Stand and landscape diversity as a mechanism of forest resistance to insects

Plant vulnerability to herbivorous insects depends on both suitability (determined by nutritional and defensivefactors) and exposure (determined by plant location relative to herbivore population sources). The importance of plantbiochemical defenses against herbivores has received much attention (Rosenthal and Janzen 1979, Cates and Alexander 1982.Harborne 1982, Coley et al. 1985). Herbivore populations typically are aggregated on particular trees that often differ fromtheir neighbors in chemical composition or other factors that indicate greater susceptibility to herbivores (Alstad andEdmonds 1983, Lorio 1993). Plant compounds are important as feeding deterrents or toxins for herbivores unable to adaptappropriate avoidance or detoxification mechanisms. However, herbivores have adapted various strategies for avoiding ordetoxifying the chemical defenses of their hosts (Bernays and Woodhead 1982, McCullough and Wagner 1993).
Variation in suitability among plants may be important both for reducing selection for herbivore adaptations and forminimizing host exposure to adapted herbivores. Herbivores seeking new hosts must both distinguish and be able to reachsuitable hosts. Therefore, plant suitability, apparency, and distance from herbivore populations function interactively todetermine colonization by herbivores (Courtney 1986, Schowalter and Stein 1987).
My objective in this paper is to evaluate plant diversity as a means of reducing host exposure to herbivores. I willconsider diversity at the level of genetic variability within a particular species, and at the level of stand and landscapevariability in community composition. These three levels constitute a nested hierarchy of diversity that can limit populationgrowth of herbivores within stands and across landscapes.