Allelopathy and secretion of antibioticsSpec C4.1.18
In short
Allelopathy is the release of chemicals by a plant that inhibit the germination or growth of other plants nearby, such as juglone from black walnut. Secretion of antibiotics is the release by microorganisms, such as Penicillium fungi, of chemicals that kill or inhibit bacteria. Both release a substance into the environment to deter potential competitors.
Both processes are a form of chemical competition: an organism releases a substance into its surroundings that harms competitors, giving it more access to resources such as light, water, mineral ions or nutrients.
| Allelopathy | Secretion of antibiotics | |
|---|---|---|
| Organisms | Plants | Microorganisms (fungi and bacteria) |
| Effect | Inhibits germination or growth of nearby plants | Kills or inhibits the growth of bacteria |
| Example | Black walnut trees (Juglans nigra) release juglone from roots, leaves and fallen husks, which inhibits the growth of many plants beneath them, such as tomatoes | The mould Penicillium secretes penicillin, which stops bacteria building cell walls, so they burst; soil Streptomyces bacteria secrete streptomycin |
Local example (Arabian Gulf): extracts of leaves of the invasive mesquite, Prosopis juliflora, reduce the germination or root growth of several native plants in laboratory tests, including the ghaf tree, and fewer native annuals grow beneath mesquite in the UAE. In the field it is hard to separate this chemical effect from shading and competition for water, so black walnut is the clearer example to learn.
Make the link to competition explicit: the chemical reduces the growth of competitors, so the producer gains more of a limited resource.
Quick check
What is the Lincoln index formula?
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Population size estimate = (M × N) ÷ R: M marked first, N caught second time, R marked in the second sample.
Why must quadrats be placed randomly?
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To avoid bias, so that every part of the area has an equal chance of being sampled.
Give three density-dependent factors.
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Competition for limited resources, predation, and the transfer of pathogens or pests.
What benefit do zooxanthellae get from hard corals?
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Carbon dioxide and nitrogen compounds from the coral, and a protected, well-lit position.
What is the critical value of χ² for a 2 × 2 table at p = 0.05?
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3.84 (1 degree of freedom).
Written and checked against the IB Biology SL specification · Updated October 2026