ABSTRACT
Terrestrial herbivorous insects alone represent about 25% of all animal and plant species,1 and many other organisms (invertebrate and vertebrate groups in marine, aquatic, and terrestrial habitats) have herbivorous lifestyles as well. The large impact
of plants on herbivores seems clear, but the impact of herbivores on plants (especially at plant distribution and community levels) has not been studied as extensively.2 The impact of herbivory on plants has been a controversial topic,3 proponents of the main view suggesting that herbivory is detrimental to plants while others maintain that herbivory generally has minor effects compared to abiotic factors, no impact, or even benefits plants.4-6
It is clear that herbivory has exerted a strong evolutionary impact affecting plant evolution over time, so that how we see plants today is at least partly due to herbivory. For example, plant chemicals have been suggested to drive the coevolution of plant-insect interactions,7 either as pairwise8 or diffuse9, 10 coevolutionary plantherbivore interactions. Several lines of evidence suggest that herbivory has not generally had a favorable impact on plant fitness. This evidence includes the widespread presence of secondary plant defensive compounds, plant morphological defensive structures, and induced plant defenses, all potentially defensive mechanisms against herbivores.11-16
In addition, considering plants and herbivores in the context of tritrophic interactions with primary predators, plant responses can interact with the effects of predators (and parasitoids) in several ways to affect herbivores.17-18 Plants may have lower nutritional quality to delay herbivore development so that predators have more time to consume herbivores.17 It also seems that plants often provide predators more effective physical access to herbivores,18 and plants will sometimes release compounds following herbivory that attract predators of the herbivores.17-20
Many of the authors who have written about the impact of herbivory on plants work with plants in temperate, seasonal environments. Coley and Barone16 offer a different perspective, suggesting that herbivory pressure is most intense when herbivores and plants have a continuous, year-round association. In support of this idea, tropical plants tend to be more chemically, mechanically, and/or spatially defended than temperate plants.16 Also, the pattern of resource investment to plant defenses tends to differ between seasonal and aseasonal plant species. Because leaves often remain on aseasonal plants longer than on seasonal plants, leaves appear to be more valuable to aseasonal plants. These data suggest that the degree of plant defenses tends to increase as the severity of herbivory pressure increases. This in turn suggests that many plants “resist” herbivory injury not only because of a general negative evolutionary impact of herbivory on plant fitness, but also because of the current negative effects of herbivory on many plants.2, 16
The impact of herbivory on plants has importance for both applied and theoretical reasons. Farmers, plant breeders, and scientists need to understand how herbivores affect plants if they are to maximize crop yields, while selecting plants maximally resistant to herbivores and thus suffering reduced yield losses. Useful insights can be obtained by reviewing how plants respond to herbivory injury by drawing on studies from agricultural systems as well as basic ecology studies. In natural systems, i.e., environments without human interference, plants are subjected to a broad array of biotic and abiotic stresses. Compared to standard agricultural settings, natural systems are diverse, with high genetic variation. Also, natural systems may experience greater degrees of abiotic stresses than agricultural systems. These
conditions of stress are themselves unfavorable to plants and may compound with insect injury,21 in space and time.22
