"Fungi prevent any single species from dominating rainforests as they spread more easily between plants and seedlings of the same species. If lots of plants from one species grow in the same place, fungi quickly cut their population down to size, leveling the playing field to give rarer species a fighting chance." says Own Lewis. (Credit: Alexander Torrenegra/Flickr)

Fungi are the biodiversity police of rainforests

Fungi, often seen as pests, play a crucial role policing biodiversity in rainforests by making dominant species victims of their own success.

New research shows how fungi spread quickly between closely packed plants of the same species, preventing them from dominating and enabling a wider range of species to flourish.

“When we think about the diversity of tropical rainforests, we often focus on plant or insect species—paying little attention to the microscopic fungi and other microbes living in and on everything,” says Rachel Gallery, assistant professor of microbial ecology in the University of Arizona’s who did part of the research as a postdoc under Owen Lewis of Oxford University, the leader of the research project.

“Our study is the first tropical experimental test of the long-standing hypothesis that plant pests can drive plant community diversity through their disproportionately negative effects on locally abundant plant species,” she says.

“In the plant world, close relatives make bad neighbors,” Lewis says. “Seedlings growing near plants of the same species are more likely to die, and we now know why. It has long been suspected that something in the soil is responsible, and we’ve now shown that fungi play a crucial role. It’s astonishing to see microscopic fungi having such a profound effect on entire rainforests.

“Fungi prevent any single species from dominating rainforests as they spread more easily between plants and seedlings of the same species. If lots of plants from one species grow in the same place, fungi quickly cut their population down to size, leveling the playing field to give rarer species a fighting chance.”

Natural enemies

The study, published in the journal Nature, looked at seedling plots across 36 sampling stations in the Chiquibul Forest Reserve in Belize. Researchers sprayed plots with water, insecticide, or fungicide every week for 17 months. They found that the fungicide Amistar dealt a significant blow to diversity, reducing the effective number of species by 16 percent. While the insecticide did change the composition of surviving species, it did not have an overall impact on diversity.

While 16 percent may not sound like much, “16 percent of a large number would be a large number. If these results are similar in an Amazon forest, for example, where researchers have found 250 tree species coexisting in a few acres, then the role of fungi maintaining diversity should not be underestimated,” Gallery says.

“We expected that removal of both fungi and insects would have an effect on the tree species,” says Rob Freckleton of the University of Sheffield. “However, what was unexpected was that removal of the fungi affected diversity, but eliminating insects didn’t. Ours is the first study to unpick the effects of the different natural enemies.”

Soon after, they became dominated by a few species at the expense of many others, leading to a marked drop in diversity.

“These results surprised us somewhat,” Gallery says. “Based on decades of research, we expected pests such as insect herbivores and other microorganisms to also have a diversity enhancing role. What we found was that fungi are the important pests driving diversity in this forest.”

“Just like humans, plants have their own microbiomes—all the microbes that live in and on an organism—that include fungi and bacterial living in an on their roots, stems, leaves, flowers, and seeds. Plants and fungi form all sorts of relationships from beneficial to antagonistic and we are only beginning to understand these complex relationships in tropical forests.”

Fungi keep species in check

Scientists had suspected that fungus-like microorganisms called oomycetes might also play a part in policing rainforest diversity, but this now seems unlikely.

“Oomycetes are potent pathogens that can cause seeds and seedlings to rot,” says Sarah Gurr, formerly of Oxford University and now at the University of Exeter. “They were responsible for the 1840s potato famine in Ireland.”

To see if oomycetes play a role in promoting rain forest biodiversity, the researchers sprayed plots with Ridomil Gold, which protects plants against oomycetes. Ridomil Gold had no significant effect on the number of surviving plant species, suggesting that true fungi and not oomycetes are driving rainforest diversity.

The findings show that fungi play a vital role in maintaining the biodiversity of rainforests, preventing a few highly competitive species from dominating. It helps to explain why tropical rainforests are so much more diverse than forests in temperate countries.

“We suspect that the effect of fungi will be strongest in wetter, hotter areas because this is where they thrive,” says lead author Robert Bagchi, who began the study at Oxford University and completed it at ETH Zurich. “This has important implications for how rain forests will respond to climate change, which is often predicted to reduce overall rainfall, making it harder for fungi to spread.”

The researchers concluded that without fungi to keep species in check, much of the diversity that makes rainforests special could be lost.

“Our analyses are ongoing,” Gallery adds, “but what were able to do with this study was identify certain species of fungi that appear to be heavy hitters in this forest. Our preliminary results are showing that these fungal pests are able to infect and kill a large number of species in this forest, but our results also show that plant species respond differently to infection. For some plant species, the infection might be more like a mild cold whereas for others the infection is fatal.”

“It is now critically important to examine how these plant-fungal interactions might change under the warmer and drier climate projected in this region and what that might mean for tropical plant and fungal communities.”

The UK’s Natural Environment Research Council funded the study.

Source: University of Arizona

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