Microbes feature as some of the most abundant and important mutualists with plants. The fact that microbes have long been ‘invisible’ to many community ecologists may be one important reason why mutualisms have often been overlooked at the community level. Here, I synthesize recent experimental research (by myself and others) on the community and ecosystem consequences of pair-wise mutualisms between grasses and fungal endophytes. In grasses, systemic endophytes can ameliorate abiotic (e.g., drought) and biotic (e.g., herbivory) stress, and thereby promote host performance. Work with non-native grass-endophyte systems has revealed that these mutualisms generate strong, cascading effects on communities and ecosystems – reducing plant and arthropod diversity at large spatial scales, slowing the process of decomposition, and altering the abundance of other microbial community members. Further, endophyte symbiosis can promote the invasion of non-native host grasses into diverse, established plant communities. These effects are not easily predicted from the responses of individual community members, and detection requires a community-level perspective. Altogether current results suggest that exclusive, pair-wise mutualisms -- where the benefits of association do not extend to other community members -- can have negative impacts on biodiversity and ecosystem functioning, particularly when occurring with dominant, invasive species.