Species Interactions and Fungal Community Structure: Is Competition Important?
Many mushrooms present in Californian forests are produced by a group of fungi known as ectomycorrhizal fungi. These fungi are important symbionts of trees and exchange nutrients and water for carbon produced by plant photosynthesis. One question that has long interested mycologists has been what determines the presence or absence of a fungus in a certain location. Environmental factors such as soil moisture, nutrient status, and organic matter content are all known to affect the distributions of most fungi. Another important factor for ectomycorrhizal fungi in particular is host specificity. For example, many mycologists have observed species such as chanterelles (Cantharellus species) can be found under conifers as well as oaks, but others such as false truffles (Rhizopogon species) are only found under conifers. Aside from host specificity, however, little is currently known about other types of species interactions that affect whether certain fungi are present in a community.
One factor that has a large effect on plant and animal distributions is competition, but this factor has received very little study in ectomycorrhizal fungi until recently. The main reason why competition has not been better studied is that examining interactions among ectomycorrhizal fungi is difficult. Because these fungi require carbon from the plant, most cannot be grown without their host (some species can be grown on alternative carbon sources, but only a small minority). This situation eliminates the possibility of growing most species in laboratory settings such as Petri dish cultures, where many competition studies have been conducted on other groups of fungi such as wood rotters. Fortunately, molecular tools such as DNA sequencing have allowed mycologists to directly examine ectomycorrhizal communities in field soil. By taking small quantities of DNA from the soil, mycologists can now determine which species are present in a given location.
A recent study by Roger Koide and his colleagues took advantage of these molecular techniques to look for evidence of species interactions within an ectomycorrhizal fungal community in a Pennsylvania pine forest. They reasoned that if species interactions were occurring the distribution of species should not be random in the soil. For example, if certain species were strong competitors they would be less likely to be encountered in the same soil area. In contrast, if the presence of one species made the environment more hospitable for a second species, then those two species should be encountered in the same area of soil more often than by random chance alone. They also hypothesized that the addition of either nitrogen or tannins (the latter being thought to affect soil chemistry such as pH) might influence the outcome of species interactions. To assess fungal community composition, they took a number of small soil cores (0.25 ml pr core) at monthly intervals for one year, brought them back to the lab and identified all species within each core using molecular techniques.
They found that the community seemed to show strong evidence of species interactions. In general, it appeared that species tended to avoid each other more often than by random chance, suggesting that competition was an important factor structuring the spatial distributions of many of the fungi in their community. This was particularly the case for the two dominant species Cenococcum geophilum and Clavulina cinnerea, which showed a negative association when examining both the soil hyphae and root tips occupied by the two species. There was also evidence of positive associations between a small number of species pairs, but this appeared to be a less common phenomenon than competitive interactions between species. The addition of both nutrients and tannins did not measurably affect the nature of species interactions, but there were a higher number of species in the nitrogen addition than control plots. This was interesting because it suggested that competitive interactions may be driven by soil nutrient status, with some species unable to persist in areas where other species had already extracted the majority of the nutrients.
This study is one of the first to demonstrate that species interactions such as competition may have an important influence on the distribution of ectomycorrhizal fungi. While it provides evidence of competition, more studies are needed to assess the frequency, intensity, and mechanisms of competitive interactions among ecotmycorrhizal fungi. Understanding the role of competition also has practical implications for agroforestry because when seedlings inoculated with one ectomycorrhizal species are out-planted into natural settings, native ectomycorrhizal species typically quickly replace the inoculated species on seedling roots. By understanding how this competitive replacement takes place, mycologists may one day be able to cultivate high-value edible ectomycorrhizal fungi such as truffles, chanterelles, matsutake, and Boletus spp., by mimicking the conditions that favor the presence of only their desired fungi.
- R.T. Koide, B. Xu, J. Sharda, Y. Lekberg, and N. Ostiguy. 2005. Evidence of species interactions withinan ectomycorrhizal fungal community. New Phytologist 165:305-316.