For studying the ecology of herbivore-induced plant responses, it is necessary to address natural ecosystems. For instance, in wild radish induced responses to her-bivory increase plant fitness in natural environments (Agrawal 1999). Herbivory increases trichome density and subsequently reduces preference and performance of several herbivores. The effectiveness of induced defenses is clear from a field study on N. attenuata where herbivory was reduced by as much as 90% by the addition of defense compounds (Kessler and Baldwin 2001). In a later study, Kessler et al. (2006) showed that volatiles from damaged sagebrush can prime responses in N. attenuata and reduce herbivore damage to the exposed plants. However, despite the use of natural populations of N. attenuata, the ecological relevance of this study is questionable because the sagebrush that was used for elicitation occurs in a different successional stage than N. attenuata. Furthermore, the effect was only detected at a short range, when the plants grew within 15 cm from each other.
In naturally occurring milkweed populations, early-season herbivory affects subsequent herbivory throughout the season, thereby affecting community structure (Van Zandt and Agrawal 2004). In a natural community, Agrawal (2004) studied the complex interactions between milkweed and competing grass, in presence or absence of root and leaf herbivory. He concluded that the genetic differences, competition, and herbivory resulted in complex interactions that may result in diffuse co-evolution between milkweed and its herbivores. To improve our understanding of the evolution of induced defenses and resistance of herbivores more field studies are needed.
The use of elicitors in the field can provide information on the ecological relevance of pathways and certain steps therein. For example, the application of jasmonic acid to tomato plants in the field increased parasitism of herbivores and thus showed the involvement of jasmonic acid-induced changes in the attraction of carnivores (Thaler 1999). The application of methyl jasmonate to tobacco plants demonstrated the costs of jasmonate-induced responses. In environments with herbivore pressure, induced plants suffered less from herbivore attack and produced more viable seeds than uninduced plants. However, undamaged plants produced more seeds when they were not induced compared to jasmonate-induced plants (Baldwin et al. 1998).
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