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Herbivory is an exploitative interaction between two individuals which produces a positive response for one individual and negatively impacts the other. In comparison to other exploitative interactions, such as predation and parasitism, herbivory differs because it is between plant-eating species (called herbivores) and plant material.

Herbivory is an interaction which can be thought of as an exploitative arms race. An arms race basically means that there is an ongoing competition between plants and non-plant species to be the individual on the positive end of this interaction. For example, the Prickly Pear, Opuntia stricta is a type of cactus which can disperse its seeds easily and grows rapidly. It became a major pest in Australia causing many infestations. In order to control these infestations, Cactoblastis cactorum, a cactus moth was introduced as a bio-control agent and this predator was able to control the numerous infestations of these pest populations to a low rate, preventing this pest from producing rapidly and proliferating. The arms race in this case is between the Prickly Pear pest, sprouting different populations throughout Australia, and the moth species, which takes about a year to come in and reduce the pest to a low frequency.

Since this interaction is exploitative, it causes adaptations to evolve in both the predator and prey species. Plants develop defenses to cause harm to the predator feeding upon them. These defenses can either be:

  1. Constitutive: Defenses which a plant continuously possesses, not just in response to herbivory.
  2. Induced Defenses: Occur in response to an herbivore attack. For example, a plant may increase the concentration of defense toxins it produces after the first round of attacks from a predator.

In return, herbivores have evolved methods to learn which plants contain less toxins and which plants are more nutritious to feed on. For example, humans use their sense of smell to detect whether vegetables have gone bad or are still healthy to consume. 



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