Johnson Ch16.qxd

527 INTRODUCTION Native forest insects are the greatest forces of change in forest ecosystems of North America. In aggregate, insect disturbances affect an area that is almost 45 times as great as that affected by fire, resulting in an economic impact nearly five times as great (Dale et al., 2001). Of these natural agents of ecosystem disturbance and change, the bark beetles are the most obvious in their impact, and of these, the mountain pine beetle (Dendroctonus pon-derosae Hopkins) has the greatest economic importance in the forests of western North America (Samman and Logan, 2000). The primary reason for this impact is that the mountain pine beetle is one of a handful of bark beetles that are true predators in that they must kill their host to successfully reproduce, and they often do so in truly spectacular numbers. Although the mountain pine beetle is an aggressive tree killer, it is a native component of natural ecosystems; in this sense, the forests of the American West have co-evolved (or at least co-adapted) in ways that incorporate mountain pine beetle disturbance in the natural cycle of forest growth and regeneration. Such a relationship in which insect disturbance is " part and parcel of the normal plant biology " has been termed a normative outbreak by Mattson (1996). This normative relationship between native bark beetles and their host forests is undergoing an apparent shift, exemplified by an unusual sequence of outbreak events. Massive outbreaks of spruce beetle have recently occurred in western North America ranging from Alaska to southern Utah (Ross et al., 2001; Munson et al., 2004). A complex of bark beetles are killing ponderosa pine in the southwestern United States at levels not previously experienced during the period of European settlement. Pinyon pines are being killed across the entire range of the pinyon/juniper ecotype, effectively removing a keystone species in many locations. Mountain pine beetle outbreaks are occurring at greater intensity, and in locations where they have not previously occurred (British Columbia Ministry of Forests, 2003). Any one of these events is interesting; that they are occurring almost simultaneously is nothing short of remarkable. In many of these instances the outbreaks are anything but normative; they are occurring in novel habitats with potentially devastating ecological consequences (Logan and Powell, 2001; Logan et al., 2003). What is going on here? The root of these unprecedented outbreaks appears to be directly related to …