CoachAn interaction between two species is classified by its effect on each: (benefit), (harm) or . Competition (): both use a resource in short supply. Predation (): one kills and eats the other; herbivory () when the eaten is a plant and usually survives; parasitism () when the exploiter lives on or in a host it does not kill at once. Mutualism (): both benefit. Commensalism (): one benefits, the other is unaffected. Amensalism (): one is harmed, the other unaffected. Every interaction is a selective pressure on both partners, and their reciprocal adaptation over generations is coevolution.
Examples
Example 27.2 (One tree, all six)
An oak competes with its neighbours for light; its acorns are eaten by jays (predation) and its leaves by caterpillars (herbivory); mistletoe draws its sap (parasitism); its roots exchange sugars for phosphate with a fungus (mutualism); ferns grow on its bark, using it only as a perch (commensalism); and its shade kills the grass beneath it (amensalism).