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C4.1.18—Allelopathy and antibiotics

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

Syllabus
First assessment 2025
Objective
C4.1.18
Level
SL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper1A
Typical marks1

Common command terms

  • Explain
  • Identify

Scoring notes

Common mistake
Including abiotic factors in the definition of community instead of reserving them for ecosystem.

Recent exam appearances

May 2025Paper1A ["SL"] · TZ127[ 1 ]C4.1.18—Allelopathy and antibiotics
Practice this objective

Coverage 2025–2025 · Updated 16 Jul 2026

Allelopathy and antibiotics are chemical interference

Allelopathy and antibiotic secretion both release chemicals into the environment that deter potential competitors.

Process Specific example Producer and target effect
Allelopathy Juglone released by black walnut Inhibits germination or growth of susceptible neighbouring plants
Antibiotic secretion Penicillin from Penicillium fungus Inhibits susceptible bacteria competing in the same environment

The ecological outcome depends on chemical concentration, transport, breakdown and target susceptibility; secretion can improve the producer's access to space or resources.

A laboratory inhibition zone shows chemical activity but does not by itself prove an ecologically important field concentration. Antibiotic secretion here is competition between organisms, not clinical prescribing.

Allelopathy and antibiotics

Assessment in practice

1–3 marks
How it is assessed

This objective is assessed through structured response, commonly using Explain / Identify.

Command terms

Explain / Identify

What earns marks

Build the answer around this relationship: Allelopathy and antibiotics must be linked to the correct ecological unit, method or species interaction.

Watch for

Including abiotic factors in the definition of community instead of reserving them for ecosystem.

Representative question

Question 1

[Maximum number: 3]

Explain how a named plant can reduce competition by allelopathy.

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

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