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Herbivory describes the consumption of plants by insects and other animals, and it is another interspecific relationship that affects populations. Unlike animals, most plants cannot outrun predators or use mimicry to hide from hungry animals. Some plants have developed mechanisms to defend against herbivory. Other species have developed mutualistic relationships; for example, herbivory provides a mechanism of seed distribution that aids in plant reproduction.

Defense mechanisms against predation and herbivory

The study of communities must consider evolutionary forces that act on the members of the various populations contained within it. Species are not static, but slowly changing and adapting to their environment by natural selection and other evolutionary forces. Species have evolved numerous mechanisms to escape predation and herbivory. These defenses may be mechanical, chemical, physical, or behavioral.

Mechanical defenses, such as the presence of thorns on plants or the hard shell on turtles, discourage animal predation and herbivory by causing physical pain to the predator or by physically preventing the predator from being able to eat the prey. Chemical defenses are produced by many animals as well as plants, such as the foxglove which is extremely toxic when eaten. [link] shows some organisms’ defenses against predation and herbivory.

Photo (a) shows the long, sharp thorns of a honey locust tree. Photo (b) shows the pink, bell-shaped flowers of a foxglove. Photo (c) shows the pink, bell-shaped flowers of a foxglove. Photo (d) shows a millipede curled into a ball.
The (a) honey locust tree ( Gleditsia triacanthos ) uses thorns, a mechanical defense, against herbivores, while the (b) Florida red-bellied turtle ( Pseudemys nelsoni) uses its shell as a mechanical defense against predators. (c) Foxglove ( Digitalis sp.) uses a chemical defense: toxins produced by the plant can cause nausea, vomiting, hallucinations, convulsions, or death when consumed. (d) The North American millipede ( Narceus americanus ) uses both mechanical and chemical defenses: when threatened, the millipede curls into a defensive ball and produces a noxious substance that irritates eyes and skin. (credit a: modification of work by Huw Williams; credit b: modification of work by “JamieS93”/Flickr; credit c: modification of work by Philip Jägenstedt; credit d: modification of work by Cory Zanker)

Many species use their body shape and coloration to avoid being detected by predators. The tropical walking stick is an insect with the coloration and body shape of a twig which makes it very hard to see when stationary against a background of real twigs ( [link] a ). In another example, the chameleon can change its color to match its surroundings ( [link] b ). Both of these are examples of camouflage    , or avoiding detection by blending in with the background.

Photo (a) shows a green walking stick insect that resembles the stem on which it sits. Photo (b) shows a green chameleon that resembles a leaf.
(a) The tropical walking stick and (b) the chameleon use body shape and/or coloration to prevent detection by predators. (credit a: modification of work by Linda Tanner; credit b: modification of work by Frank Vassen)

Some species use coloration as a way of warning predators that they are not good to eat. For example, the cinnabar moth caterpillar, the fire-bellied toad, and many species of beetle have bright colors that warn of a foul taste, the presence of toxic chemical, and/or the ability to sting or bite, respectively. Predators that ignore this coloration and eat the organisms will experience their unpleasant taste or presence of toxic chemicals and learn not to eat them in the future. This type of defensive mechanism is called aposematic coloration    , or warning coloration ( [link] ).

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Source:  OpenStax, 101-nya-05 - general biology i. OpenStax CNX. Jul 22, 2015 Download for free at http://legacy.cnx.org/content/col11849/1.2
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