Not that many fungi

Fungi play major roles in ecosystem processes, but the determinants of fungal diversity and biogeographic patterns remain poorly understood. Using DNA metabarcoding data from hundreds of globally distributed soil samples, we demonstrate that fungal richness is decoupled from plant diversity. The plant-to-fungus richness ratio declines exponentially toward the poles.

This is part of an abstract of a new study published in Science and led by the researchers of the University of Tartu Natural History Museum. Together with 35 research institutions they discovered that the most species-rich fungal communities occur in tropical rainforests. The estimated global species richness of fungi, 1.5-5.1 million species, however, seems to be a vast overestimation, according to their data.

The team collected approximately 15,000 soil samples from various areas across the world. The samples were analysed using next generation sequencing of smaller ITS (fungi barcode standard) fragments. When analyzing the soil samples, the colleagues found more than 45,000 species which is about half of the described fungal diversity on Earth. 

One of the main messages of this study is the finding that species richness patterns of fungi in general follow these of plants and animals, i.e. the species richness is the highest in tropical rainforests. In the past, it was also commonly held that general rules of biogeography do not apply to microorganisms in other words all forms are present everywhere only separated by substrate. The researchers could show that endemism is also very common among fungi. 

In addition to the finding that fungi might be much less diverse than previously thought the researchers also made some more alarming observations:

Climatic variables explained the greatest proportion of richness and community composition in fungal groups by exhibiting both direct and indirect effects through altered soil and floristic variables. The strong driving climatic forces identified here open up concerns regarding the impact of climate change on the spread of disease and the functional consequences of altered soil microorganism communities. The observed abrupt functional differences between fungal communities in forested and treeless ecosystems, despite spatial juxtaposition, suggests that plant life form and mycorrhizal associations determine soil biochemical processes more than plant species per se. Loss of tree cover and shrub encroachment resulting from drying and warming may thus have a marked impact on ecosystem functioning both above- and belowground.