130 www.frontiersinecology.org © The Ecological Society of America T he evolutionary history of insects predates that of modern forests, and forest ecosystems originated and evolved under substantial insect herbivore pressure (Farrell et al. 1992). In spite – or perhaps because – of their long-standing ecological association, most insects live in a benign, or even beneficial, relationship with their host trees. However, a few species are typified by explosive population eruptions that have profound eco- logical and economic implications (Figure 1). Taken together, insect outbreaks are the major agent of natural disturbance in North American forests. The forest area impacted by insects and pathogens in the US is approxi- mately 45 times that of fire, with an economic impact that is almost five times as great (Dale et al. 2001). Since dead trees serve as fuel for catastrophic wildfires, insects and pathogens often play a key role in the occurrence and severity of the forest’s second greatest disturbance agent, fire (Bergeron and Leduc 1998). Insect outbreaks may also have significant adverse effects on nutrient cycling, carbon sequestration, and biodiversity (Ayres and Lombardero 2000). Even though insect outbreaks greatly affect forest ecosystems, they may not be detrimental from a long-term ecological perspective. Such disturbances may in fact be crucial to maintaining ecosystem integrity, a situation that Mattson (1996) has described as “normative outbreaks”. The term “pest” is a pejorative given to organisms that successfully compete with humans for valued resources. We are primarily interested in the potential for climate change to disrupt current associations between important herbivores and their forest hosts. In this paper, we con- sider forest insect pests because of their substantial eco- nomic and social impact, and also because the disruption of co-evolved normative relationships could have devas- tating ecological consequences that will eventually impact the survival of their host trees (Loehle and Leblanc 1996; Logan and Powell 2001). Climate change, and particularly global warming, will have a dramatic impact on pest insect species. As “cold- blooded” organisms, they have a life history that hinges on temperature; the thermal habitat largely sets the bound- aries of their geographic distribution. Forest pests are typi- cally highly mobile insects with short generation times and high fecundity. Population dynamics provide a sensi- tive indicator that integrates the complex climate signal into a measurable (and often spectacular) response. Insect trapping efforts in the UK have already provided evidence that the timing of critical life history events (phenology) – the most easily observed response of ectothermic organ- isms to a warming climate – is occuring earlier among insects than previously recorded (Harrington et al. 2001). Ayres and Lombardero (2000) and Harrington (2002) REVIEWS REVIEWS REVIEWS Assessing the impacts of global warming on forest pest dynamics Jesse A Logan 1 , Jacques Régnière 2 , and James A Powell 3 Forest insects and pathogens are the most pervasive and important agents of disturbance in North American forests, affecting an area almost 50 times larger than fire and with an economic impact nearly five times as great. The same attributes that result in an insect herbivore being termed a “pest” predispose it to disruption by climate change, particularly global warming. Although many pest species have co-evolved relationships with forest hosts that may or may not be harmful over the long term, the effects on these relationships may have disastrous consequences. We consider both the data and models necessary to evaluate the impacts of cli- mate change, as well as the assessments that have been made to date. The results indicate that all aspects of insect outbreak behavior will intensify as the climate warms. This reinforces the need for more detailed mon- itoring and evaluations as climatic events unfold. Luckily, we are well placed to make rapid progress, using software tools, databases, and the models that are already available. Front Ecol Environ 2003; 1(3): 130–137 1 USDA Forest Service, Rocky Mountain Research Station, Logan Forestry Sciences Laboratory, 860 N 1200 East, Logan, UT 84321 (jlogan@cc.usu.edu); 2 Natural Resources Canada, Canadian Forest Service, PO Box 3800, Sainte-Foy, Quebec, G1V 4C7, Canada; 3 Department of Mathematics and Statistics, Utah State University, Logan, UT 84322. In a nutshell: • Forest insects and pathogens, which have a considerable effect on the forests of North America, are particularly vulnerable to disruption by climate change • Current data and models suggest that global warming will result in the redistribution of insect pests, resulting in the invasion of new habitats and forest types • Unusually hot, dry weather patterns are already responsible for increased insect outbreaks in forests from the US Southwest to Canada and Alaska • The necessary tools are in place to assess the impacts of cli- mate change on forest pests and their host trees