Herbivory, the feeding on living plant parts by animals, is a key ecosystem process that has widely recognized effects on primary production and on vegetation structure and composition. The effect of herbivory depends on herbivore feeding type and intensity. Different types of herbivory affect different tissues and the production, translocation, and accumulation of photosynthates to varying degrees.

A number of methods have been used to measure the intensity and effects of herbivory. The most common method for measuring intensity has been estimation of consumption rates by individual herbivores and extrapolation to population size. This method can be used to measure consumption by sap-sucking herbivores as well as folivores. A second method is measurement, by various means, of missing plant biomass. This method does not account for completely consumed (and unobserved) parts or for compensatory growth. Measurement of turnover of marked plant parts is the most accurate, but labor-intensive, method for estimating herbivory. Estimates of herbivory can differ by 2-5 times among methods, making standardization a key to comparison among ecosystems. Evaluating the effect of herbivory requires measurement of a variety of plant and ecosystem responses, not simply plant growth or productivity.

The intensity of herbivory varies widely, but a trend is apparent among ecosystem types. Herbivory generally is lowest (<2% reduction in primary production) in some forests and highest (most primary production consumed daily) in aquatic ecosystems. Insects are the primary herbivores in forest ecosystems and may account for the bulk of herbivory in grasslands, although vertebrate grazers are more conspicuous.

Herbivory has well-known effects on survival, productivity, and growth form of individual plants. However, the traditional view of herbivory as a negative effect on plants is being replaced by a view that recognizes more complex effects of variable intensity and timing. Moderate intensities of herbivory often stimulate production, through compensatory growth, and flowering, thereby increasing fitness. A given intensity of herbivory can have different effects at different times during the growing season or under different environmental conditions. Herbivory can affect the growth form of plants by terminating shoot growth and initiating branching and by affecting shoot-to-root ratios. Changes in survival, productivity, and growth of individual plant species affect vegetation structure and community dynamics. Herbivores often determine the geographic or habitat patterns of occurrence of plant species and facilitate successional transitions.

Few studies have addressed effects of insect herbivores on biogeochemical cycling or other abiotic conditions. However, herbivores affect, often dramatically, the turnover of plant nutrients to litter as plant fragments, feces and animal tissues, and nutrients leached from chewed surfaces. Folivory alters seasonal patterns of nutrient fluxes by transferring material prior to plant resorption of nutrients from senescing parts. Sap-sucking insects transfer copious amounts of labile carbohydrates (as honeydew) that stimulate growth and nutrient uptake by microbes. Herbivory also may affect climate and the likelihood and intensity of future disturbances. Reducing vegetation cover greatly affects the penetration of light, precipitation, and wind to the understory and soil, affecting soil warming and water content, relative humidity, erosion, transpiration, etc. Reduced vegetation biomass or litter accumulation affects abundance of fuel to support fire and affects water-holding capacity and vegetation demand for water during drought. Therefore, herbivory can influence ecosystem stability substantially (Chapter 15).

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