Allomones are chemicals which give adaptive advantage to the organism that produces the chemical. Examples include repellents, which provide a defense against attack, such as the odour of a skunk; escape substances, such as the ink that is used an octopus to confuse predators; venoms and toxins, as would be found in snakes and scorpions; and attractants, which are used by a predator to attract a prey, such as those used by carnivorous plants and fungi or flower scents used to attract pollen carriers.
Kairomones are chemicals that give adaptive advantage to the receiving organism. These include chemicals given off by food sources, such as chemical cues used by parasites or predators to find their victims; the chemical released by nematode worms, which stimulates certain fungi to develop traps for these worms; signals that warn of danger or toxicity to the receiver, such as the odour of a cat smelled by a mouse.
Depressants are a class of ailelochemicals that inhibit opposition the receiver without benefit to the releasing organism, such as some bacterial and fungal toxins. Depressants may be inorganic chemicals such as acids or bases which are produced by pioneering organisms and which reduce the competition for nutrients, light, and space, between the mensal species and inhibitor species.
Depressants may be simple organic toxins which inhibit growth of seedlings of other species in the vicinity. Finally, depressants may be antibiotics, toxic substances produced by an organism which in low concentrations, can inhibit or kill the growth of another organism.
The majority of inhibiting chemicals are produced as secondary substances by plants and released into the soil through the roots or leaf wash. The suppression of growth through the release of chemicals by a higher plant is known as allelopathy. For example, walnut tree produces a non-toxic substance, juglone (Bode, 1958) which is found in its leaves, fruits an other tissues.
When the leaves or fruits fall on the ground, juglone is released to soil, where it is oxidized to a substance that inhibits the growth of certain under story species and garden plants such as heaths and broad-leaf herbs, and favors others such as bluegrass and blackberries.
Antibiotics produced by bacteria, fungi, actinomycetes, and lichens are widespread in nature and may be one of the reasons why bacteria pathogenic to man cannot multiply well in soils. A number of antibiotics such as penicillin have been used extensively in human medicine.