Insects around the world play prominent roles in ecology. They belong to a group of organisms that are much, much older than any mammal, so they have had a longer time evolving specialized relationships. Besides pollinating the majority of flowering plants, and being a critical trophic component in ecosystems all over, these flying and crawling insects spread the spores and mycelia of fungi that decompose wood. Without them, decomposition rates would slow and there would be a larger reservoir of nutrients left inaccessible to each ecosystem. Generally speaking, there would be a decline in species diversity if insect biomass is reduced.
Through the vast expanses of time, trees have evolved a hardy, resinous, protective layer that protects their vulnerable wood from hungry fungi and insects. We know that layer as bark, and it’s better at keeping fungi out than insects. Although a single fruiting body can produce billions of spores a day, those spores float around aimlessly, being carried around by wind. Insects on the contrary, actively disperse to new substrate with purpose and intensity. Fungi have paired up with species within the class Insecta time and time again for this reason. The insects are better adapted at penetrating the protective bark to get to its yummy interior-a place insatiable for saprotrophic fungi.
Oak and maple trees with their protective bark overlooking the Genesee River.
To better understand the relationship between saprotrophic fungi and insects, in 2015, a long-term study was carried out by Rannveig Margrete Jacobsen, Tone Birkemoe and Anne Sverdrup-Thygeson. Sixty aspen trees were killed on two separate landscapes, and using insect trapping methods they quantified the different species flying to and from the pile of woody debris. They did this for the first four years, and at the end of the fourth year they analyzed the fruiting bodies growing from the wood. At year twelve, fungal growth was quantified again so the researchers could see the interplay of insect and fungal succession through time.
A staggering species diversity was seen in the first four years after aspen tree death. A total of 552 beetle species were collected of which 277 species were wood-living beetles associated with deciduous trees! Unlike the initial insect diversity, the first fungal registry at the end of year four only quantified 14 species of fungi. Though, at year twelve, the fungal richness growing on aspen wood increased to 62 species.
Top view of Glischrochilus quadripunctatus
This study showed some rather noteworthy ecological trends. First, they found a positive correlation between the bracket fungus Ganoderma applanatum and the abundance of fungivorous beetles. The wood decomposing G. applanatum was more likely to be found fruiting out of dead aspen when the fungivorous beetle Glischrochilus quadripunctatus was abundant during the first four years of colonization. These insects actively transport the fungus to suitable substrate, faster and more efficiently than spore dispersal via wind.
Prediction of G. applanatum presence as a function of G. quadripunctatus abundance.
The small yellow cup fungus Bisporella citrina was positively correlated not with the presence of fungivorous beetles, but with the presence of wood-boring beetles. Unlike the last interaction, the wood-boring beetles are not transporting B. citrina spores, but are creating more habitat for the fungus. The aspen trees that had a greater abundance of wood-boring beetles, also had more bark removed. B. citrina is sensitive the antifungal properties of bark, so if you google image the fungus, you will hardly see it fruiting from the resin filled plant structures. Instead, you will almost always find the species fruiting from exposed outer wood beneath. In this instance, the wood-boring beetles physically alter the substrate, resulting in some saprotrophic fungi like B. citrina to take full advantage.
Bisporella citrina growing on wood with bark removed.
Wood-boring beetles reduce bark cover. Jacobsen et al. 2015.
Bisporella citrina is enhanced with the presence of wood-boring beetles. Jacobsen et al. 2015.
Wood-boring and fungivorous beetles are critically important to the succession of saprotrophic fungi. The decomposition of woody substrate takes several decades, even with species rich assemblages of fungi and insects intact. Any reduction in species diversity or insect biomass may alter decomposition rates and ultimately shift ecosystem functioning on a grand scale. A new paper published just a few days ago revealed an alarming decline in insect biomass in Germany. Using another long-term study, Caspar A. Hallmann and his team found a 75% reduction in total flying insect biomass in just 27 years. These results are disturbing to say the least.
Flying insect biomass reduction through time. Hallmann et al. 2017.
There is no doubt that insects play critical roles in ecosystems across the globe. In this post, I really only mentioned their influence on saprotrophic fungi. They help fungi colonize woody substrate by transporting their spores, and structurally enhance their habitat. Flying insects facilitate fungi that carry out a diverse array of functions crucial ecosystem resilience and resistance. Together, we need to help maintain conservation efforts and rally against political movements that place science on the backburner. Could this new, alarming study get the ball rolling for environmental advocacy? Or is it too late? Regardless, the health of Earth's ecosystems is at stake, and their longevity hangs in peril.