Prairie fens are unique in their high plant and animal diversity; however, many are currently threatened by the invasion of exotic glossy buckthorn (Frangula alnus). Land managers and conservationists have observed a variety of qualitative changes in prairie fen after F. alnus invasion, but these observations have not been previously quantified. Here, we contrast abiotic and plant community parameters in areas of a Michigan prairie fen invaded by buckthorn with that of uninvaded references sites within the same fen. Our results document a variety of significant differences including: lower soil pH, fewer vegetative hummocks, less light availability, lower plant coefficient of conservatism, less total plant cover, and lower graminoid relative abundance in invaded versus uninvaded areas. We also investigated the buckthorn invasion process by examining abiotic and plant community differences within partially invaded plots. In such plots, F. alnus-invaded areas had significantly greater percent soil organic matter, fewer vegetative hummocks/m², and lower mean coefficient of plant conservatism. We propose that F. alnus may be facilitating its own invasion of fen habitats and that resulting hummock degradation may pose a long-lasting detriment to fen biodiversity. We anticipate that wetland managers will be able to use this study to provide a baseline against which fen restoration success can be gauged.