Q.Name the association in which one species produces poisonous substance or a change in environmental conditions that is harmful to another species.

This is a specific type of negative interaction where one organism actively makes the surroundings hostile for another. The key is that the harming species does not directly attack or consume the other; instead, it alters the environment in a way that is toxic or unfavourable.

A well-known real-world example (not one the NCERT chapter itself gives -- it defines amensalism only via its interactions table, with no worked example) is the weed Parthenium (congress grass) and the tree Eucalyptus, both of which release chemicals into the soil that inhibit the germination and growth of other plants around them. …

In nature, no species lives in complete isolation. Every organism constantly interacts with others — sometimes to its benefit, sometimes to its harm, and often in ways that are indirect but powerful. These interactions shape populations, communities, and entire ecosystems. Among the many types of ecological relationships, one of the most one-sided is amensalism.

Amensalism is a type of negative interaction where one species is harmed or inhibited, while the other species remains completely unaffected — neither helped nor hurt. The harm comes not from direct attack or competition, but from the release of a toxic chemical or a change in the physical environment caused by the first species. A well-known example (though not one the NCERT chapter itself illustrates -- it names amensalism only in its interactions table, with no worked example) is the mould Penicillium, which secretes the antibiotic penicillin and kills surrounding bacteria. The mould gains nothing from this killing — it simply produces the substance as a by-product of its own metabolism. The bacteria, however, are destroyed. That is amensalism in action.

Another well-known case, though again not one the NCERT chapter itself describes, is the black walnut tree (Juglans nigra). Its roots release a chemical called juglone into the soil. This substance is toxic to many other plants — tomato, potato, alfalfa, and pine trees — that try to grow nearby. The walnut tree simply grows as it always does, but the soil around it becomes a death trap for sensitive species. Again, the walnut is neither helped nor harmed by this; it is just a side effect of its own chemistry.

Separate the cause (a toxic secretion or environmental change produced by one species) from the effect (harm to a bystander species that gains nothing from causing it) …