Biotic Stress and Yield Loss

injury levels. 5, 60, 61 McNaughton 58 pointed out that herbivory injury often does nottranslate into linear forms of damage to plants because of plant compensation, inwhich case herbivory generally does cause some plant damage but not as much asexpected based on the injury received. However, with overcompensatory plantresponses, plant vegetative growth and/or reproductive production increases followingherbivory injury, in which plant fitness would be expected to be higher in plantssubject to herbivory relative to plants not subject to herbivory. However, overcompensationto herbivory may not be the result of herbivory selection pressure on plants,but rather plant regrowth responses to other factors that happen to also be stimulatedby herbivory. 61 Hence, Belsky et al. 61 suggest that overcompensation may not indicatea plant–herbivore mutualistic coevolutionary relationship, 62 and several assumptions behindthe idea that herbivory benefits plants may not be true.Yet, several examples of overcompensation have been suggested. Some examplesinclude a history of management pressures by humans on a biennial, Gentianellacampestris, 63 possibly between herbivores and a cyanogenic grass, Cynodon plectostachyus,59 between grazing herbivores and some grasses, 3, 5, 59 and with the biennialscarlet gilia, Ipomopsis aggregata. 5, 61, 64–66 Additionally, it has been suggestedthat overcompensation may not be quite so rare a form of plant response to herbivory.66 Yet, despite finding overcompensatory responses in I. aggregata followingshoot consumption by deer, other researchers with the same study organism have notbeen able to detect overcompensation 67, 68 to the same types of injury. Bergelson etal. 69 reported partial plant compensation (not overcompensation) for herbivory injuryin I. aggregata, and injury from each of several different herbivores could have negativefitness consequences. 69 With this example, I. aggregata may have geographicalvariation in both compensatory responses to herbivory injury and pressure from differentnumbers of herbivores, 68, 69 and interactions between different types of injurythat can result in either plant damage or plant overcompensation. 66The debate about plant overcompensation remains unclear because variation inenvironmental conditions and genetic variation in plant ability to compensate forinjury seem to exist even within a single plant species. Thus, making general statementsabout plant responses to herbivory may be difficult even at the level of aspecies, let alone for general plant responses to herbivory. It seems that herbivoresusually have negative effects on plant growth and fitness, 2, 3, 16, 62 although in somelimited number of situations herbivory seems to benefit plant fitness. 66 At a broaderlevel, some herbivores may serve as controlling factors of plant population dynamicsand distributions.2, 46, 70–739.4 HERBIVORY: DIFFERENT PLANT TISSUESSeveral different plant tissues can be consumed directly by herbivores. These tissuesinclude roots, stems, leaves, flowers, fruits, and seeds. In addition, multiple herbivoresmay consume tissue or fluids from all of these plant tissues from one plantspecies. 70, 71 Some herbivores consume plant tissues, other herbivores suck plant fluids(phloem or xylem sap) from roots, leaves, and stems, and some insects develop in