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Herbivory is common and widespread in estuarine systems, and numerous invertebrate and vertebrate taxa can act as herbivores in these habitats (see Herbivorous Grazers). Estuaries worldwide vary substantially in herbivore densities and consumption rates, leading to corresponding variation in the trophic impact and ecosystem-level importance of herbivory. Herbivory can shape the abundance, distribution, and species composition of primary producers. Herbivores in low-nutrient systems can serve as a “biological control” for algal blooms, for example, but may have little impact on algal biomass under other conditions (Hauxwell et al., 1998; Worm and Lotze, 2006). In many estuarine systems, primary producer biomass is mainly degraded via detrital pathways, with herbivory having a much more limited role (e.g., Conover, 2011; Guidone et al., 2012). In systems where herbivory plays an important role, primary producers frequently utilize morphological adaptations and/or produce secondary metabolites as defenses against consumers. Morphological adaptations include spines, scales, or thick and tough outer tissue layers, while secondary metabolites can make plants and algae distasteful or toxic to grazers (Hay et al., 1994; Van Alstyne et al., 2007).

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Encyclopedia of Earth Sciences Series