Plant Defense Against Predators

Biology Strategic Defense in the Plant Kingdom The purpose of this literature review is to detail the different ways plants protect themselves from predators. Knowledge of plant defenses can help boost understanding of more effective means of minimizing pesticide and herbicide use in agriculture or alternatively, to help develop more effective and targeted chemical pesticides and herbicides. Understanding plant defenses requires knowledge of plant biology and their role in their respective ecosystems. Moreover, their defenses ensure the survival not only of individual plants, but of whole ecosystems.

Plants have no immune system, in the same way animals do (Freeman & Beattie, 2008). Instead, they boast "a stunning array of structural, chemical, and protein-based defenses designed to detect invading organisms and stop them before they are able to cause extensive damage," (p. 1). The defenses plants have evolved to ward off predators can be loosely grouped into three categories: surface-based protections (morphological protections such as thorns), polymers and other substances that diminish digestibility (molecular mechanisms, such as the development of indigestible leaves), and actual toxins (biochemical poisons).

Additionally, plants can also attract species that prey on herbivores in an indirect form of self-protection ("Plant Defense Against Being Eaten," n.d.; War, et al. 2012). Many of these types of protection work congruously and simultaneously, allowing plants to develop a systematic strategic defense plan suitable for their environment. The production and release of volatiles epitomizes the ways plants use a number of different protection mechanisms at the same time. Plants release more volatiles when they are under attack by herbivorous insects (Pare & Tumlinson, 1999).

At the same time, many airborne volatiles attract other insect species that prey on the herbivores. Pare & Tumlinson (1999) call the process of attracting predators a "distress call," because the plant will only release the volatile compounds when they are under attack by herbivorous insects. The volatile compounds are airborne. Another example of how plants combine the effects of two or more defenses is with the use of polymers plus poisons.

Polymers on their own form a cohesive defensive front for plants, because this group of substances includes cellulose, lignin, tannins, and silicates (Schardl, 2002). Polymers minimize the digestibility and/or nutritive content of the plant's materials. Yet by combining polymers with poison, plants launch a particularly sneaky attack on herbivorous insects. The insects are forced to eat more of the plant because of the polymers, but the plant simultaneously protects itself by releasing toxins (Schardl, 2002).

Because insects will evolve methods of adapting to plant defense mechanisms, such as immunity to toxins or the ability to eat through polymers, plants must respond in turn with more robust defenses. Moreover, many of the defense mechanisms in plants work in a targeted manner. Carnivorous plants like the Venus flytrap emit resins that can trap and kill herbivores. Some compounds cause digestive upset in grazing animals, prompting those animals to seek alternative sources of food. Plant defenses are essential for life on earth.

Without their defenses, plants would be "driven to extinction, with the consequent extinction of myriad herbivores, predators, parasites, and saprotrophs," (Schardl, 2002, p. 1). Plants develop methods of defense that target the immediate predators in their ecosystem, and if those ecosystems change, so too will the nature.