Fungal Community Ecology: A Hybrid Beast with a Molecular Master

F play pivotal roles in all terrestrial environments (figure 1). They are amajor component of global biodiversity and control the rates of key ecosystemprocesses. Fungi are perhaps best known for their role as decomposers, dominating the decomposition of plant parts, and particularly of lignified cellulose. They produce a wide range of extracellular enzymes that break down complex organic polymers into simpler forms that can be taken up by the fungi or by other organisms. This process is an essential step in the carbon cycle; without it, plant detritus would quickly tie up available carbon and mineral nutrients. It is not surprising, therefore, that eliminating fungi results in a significant reduction in both carbon and nitrogen depletion from litter (Beare et al. 1992). In fact, fungal hyphae often account for the greatest fraction of soil biomass (Wardle 2002) and can reach lengths of hundreds to thousands of meters per gram of soil (Taylor and Alexander 2005). In addition to their central roles in carbon and nutrient cycling, fungi are also a major component of terrestrial food webs. Fungal mycelia serve as the primary carbon source in a number of soil food webs (Wardle 2002), and fungal fruiting bodies can serve as a significant food source for large vertebrates, including humans. Some fungi can also act as predators: Perhaps the best-known examples are nematodetrapping fungi, but fungi also trap, poison, or parasitize and feed on other groups of soil invertebrates, including tartegrades, collembola, copepods, and rotifers (Thorn and Barron 1984). Interestingly, fungi express these behaviors in nitrogen-poor environments, suggesting that they seek nitrogen rather than carbon from their predation. Fungi directly shape the community dynamics of plants, animals, and bacteria through a range of interactions. They are themost common and important plant pathogens, causing serious crop loss and shaping the composition and structure of natural plant communities in many significant but often underappreciated ways. For example, seedlingmortality is often highest close to parent plants because of hostspecific fungal pathogens that reside on the parents. Such distance-dependent mortality has been hypothesized as a major mechanism preventing competitive exclusion and maintaining plant species diversity (Gilbert 2002).The recent worldwide spread of the frog pathogen Batrachochytrium