Assessing the impacts of global warming on forest pest dynamics

www.frontiersinecology.org © The Ecological Society of America T 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 ecological 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 approximately 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 consider forest insect pests because of their substantial economic and social impact, and also because the disruption of co-evolved normative relationships could have devastating 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 “coldblooded” organisms, they have a life history that hinges on temperature; the thermal habitat largely sets the boundaries of their geographic distribution. Forest pests are typically highly mobile insects with short generation times and high fecundity. Population dynamics provide a sensitive 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 organisms 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