Arbuscular Mycorrhizal Fungi: More Diverse than Meets the Eye, and the Ecological Tale of Why

H ecologists focused on interspecific competition as the critical factor structuring plant communities. Interactions between plants, however, are likely to be mediated by myriad interactions with soil organisms (Bever et al. 1997). The vast majority of plants, for example, take up nutrients through interactions with root symbionts. Of these root symbionts, arbuscular mycorrhizal (AM) fungi are perhaps the most common, likely forming associations with the majority of plant species, and are probably among the most important because they facilitate plants’ uptake of phosphorus, a limiting nutrient in many soils. While acknowledging the potential importance of AM fungi, ecologists are only beginning to understand the diversity and dynamics of these soil symbionts. Research on plant–fungal interactions has always been hampered by a basic asymmetry: Whereas plants show themselves and wait to be counted, fungi are much more cryptic. Over the past several years, we have worked intensely on the ecology of the plant–AM fungal interactions within a one-hectare field in North Carolina. This work provides a window into an underground world that is surprisingly diverse and dynamic. In this article, we describe the process of discovering this diversity, detail mechanisms that might maintain fungal diversity, and then discuss our understanding of what this diversity means for ecology as a whole.