In 2004, the Catamount Biological Field Station published a research brief by Peter Marchand and Anne Havemann (http://www.catamountcenter.org/research-and-projects/research-briefs/phellinus-tremulae-and-nest-cavity-resources-in-the-upper-montane-forests-of-the-pikes-peak-region/) that reported that aspen tree holes are strongly favored over conifers in terms of nesting sites for birds. In a survey of montane breeding birds they found 14 cavity-nesting insectivorous bird species (29 pairs), all but two of which nested in aspen tree holes, even when conifer tree holes were available. They further discovered that 87.5% of the tree holes studied were visibly infected by Phellinus tremulae, a heart rot fungus that seems to be transmitted from tree to tree by birds, bats, and probably insects. This led Peter Marchand to embark on a separate study of the significance of Phellinus on the health of aspen stands at and around the Catamount Field Station. Peter’s study was apparently never published, but the upshot was fascinating to me.
It all starts with an aspen tree hole, into which Phellinus fungus is introduced, probably mainly by woodpecker beaks that have previously probed and hammered infected tree holes.
The fungus makes the tree easier to excavate as time goes on, so tree hole inhabitants can find nesting spaces. The presence of hungry birds reduces the numbers of moth larvae that feed on aspen leaves. This is good for the aspen!
Thus, aspen, indirectly, seem to benefit from having heart rot fungus to make the tree holes livable. Of course, too much fungus would threaten the whole aspen stand. But having some trees infected is apparently beneficial to the degree that they attract the insectivorous birds. Balance is everything. The 2004 study found that about 12% infection by heart rot fungus provides an adequate number of tree holes to host hungry birds to provide protection for the aspen stand.
Benefiting from a parasitic fungus seems unusual to me. It would be like finding that athlete’s foot attracts a beneficial microbe, so humans would maintain a significant infection rate to gain the benefit. Are there parallels in our species? I mean, we have mutualists galore. But what about “beneficial” parasites?
The first feedback loop is a simple relationship between aspen and the herbivores that defoliate and damage it. As moths increase, aspen decrease. Bad for the aspen!
But, with the introduction of the Phellinus fungus, aspen find an indirect benefit due to the increase in tree holes and their nesting birds, which depress moth populations. Good for the aspen!
Remember that the decrease in moths creates an increase in aspen. It might seem that since the nesting birds positively affect the aspen, they would positively affect the fungus, the tree holes, and themselves. So what prevents a runaway, reinforcing loop for the tree-hole-nesting birds? I imagine, predators:
This balancing set of effects seems to create a dynamic advantage to all species involved, and maintains ecosystem health.