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C4.1.11—Interspecific relationships

Interspecific relationships explains how ecological evidence links organisms, resources and interactions to population size, distribution or community structure in a habitat.

Syllabus
First assessment 2025
Objective
C4.1.11
Level
HL

Exam analysis

Chance of appearing10%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper3
Typical marks1–2

Common command terms

  • Identify
  • Discuss
  • Describe
  • State
  • Suggest
  • Predict
  • Deduce
  • Evaluate

Scoring notes

Common mistake
Confusing a population with a community or with all organisms in an ecosystem.

Recent exam appearances

May 2025Paper2 ["HL"] · TZ35(b)[ 2 ]C4.1.11—Interspecific relationships
November 2024Paper3 ["HL"] · TZ016(b)[ 3 ]C4.1.11—Interspecific relationships
May 2024Paper2 ["HL"] · TZ11(j)[ 1 ]C4.1.11—Interspecific relationships
May 2024Paper2 ["HL"] · TZ11(i)[ 1 ]C4.1.11—Interspecific relationships
May 2024Paper2 ["HL"] · TZ11(c)[ 1 ]C4.1.11—Interspecific relationships

Coverage 2012–2025 · Updated 16 Jul 2026

Interaction Signs Classify Effects on Two Species

Relationship Species A Species B Defining transfer or effect
mutualism + + each gains a resource or service
competition − − both lose access or spend resources contesting
predation + − predator kills and consumes prey
herbivory + − animal consumes plant or algal tissue
parasitism + − parasite gains resources while living on or in host
pathogenicity + − pathogen reproduces while damaging host

The signs describe effects relative to no interaction. They do not mean every individual receives the same benefit or cost, and the outcome can change with conditions.

Examples classify mutualism as positive-positive, competition as negative-negative, and predation, herbivory, parasitism and pathogenicity as positive-negative interactions.

Explain Community Change with Signs, Mechanism and Evidence

Question What a strong explanation identifies
Who interacts? populations and whether each gains or loses
What is transferred or limited? food, habitat, nutrients, protection, disease or a chemical
Where does change begin? individual pair, predator level, producer/resource level or introduced species
How does it propagate? competition, feedback, trophic cascade or niche restriction
What supports causation? manipulation, control, replication and a predicted response
What supports association? representative sampling, expected counts and a statistical decision

Use observations to find a pattern, statistics to judge whether it is unlikely under a null model, and experiments or field manipulations to test the proposed mechanism.

Community structure emerges from many population interactions. Naming a relationship is only the start; explanation requires its mechanism, direction of effect, feedback through the network and the limits of the evidence.

Interspecific relationships

[ 3 ]

Evaluate whether crop yield may be improved through enhancement of pollinator richness.

Concept essentials

  • Interspecific relationships must be linked to the correct ecological unit, method or species interaction.
  • The evidence for interspecific relationships depends on measurable abundance, distribution, survival or resource patterns.
  • Interspecific relationships is clearest when cause, ecological process and population outcome are kept distinct.
  • Field or graph evidence for interspecific relationships needs biological interpretation, not values alone.