Herbivory affects a variety of ecosystem properties, primarily through differential changes in survival, productivity, and growth form among plant species. Her-bivory is not evenly distributed among plant species or over time. Rather, some species are subject to greater herbivory than are others, and relative herbivory among plant species varies with environmental conditions (e.g., Coley 1980, Coley and Aide 1991, Crawley 1983, Schowalter and Ganio 1999). These differential effects on host conditions alter vegetation structure, energy flow, and bio-geochemical cycling and often predispose the ecosystem to characteristic disturbances.
The observed severity of herbivore effects in agroecosystems and some native ecosystems has led to a widespread perception of herbivory as a disturbance (see Chapter 2). This perception raises a number of issues. How can a normal trophic
| Herbivore damage to plants in young, intermediate, and old successional sites in sand dune vegetation in Michigan in June (A) and August (B) 1988. Percentages are averages for leaves on upper and lower canopy branches by damage category: 0, 0% damage; 1,1-5%; 2, 6-25%; 3, 26-50%; 4, 51-75%; 5, 76-100%; and 6, no leaves remaining. From Bach (1990) with permission from the Ecological Society of America.
process also be a disturbance? Is predation a disturbance? At what level does herbivory become a disturbance? Do the normally low levels of 5-20% loss of net primary productivity (NPP) constitute disturbance? Although debate may continue over whether herbivory is a disturbance (Veblen et al. 1994, P. White and Pickett 1985) rather than simply an ecosystem process
(Schowalter 1985, Schowalter and Lowman 1999, Willig and McGinley 1999), herbivory can dramatically alter ecosystem structure and function over large areas.
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