Forest pathogens : friend or foe to biodiversity ? 1

Biological diversity in forest ecosystems results from evolutionary processes driven by ecological imperatives linked to pathogens, symbionts, fire, climate, and competition or impacts from other agents of disturbance. To understand the behavior of microorganisms and microbial pathogens, it is necessary to have a comprehensive appreciation for the diversity of their functional attributes in their natural habitats. Where niches are complex, evolution is guided by more than simple host–pathogen relationships. Moreover, the negative attributes of a pathosystem are not always obvious at different scales or in different contexts. Deleterious impacts on one species (e.g., mortality, parasitism) may benefit another species through reduced competition or enhanced nutrient cycling. Species with pathogenic behavior also display mutualistic and symbiotic benefits (e.g., mycorrhizal fungi and endophytic antagonists found in grasses) or useful attributes as biological control agents. Preserving the diversity of natural areas while controlling forest pests is both a challenge and an opportunity. The challenge is to understand pest niches more thoroughly, while the opportunity is to have a wiser use of both timber and nontimber forest resources.