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Successful management of vegetation and the restoration of threatened or endangered plant populations clearly depend on the unambiguous identification of the factors that determine and limit plant abundance and distribution (e.g. Harper 1977, Jordan, Gilpin & Aber 1987). Physical conditions, plant physiological responses, and plant competitive interactions are often important (e.g. Harper 1977, Chabot & Mooney 1985, Pickett & White 1985, Grace & Tilman 1990), and these factors are usually evaluated. However, trophic interactions, such as those involving plant consumption by insects and pathogens, may also be critical in the growth, reproduction, and population dynamics of native plants (e.g. Harper 1969, Whittaker 1979, Rausher & Feeny 1980, Dirzo 1984, Howe & Westley 1988, Burdon 1987, Hendrix 1988, Burdon, Jarosz & Kirby 1989, Louda 1989), but are seldom evaluated in plans for the management or restoration of threatened plant species.
In this chapter, after presenting some background on insect herbivory in plant dynamics, I review several key results from two field experiments that excluded flower- and seed-feeding insects from native plants. I then use these results to: challenge the view that insect damage can be assumed to be irrelevant to plant success, suggest patterns of significant insect damage to seeds of native plants, and make two recommendations involving assessment of insect herbivory to improve the management and restoration of rare and threatened plant species. The evidence suggests that evaluation of insect herbivory is generally warranted. Insect herbivory can be a critical, limiting factor, particularly in determining the abundance and distribution of relatively short-lived, herbaceous, perennial plants. This appears especially true where both insect damage and regeneration failure are observed in declining populations of a rare or threatened plant.