A leaf falls from the rainforest canopy, but it never hits the ground. Instead, it becomes trapped by nets of sticky fungi. While other lost leaves litter the forest floor, this one has joined the jungle’s mezzanine level – a layer of litter suspended in mid-air and hanging by a thread.
The fungi belong to a single genus called Marasmius, which extend networks of root-like filaments through the air. They act like a web that catches falling matter from the branches above. They have gone unappreciated, but Jake Snaddon from the University of Oxford has found just how important they can be.
By snaring leaves, the fungi provide room and board to insects, spiders and other canopy creepy-crawlies that might otherwise be confined to the ground. When Snaddon removed the fungi, the numbers of these animals plummeted by 70 percent.
The Marasmius fungi have been ignored for a long time. Even though they’re found throughout the tropics, they are very hard to see as they mostly consist of fine threads. They rarely produce obvious mushroom-like structures and when they do, these “fruiting bodies” are small and non-descript. You would only really notice them because of the leaves that they trap, and most rainforest scientists just move them out of the way. People have probably been brushing these important fungi aside for decades.
Snaddon was one of them. He was more interested in ferns, bromeliads and other epiphytes (plants that grow on other plants). People used to think that if falling leaves were intercepted in the canopy, they would be caught within the folds of such plants. “While working with these ferns [in the rainforests of Borneo], I noticed that there seemed to be more leaf litter outside of them, just hanging in the canopy,” he says.
When Snaddon shifted his focus to these hanging leaves, he soon realised their importance. In every hectare (the size of a rugby pitch, or London’s Trafalgar Square), the fungi hold around 260 kilograms of leaves. They hold 2-3 times more than other epiphytes can, and they’re more evenly dispersed.
These litter-traps are suspended worlds. Snaddon counted around 340 different species of insects and other arthropods among the dead leaves. If he removed the fungi, the number of species in the lower canopy fell by 57 percent, and the total number of individuals fell by 70 percent. That’s a huge figure, especially when you consider that around 60 percent of the canopy’s arthropods live in its lowest parts. Clearly, our knowledge of the rainforest was missing a crucial layer.
Now, Snaddon wants to look at these litter-trap communities in more details, to see if the arthropods have developed any intimate partnerships with the fungi that give them support. He adds, “With increasing degradation and loss tropical forests it would be interesting to see how these systems are influenced by changes associated with logging and forest clearance.”
Reference: Snaddon, Turner, Fayle, Khen, Eggleton & Foster. 2011. Biodiversity hanging by a thread: the importance of fungal litter-trapping systems in tropical rainforests. Biology Letters http://dx.doi.org/10.1098/rsbl.2011.1115
All photos by Jake Snaddon