Amensalism

Amensalism is a type of biological interaction in which one species is inhibited or destroyed by another, while the inhibiting species remains unaffected by the presence of the harmed species. It is an asymmetrical interaction in which the impact is unidirectional: one organism experiences a negative effect, and the other experiences no direct benefit or harm.

Common mechanisms of amensalism include chemical inhibition and physical interference. In plants, allelopathy is a well-known chemical form, where compounds released by one species suppress the growth of neighboring plants; a classic example is juglone produced by black walnut trees that can inhibit many surrounding species. In microbial communities, producers of antibiotics or antimicrobial compounds can suppress competitors without gaining a direct benefit themselves. Physical effects, such as shading or habitat modification by a tall plant, can also harm understory species while the tall plant remains unaffected.

In practice, amensalistic relationships are sometimes difficult to demonstrate conclusively in nature, because interactions are often context dependent and influenced by environmental conditions. They are related to, but distinct from, parasitism and predation, which involve direct benefit to the actor or a direct transfer of resources. Some researchers view amensalism as a special case of interference competition or as a component of more complex interaction networks, where asymmetrical effects may vary over time and space.