Biotic Stress and Yield Loss

important consequences to plant fitness, while chronic effects of herbivory may takeyears to detect. Finally, herbivores that feed on annuals, biennials, and long-livedplants, along a gradient may have temporary to continuous pressure on their planthost(s). Thus, the present selection pressure that herbivores impose on plants variesgreatly, 61, 66, 135 and plants will evolve only when they possess additive genetic variationin traits that affect plant fitness.Just because herbivory injury affects individual plant performance does notnecessarily mean that injury affects plant population dynamics like recruitment, density,distribution, and community structure of plant species. High seed predation canlimit plant populations, 71, 143 but effects on seedling or adult survivorship stronglyaffect plant recruitment in a habitat. Although multiple herbivores may affect individualsof a plant species, not all of the herbivores will necessarily affect populationdynamics of the plant species. Thus, certain keystone herbivores 45, 143 can have largeeffects even on plant community structure while other herbivores may have minor ornegligible effects on present-day plant performance parameters.The effect of herbivory on plants is not a simple issue. A number of factors oftenrequire study before a complete picture can be made for the effects of herbivory on asingle plant species. However, researchers have examined a number of levels of plantperformance parameters. These levels include plant physiological responses, growthchanges, yields, effects on survivorship and fecundity traits that affect plant fitness,the selection pressure of multiple herbivores on a plant species, the responses of aplant species across a variety of environmental conditions, and several aspects ofplant population and community level parameters. Studies at all of these levels willcontinue to help in how we understand plant–herbivore interactions from theperspective of how herbivores affect plants.REFERENCES1. Strong, D. R., Lawton, J. H., and Southwood, T. R. E., Insects on Plants, BlackwellScientific, Oxford, U.K., 1984.2. Crawley, M. J., Insect herbivores and plant population dynamics, Annu. Rev. Entomol.,34, 531, 1989.3. Belsky, A. J., Does herbivory benefit plants?, Am. Nat., 127, 870, 1986.4. Boyer, J. S., Plant productivity and environment, Science, 218, 443, 1982.5. Paige, K. N., and Whitham, T. G., Overcompensation in response to mammalianherbivory: the advantage of being eaten, Am. Nat., 129, 407, 1987.6. Sacchi, C. F., and Conner, E. F., Changes in reproduction and architecture in floweringdogwood, Cornus florida, after attack by the dogwood club gall, Resseliella clavula,Oikos, 86, 138, 1999.7. Ehrlich, P. R., and Raven, P. H., Butterflies and plants: a study in co-evolution, Evolution,18, 586, 1964.8. Hougen-Eitzman, D., and Rausher, M. D., Interactions between herbivorous insects andplant-insect coevolution, Am. Nat., 143, 677, 1994.9. Rausher, M. D., Genetic analysis of coevolution between plants and their naturalenemies, Trends Ecol. Evol., 12, 212, 1996.10. Iwao, K., and Rausher, M. D., Evolution of plant resistance to multiple herbivores: quan-