Fungi play a crucial ecological role in all terrestrial ecosystems, including those in Olympic National Park. What is that role? A. They are photosynthesizers, providing energy and nutrients to the other organisms in the ecosystem. B. They are important fermenters, used by humans to make foods and beverages. C. They are a primary food source for vertebrate consumers; without them, vertebrate diversity would be very low. D. They are decomposers, breaking down dead organisms and helping to recycle their nutrients. E. All of the above.