Fungi play a few important roles with herbivores. First, for ruminants, one of the main organisms that breaks down cellulose in the rumen are anaerobic fungi. Secondly, they contribute to the breakdown of dung in the environment. Third, they can cause illnesses, such as Candidiasis.
We recently did a test run examining the fungus of dung from South Dakota bison. The most abundant fungus was from Caecomyces, one of the anaerobic fungi found in the rumen. The second most abundant fungi were from the Sporormiaceae family, which is typically considered a dung fungus. Third most was another dung fungus (Panaeolus sphinctrinus–who named that?) that is related to some psychoactive mushrooms (but isn’t itself).
In all, we’re just starting to understand the assemblages of fungi in the digestive system of herbivores (and dung). As our understanding advances, we’re going to be able to recognize what “healthy” fungal assemblages look like as well as how they change seasonally. For animals that eat fungi–mushrooms or lichens–sequencing fungi in DNA also reveals under-appreciated contributions to diet.