Amensalism

Amensalism is an asymmetric interaction between two species in which one is harmed or killed while the other is unaffected, one of the six classic categories of interspecific interaction defined by how each species' fitness is affected. It can take two forms: competition, in which a larger or stronger organism deprives a weaker one of a shared resource such as sunlight, water or nutrients, and antagonism, in which one organism damages or kills the other by releasing a chemical. A mature tree that shades out a sapling and depletes the soil around it while remaining unaffected itself is a competitive example, as is the black walnut tree, which secretes a toxin called juglone that kills many neighboring herbaceous plants; the Spanish ibex, which eats vegetation that weevils depend on and so harms weevil populations while remaining largely unaffected itself, is another documented case. Amensalism differs from mutualism, where both species benefit, and from parasitism, where one benefits directly at the other's expense, because the organism causing the harm gains nothing from the interaction.