Ecosystems near the equator function much differently that the more temperate ecosystems I am familiar with. Near the equator, the climate is warmer and wetter, which is why these abiotic factors drive species diversity and overall plant production. With the majority of nutrients being allocated to the competitive living system, few soil resources remain underground for long. Fungi from the family Marasmiaceae have evolved some remarkable structures to compete for nutrients before plant debris even reaches the forest floor. Litter trapping fungi collect and digest fallen material using aerial rhizomorphs. A newer study indicates that these species directly enhance tropical biodiversity, so yeah, biodiversity hangs by a thread; an aerial fungal thread.
These litter trapping fungi extend dense networks of rhizomorphs through the vegetation in the lower canopy. The lower canopy is a dense, bioactive region that maintains the majority of arthropod diversity, so in 2011, Jake Snaddon and his team conducted a manipulative experiment to better understand this fungal community.
Aerial rhizomorphs from Snaddon et al. 2011.
Initially, these researchers found that these networks collected a huge amount of organic litter. For every hectare, these fungi trapped more than 250 kilograms of fallen plant material! For all of my American readers, that’s around 225 lbs. per acre! These marasmioid species are quite voracious but it’s not really how these species interact with the carbon cycle that promotes arthropod diversity. These aerial networks provide another dimension to the forest floor that increase the number of habitats these insects can make a living from.
To sample the arthropods, the researchers used 60 paired sites in the Danum Valley Conservation Area, Sabah, Malaysia. Each paired site was 4 m2 and 3 m high. The fungi trapping network in one of the paired sites was removed while the other was kept intact. After these sites were set up, researchers removed arthropods and waited 10 days to quantify the diversity of re-colonizers. After the 10 days, the plots were fumigated, and the branches in this understory area where gently shaken to remove all of the insects.
Aerial rhizomorphs and fruiting body of a Crinipellis spp.-family Marasmiaceae. Photo by Danny Newman.
Previous work in this exact location shows that around 60% of the arthropod species occur in the lower 16 meters of the forest canopy. This removal experiment showed the true importance of these fungal networks in providing microhabitats for a diverse array of forest arthropods. By looking closer at these aerial rhizomorphs produced by litter trapping Marasmius species, these researchers showed that 43% of these canopy arthropods depend on these fungi.
Aerial rhizomorphs promotes arthropod diversity. Black circles represent plots with intact aerial rhizomorphs, white circles show plots with aerial rhizomorphs removed. Snaddon et al. 2011.
The forest floor is a complex, chaotic habitat, but in tropical ecosystems near the equator, this complexity extends from the forest floor. Litter trapping fungi that evolved to take advantage of fallen leaf litter in depleted tropical soils actually enhance tropical forest diversity as a whole. The arthropods these complex communities support, act as pollinators, herbivores, parasites, and detritivores. This 2011 study shows that without marasmioid species collecting fallen plant debris, these diverse ecosystems would severally shift in their functioning.