Positive associations between species (mutualisms and commensalisms), although ubiquitous in nature, are not well integrated within the theoretical structure of community ecology. This project will study interactions involving bark beetles (Dendroctonus) to test the consequences of mutualisms for populations and communities. The southern pine beetle has an obligate mutualism with two species of fungi. The fungi are transported by adult beetles and inoculated within the phloem of the pine trees during egg laying. The beetle larval feeds upon the fungi within the phloem. A common cause of larval mortality is poor growth of the fungi due to competition from the bluestain fungus. This fungus also has a strong mutualism but with mites (Tarsonemus), which transport the fungus ascospores between trees, and feed upon this fungus within the tree phloem. The mites, in turn, are transported by the beetles. These system properties are hypothesized to impact the entire community and influence the outbreak population dynamics of beetles and death of pines, which is a dominant source of disturbance in forests of the southeastern U.S. Studies of this beetle-mite-fungus community have also revealed marked differences between mites and beetles in their growth response to temperature change, which suggests that seasonal and regional variation in climate should modulate the community. This work will also test whether these differences among species lead to predictable responses of the intact community. Results of this project will have scientific value in clarifying the role of mutualisms in community ecology and population dynamics and the dependence of these interactions on temperature. New understanding of the processes that influence insect outbreaks will have relevance to forest management, and aid in assessing risks to forests from climate change and introductions of exotic species. Proposed activities will also have broader beneficial impacts by contributing to the training of young scientists.