Throughout the fungal kingdom, we find numerous examples of generalists. By utilizing a wide range of substrates, generalist fungi can easily find a source of carbohydrates to feed itself. Species with a wide feeding niche, need not look far for food. However, this feeding ecology does have its drawbacks. Generalist fungi have to deal with competition much more than fungi with a more specialized diet. Because of this, evolutionary processes have shifted many generalist species towards a more specialized ecology. Don’t get me wrong, there are still a wide array of generalists found in nature. Just note that it’s not their ability to use a wide variety of resources that keeps them in the generalist realm, but their ability to compete with other species.
Competition is a negative species interaction for both parties involved. Even when one species dominates another, it has allocated resources to that encounter instead of reproductive tissues for example. Competition drains fitness no matter how efficient the competitor. For this reason, species become specialized.
Baeospora myosura is quite the specialist. Commonly known as the conifercone cap, this species is found fruiting from cones of pine and spruce trees throughout North America. Instead of breaking down woody debris from diverse assemblages of trees, it has evolved a narrower feeding ecology by breaking down conifer cones. Throughout the scientific literature, Baeospora myosura is described as a saprotrophic species. I on the other hand believe that this categorization is oversimplified and sometimes incorrect.
Saprotrophic fungi break down dead organic matter. Pine and spruce cones are indeed organic matter, but are these substrates dead? Conifer cones might not be living, but they sure aren’t dead either. If the cones have been fertilized, then they are in a dormant state, waiting for the proper conditions to germinate into another generation of trees. My argument here is that a species that breaks down potential spruce and pine offspring shouldn’t be called a saprotroph, but a parasite. How viable the cones are should distinguish if the fungus is acting parasitically or saprophytically.
Regardless of what we label this fungus, it will continue carrying out its life cycle atop the forest floor on the cones of conifers. By analyzing the tissue of Baeospora myosura, we can gain even more insight about how it feeds. Last week, I talked about how isotopes are used to categorize a species feeding ecology. This species is no different, as isotopically, Baeospora myosura is what it eats. Compared to other saprotrophs, Baeospora myosura has higher values of Carbon 13.
Baeospora myosura has higher levels of Carbon 13 compared to other fungi because of the position of where its substrate grows. Cones grow higher in the tree’s canopy. It is here where elevated levels of Carbon 13 molecules accumulate in the plant. The canopy section of the tree has more available light than the understory, so it is a more photosynthetically active area. As the plant packages sugars to send to fungal symbiotes beneath the forest floor, is discriminates against the heavier Carbon 13 isotopes. The areas of the tree that produce the most sugars have the highest levels of Carbon 13. The cones that grow here utilize the sugars around them, so they become packed with these heavy Carbon isotopes. Carbon 13 then becomes absorbed by Baeospora myosura, the species that breaks down these cones.
One may think that a species that only uses pine and spruce cones of a few tree species might have a narrow distribution. However, this successful species has quite a large distribution, found widely throughout mainland North America and the Bahamas. By escaping competition, this species allocates less energy and resources to defending its substrate, and more towards its reproductive effort. Specialized fungi like Baeospora myosura reveal how selection favors strategies that reduce competition. Through time the conifercone cap enhanced its fitness by specializing in these specific cone substrates. From the plants perspective, these fungi could be considered a nuisance, if the cones they break down are fresh and viable. Though, the plants fitness is not reduced if the cones sit long enough on the forest floor and become unviable. Whether or not the species acts as a parasite or saprotroph, Baeospora myosura will exist as long as spruce and pine cones continue falling from coniferous canopies.