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Plants have evolved adaptations against herbivores for over 420 million years. Modern adaptations include avoidance, tolerance, summoning natural enemies, and producing toxins. Examples include poison ivy, foxgloves, and roses. Some plant chemicals have psychoactive effects, such as marijuana, caffeine, and opium.
Plants have evolved adaptations against herbivores as long as herbivores have existed, since 420 million years ago (in the Late Silurian) or even earlier. The earliest land plant fossils date from between 510 and 450 million years ago, during the Late Cambrian and Ordovician. The earliest fossil herbivores (millipedes) date to 428 million years ago, during the mid-Silurian, so there may have been a period during which plants could exist without evolving adaptations against herbivores by animals, although they were probably prey of fungi, bacteria, archaea and microorganisms during this period.
Modern plant adaptations against herbivores fall into four main categories: avoiding or repelling herbivores (antixenosis), tolerating the herbivore and quickly regenerating or causing the herbivore to devour non-essential parts, and summoning natural enemies of the herbivores in question , or direct comparison; produce toxins to kill the herbivore or reduce its digestibility (antibiosis). Since insects have been the most important herbivores throughout evolutionary history, most plant defenses are against them, although some plant adaptations against herbivores focus on vertebrate herbivores such as birds or mammals.
Consider three obvious examples of plant adaptations to herbivores. There is poison ivy, which produces urushiol oil, which causes contact dermatitis in many animals, including humans. A brush with poison ivy and itchy hours later, the plant is likely to be left alone. Another of the adaptations of plants against herbivores is demonstrated by the beautiful purple foxgloves, which produce numerous deadly chemicals. Even a small bite of the stems at the top of a foxglove is enough to cause nausea, vomiting, wild hallucinations, diarrhea, abdominal pain, delirium, severe headache, and death. A milder example of plant adaptations against herbivores is demonstrated by roses, which are protected from herbivores by thorns.
Some plants produce chemicals intended to defend against herbivores that are largely harmless to humans, but cause interesting psychoactive effects. This includes marijuana, which has been smoked by humans since prehistoric times, caffeine, which is consumed daily by 90% of adults in North America, and opium, which is used to make morphine, an important pain reliever.
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