QCE Biology - Unit 3 - Functioning ecosystems and succession
Species Interactions, Niches and Keystone Species | QCE Biology
Learn predation, competition, mutualism, commensalism, parasitism, niches, competitive exclusion and keystone species.
Updated 2026-05-18 - 6 min read
QCAA official coverage - Biology 2025 v1.3
Exact syllabus points covered
- Explain the following species interactions: predation, competition, mutualism, commensalism and parasitism.
- Describe the concept of an ecological niche.
- Explain the competitive exclusion principle.
- Explain the critical role that keystone species play in maintaining the structure of a community.
- Analyse ecological data (e.g. food webs, population data) to identify keystone species.
- Analyse ecological data (e.g. food webs, population data) to infer species interactions.
- Analyse ecological data (e.g. food webs, population data) to predict the outcomes of removing species from an ecosystem.
- Appreciate that some biologists have advocated for keystone species to be special targets for conservation efforts and keystone species theory has informed many conservation strategies; however, there are differing views about the effectiveness of single-species conservation in maintaining complex ecosystem dynamics.
Species interactions, niches and keystone species 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 species interactions.
Core explanation
Interaction types
Predation benefits the predator and harms prey. Competition harms both competitors by reducing access to resources. Mutualism benefits both species, commensalism benefits one with little effect on the other, and parasitism benefits the parasite while harming the host.
Ecological niche
A niche is the role and resource use of a species: where it lives, what it eats, when it is active and how it interacts with other species.
Competitive exclusion
If two species occupy the same niche and compete for the same limiting resource, one is likely to outcompete the other unless the niche is partitioned.
Keystone species
A keystone species has a larger effect on community structure than its abundance alone would suggest. Removing it can trigger trophic cascades or major changes in biodiversity.
Interaction signs and symbiosis
The effect of an interaction can be written as signs for species A and species B. This helps avoid mixing up similar-sounding relationships.
| Interaction | Effect | Example-style cue | | --- | --- | --- | | Predation | + / - | One organism kills and eats another | | Herbivory | + / - | Consumer eats plant or algae tissue | | Parasitism | + / - | Parasite benefits while host is harmed but usually not killed immediately | | Competition | - / - | Both lose access to a limiting resource | | Mutualism | + / + | Both species increase survival or reproduction | | Commensalism | + / 0 | One benefits, the other has no measurable effect | | Amensalism | - / 0 | One is harmed, the other is unaffected | | Neutralism | 0 / 0 | Species coexist with no measurable direct effect |
Symbiosis means a close, long-term association between species. Mutualism, commensalism and parasitism can all be symbiotic if the relationship is close and persistent. Predation is usually brief and is not normally described as symbiosis.
Fundamental and realised niches
The fundamental niche is the full set of conditions a species could occupy if interactions did not restrict it. The realised niche is the part it actually occupies after competition, predation, disease and mutualisms are included. Resource partitioning can reduce competition when species use different food sizes, different microhabitats or different times of activity.
Cascades and indirect interactions
Species can affect each other indirectly through food webs. If a predator suppresses herbivores, plants may increase even though the predator does not interact with the plants directly. This is a trophic cascade. Keystone species are often recognised through these indirect effects because their removal changes many other populations.
Competition can be interspecific, between different species, or intraspecific, within the same species. Intraspecific competition can be especially strong because individuals need almost identical resources. Interspecific competition can lead to competitive exclusion, but it can also lead to resource partitioning if species use different prey sizes, feeding times, root depths or microhabitats.
The realised niche can shrink when a competitor, predator or parasite is present. It can expand if that interaction is removed. This is why field distribution does not always show the full range of conditions a species could tolerate in isolation.
When classifying interactions, focus on fitness effects: survival, growth or reproductive output. A relationship that looks harmless may still be negative if it reduces reproduction, and a relationship that looks costly may be beneficial if it increases offspring survival.
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