disturbance. However, severe fire, erosion or toxic contamination can slow secondary succession by removing those biological legacies.
Succession is not always a neat march toward one fixed climax community. Disturbance frequency, climate, invasive species, grazing, fire regimes and human management can redirect the pathway. A site may remain in an early stage if disturbance repeatedly resets it before later species establish.
Facilitation, inhibition and tolerance
Succession can occur through different mechanisms. In facilitation, early species make conditions more suitable for later species, such as lichens helping form soil or grasses adding organic matter. In inhibition, early species slow the establishment of later species by monopolising light, nutrients or space. In tolerance, later species establish because they can tolerate the conditions created by earlier species and eventually outcompete them.
Disturbance regime is the pattern of disturbance frequency, intensity and type. A low-intensity fire may leave roots, seeds and soil organisms, allowing rapid secondary succession. A high-intensity fire followed by erosion may remove soil and slow recovery. Frequent disturbance can maintain grassland or shrubland even if the climate could support woodland.
Succession evidence can come from direct long-term monitoring or from a chronosequence, where sites of different known ages are compared. Chronosequences are useful but assume that the sites were similar before disturbance and differ mainly in time since disturbance. If soil type, rainfall or grazing differs between sites, the sequence may be misleading.
Restoration ecology often uses succession principles. Adding pioneer plants, stabilising soil, excluding grazing or reintroducing key species can push a degraded ecosystem toward a desired community.
A sere is a sequence of communities that replace each other during succession. Each stage is called a seral stage. The term helps when describing a whole pathway rather than one snapshot, such as bare rock to lichen to moss to grasses to shrubs to woodland.
How to use this in data questions
Start by identifying what has been measured. In Biology, a graph or table is rarely just asking for a trend; it is asking whether you can connect the trend to a process. Quote enough data to show the pattern, then use the concept language from the syllabus. If the evidence is limited, name the limitation precisely: sample size, sampling method, uncontrolled variables, measurement precision, population choice or the time scale of the data.
A useful study habit is to turn each heading into a data prompt. Ask what you would expect to happen if the relevant variable increased, decreased or was removed. For ecology topics, think about abundance, distribution, biodiversity, biomass and carrying capacity. For genetics topics, think about genotype, phenotype, gene expression, allele frequency and inheritance pattern. For evolution topics, think about variation, selection pressure, gene flow, isolation and relatedness.
When a question asks you to evaluate, do not just list problems with the experiment. Link the limitation to the confidence of the conclusion. For example, a small sample size matters because a few unusual individuals can distort the pattern. An uncontrolled abiotic factor matters because it gives another possible explanation for the same biological trend. This is the difference between naming a limitation and using it scientifically.
Worked example
Common exam traps
Other traps to watch for:
- using a general word when a syllabus term is available
- ignoring units, sample size or time scale
- treating a model as a perfect copy of the real ecosystem or cell
- writing a memorised paragraph that does not use the given data