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Amensalism is a type of biological interaction where one species causes harm to another organism without any cost or benefits to itself. It can be seen as a form of interaction or competitive behaviour among other organisms.

Modes of Amensalism

There are two modes of amensalism:

  • Competition: A larger, physically stronger organism deprives a smaller, weaker organism of food or space.
  • Antibiosis: An organism is either damaged or killed by a chemical secretion of another organism.

Example of Amensalism

When cattle trample on grass, the grass is crushed. However, the cattle do not benefit from this action nor is harmed in the process.

Another example of Amensalism is when an organism such as a goat feeds on the same type of shrub as an insect (such as a beetle). The goat is unharmed when it consumes the shrub, however, the beetle loses significant quantities of food and may accidentally be eaten by the goat. This mode of amensalism is called competition

An example of antibiosis is the interaction between Penicillium and bacteria. The mould Penicillium creates the secretion known as penicillin, which is extremely toxic to bacteria. This finding formed the basis for the first true antibiotic – called penicillin.

The classic demonstration of antibiosis is the destructive effect that the bread mould Penicillium has upon certain bacteria; the secretion, known as penicillin, has become a potent medicine in combating bacterial infections. Some higher plants secrete substances that inhibit the growth of—or kill outright—nearby competing plants. An example is the black walnut (Juglans nigra), which secretes juglone, a substance that destroys many herbaceous plants within its root zone.

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