Plants and Herbivores

Life is a constant oscillation between the sharp horns of dilemmas. 

Perhaps, the most basic case of exploitation is herbivory. Herbivores consume live plant material but do not usually kill the plants they feed on. According to the food pyramid (see Section 5.5.4), 

Nicotine is an alkaloid found in tobacco plants that have insecticidal properties. It is likely that nicotine is produced by tobacco to defend against insects that would otherwise feed on the plant. 

Allelopathy is the term used for plants that produce chemical compounds harmful to nearby competitor plants. Such plants have a growth and reproduction advantage without competition for resources. Two plants exhibiting allelopathy are black walnut trees and canola (rape). 

The herbivorous insect Helicopsyche borealis inhabits streams across most of North America (Molles, 1999). Larval Helicopsyche graze on algae and bacteria that grow on exposed surfaces of submerged stones. As the larvae grow through the summer and fall, they attain densities of over 4000 individuals per square meter and represent 25% of the total biomass of benthic animals (those animals living in water). At that density, Helicopsyche do not only reduce their food supply, but also deplete it. 

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Dowd, P. F. In Multi trophic Interactions Among Microorganisms, Plants, and Herbivores Barbosa, P. Krischik, V. A. Jones, C. G. Eds. Wiley N York, in press. 

Scrlber, J.M. In "Variable Plants and Herbivores In Natural and Managed Systems" Denno, R.F. McClure, M.S., Eds. Academic Press New York 1983 pp. 373-412. 

Our objective Is to examine some aspects of current plant herbivore theory using Douglas-flr (Pseudotsuga menzlesll) and western spruce budworm (Chorlstoneura occldentalls). Both plant and herbivore are widespread In western North America. Natural hosts of the budworm Include Douglas-flr, species of Abies, and, on occasion, other conifers (9). Variation In budworm density occurs on both a geographic and local scale. We have frequently observed differential defoliation In trees having overlapping crowns at sites In Montana, Idaho, and New Mexico. 

In undisturbed ecosystems, plant and herbivorous Insect populations coexist In a steady state condition dictated by external biological and physical factors (parasites, predators, precipitation, temperature, soil quality, etc.). But this balance is also regulated by myriads of very fundamental Interactions, many or even most of which are yet unknown, between the plant allelochemi-cals and the biochemical, physiological, and behavioral functions of the Insect herbivores ( 1). 

These examples show that several plants possess the ability to raise extrafloral nectar production in response to herbivory, but this induction is not necessarily universal and might vary depending on both plant and herbivore species (Table 2.2). Koptur (1989) could not demonstrate an effect of mechanical defoliation on extrafloral nectar production in Ipomoea carnea, Inga brenesii and Inga punctata. In... 

Denno, R. F. and McClure, M. S., Eds., Variable plants and herbivores in natural and managed systems, Academic Press, New York, 1983. 

Terpenes and phlorotannins are two structural classes of secondary metabolites that are particularly important in mediating chemical interactions between marine plants and herbivores. Terpenes are commonly found in tropical macroalgae such as members of the Dictyotales (Phaeophyta) and... 

In general, we find a series of related compounds in each plant often a few major metabolites and several minor components which differ in the position of their substituents (Fig. 9.4). Sparteine and lupanine only differ by a single keto function, but their modes of action differ substantially (Fig. 9.2). The profile usually varies between plant organs, within developmental periods, and sometimes even diurnally (e g., in lupin alkaloids60). Also marked differences can usually be seen between individual plants of a single population, even more so between members of different populations. This variation, that is part of the apparent evolutionary arms race between plants and herbivores, makes adaptations by herbivores more difficult, since even small changes in chemistry can be the base for new pharmacological activities. 

The production of toxins is only one aspect of plant defense strategy. As a result of the persistent battle of plants and herbivores, many optimized phenotypes have evolved, such as the preferential accumulation of alkaloids in tissues with a pattern that is consistent with predictions of optimal defense theory,65 i.e., the defense metabolites are allocated preferentially to tissues with a high probability of attack.66 The inducibility of pathways leading to plant secondary compounds as a strategy to minimize the costs of plant defense is a result of permanent optimization. One of a few examples of inducible alkaloid biosynthesis is the different Nicotiana species that exhibit dramatic wound-induced increases of nicotine, nomicotine, or anabasine.67... 

S. B. Malcolm, Cardenolide-Mediated Interactions between Plants and Herbivores. In Herbivores, 2nd ed. G. A. Rosenthal,... 

Takabayashi J, Dicke M, Posthumus MA. Variation in composition of predator-attracting allelochemicals emitted by herbivore-infested plants relative influence of plant and herbivore. Chemoecol-ogy 1991 2 1-6. 

Olsen P. E. (1993) The terrestrial plant and herbivore arms race a major control of Phanerozoic CO2 Abstr. Prog. Geol. Soc. Am. 25(3), 71. 

Benson WW, Brown KS, Gilbert LE. Coevolution of plants and herbivores Passion flower butterflies. Evolution 1976 29 659-80. 

Hare, J. D. 1983. Manipulation of host suitability for herbivore management, pp. 655-680. In R. F. Denno and M. S. McClure (eds,). Variable Plants and Herbivores in Natural and Managed Systems. Academic Press, NY. 

Variability occurs in both plant and herbivore populations. As a particular plant phenotype increases in numbers, any phenotypes of interacting herbivores which exist or arise and are capable of utilizing the plants will be favored also. 

Bowers, M. D., Chemistry and coevolution Iridoid glycosides, plants, and herbivorous insects, in Chemical Mediation of Coevolution (K. C. Spencer, ed.), 133-165, Academic Press, San Diego, CA, 1988. 

Malcolm, S. B., Cardenolide-mediated interactions between plants and herbivores, in Herbivores Their Interactions with Secondary Plant Metabolites, Vol. 1 (G. A. Rosenthal and M. R. Berenbaum, eds.), 251-296, Academic Press, San Diego, CA, 1991. 

Scriber, J. M. (1983) The evolution of feeding specialization, physiological efficiency and host plant races in selected Papilionidae and Saturniidae. In Variable Plants and Herbivores in Natural and Managed Systems (Denno, R. F. and McClure, M. S., eds) pp. 373-412. Academic Press, New York (in press). 

Benson, W. W., Brown, K. S. and Gilbert, L. E. (1975) Co-evolution of plants and herbivores passion flower butterflies. Evolutiony 29, 659-80. 

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