Schowalter, Tim. 1990. Differences and consequences for insects. In: Pearson, Audrey; Challenger, Derek A., eds. Forests - wild and managed: differences and consequences; 1990 January 19-20; Vancouver, BC. Vancouver, BC: University of British Columbia: 91-106.
Forest insects and pathogens do not threaten forest resources unless changes in forest conditions facilitatepopulation growth. Healthy trees in diverse forests are protected from potential pests by defensive compounds whichkill or deter plant-feeding pests and by the abundance of non-hosts which increase the distance between hosts andchemically hide host trees. Forest management practices which reduce production of defensive compounds or promotethe abundance and apparency of any tree species will increase the likelihood of pest problems.
Contrary to muntrtra tglertionS, olti•rmvth forests are highly productive and remarkably resistant topotential pests. The resistance of these forests may be due largely to the diversity of plant and predator species whichlimit the ability of potential pests to discover and colonize suitable hosts. Consequently, herbivore populations aresupported at low levels. Low levels of herbivory contribute to long-term productivity and stability by pruning healthytrees, stimulating nutrient cycling, and accelerating the decline and replacement of unhealthy trees.
By contrast, managed forests often are highly susceptible to a variety of pests. This susceptibility results Pornharvest and planting practices which promote the most commercially desirable tree species at the expense of pest-inhibiting diversity. Simplified tree farms also appear unable to sustain many of the generalist predators responsible forpest control in more diverse ecosystems. The situation has become critical in western Oregon and Washington where75% of the forest landscape has been converted to Douglas-fir monoculture over the past 50 years, creating anunprecedented resource for pests such as black-stain root disease fungus and its insect vectors.