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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
Practice this objective

Coverage 2012–2025 · Updated 16 Jul 2026

Interspecific relationships change population outcomes

Interspecific relationships are interactions between different species that change survival, growth or reproduction within a community.

Category Effect Example
Herbivory Herbivore benefits; plant is harmed Caterpillar eats leaf tissue
Predation Predator benefits; prey is killed Owl captures a mouse
Interspecific competition Both lose access to a limited resource Two plant species compete for light
Mutualism Both species benefit Bee gains nectar while pollinating a flower
Parasitism Parasite benefits; host is harmed Tick feeds on a mammal
Pathogenicity Pathogen benefits/reproduces; host is harmed Rust fungus infects a wheat plant

Classify from the mechanism and effects, not from proximity alone. Commensalism is a valid ecological term but is not one of the six categories required for this objective.

Interspecific relationships

Assessment in practice

1–2 marks
How it is assessed

This objective is assessed through essay response, commonly using Identify / Discuss / Describe.

Command terms

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

What earns marks

Interspecific relationships is assessed through predator-prey effects and population regulation, interspecific relationships in communities, capture-mark-release-recapture population estimates, chi-squared testing of species association.

Watch for

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

Representative question

Question 1

[Maximum number: 3]

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

Populations and Communities

  • A population is one species in an area; a community is all interacting populations there.
  • Estimate abundance with unbiased sampling: quadrats for sessile organisms and capture–mark–release–recapture for mobile animals, checking each method’s assumptions.
  • Density-dependent competition, predation, disease and waste create negative feedback around carrying capacity; exponential growth slows into a sigmoid curve as limits strengthen.
  • Classify interspecific relationships by costs and benefits: predation, herbivory, competition, mutualism, parasitism and pathogenicity.
  • Invasive species may escape controls and displace endemic species. Removal experiments can reveal competition and fundamental versus realized niches.
  • Chi-squared tests assess species association from observed and expected quadrat counts.
  • Predator peaks usually lag prey peaks; top-down control begins with consumers, while bottom-up control begins with resources or producers.

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.