This is not as much theory as it is a contemplation on the possible effects that the provisioning of optimal (and actively and/or regularly inhabited) dens/nests of mammalian predators or birds of prey might bear on the occupied tree’s health and regeneration.
The reason why it is not a theory lies with the very dynamic factors that are at play in each separate case.
I am not certain that generalizations could be drawn easily rather than inferred on the grounds of each case study.
If a predator of small birds, mammals or insects lives on a specific tree, this tree becomes a risky habitat for the prey populations.
Mostly, I refer to natal dens and nests rather than resting sites that are only used for brief spells although some species tend to exhibit resting site loyalty, as well.
There should be a prolonged presence of the predator species on the tree or the presence should be observed during a crucial period (e.g., seed production, seed dispersal etc.).
If the use of den/nest sites is temporal (only a few days or a week) or the sites are used on a rotational basis, the prey species would adapt to the predator’s presence and the impact of the predator might be too negligible to cause substantial effects (however, it might be of importance, for example, if the predator remains resident on the tree during the flowering season which is relatively short etc.).
The first inclination is to assume that the predator presence would be beneficial because trees provide resources for many herbivores and, perhaps more importantly, seed-eaters.
As a result, the soil seed bank or seedling densities might increase with the predator around but this is only relevant in tree species that do not rely on small birds/rodents to disperse their seeds.
Also, small birds and rodents tend to forage on buds limiting flower and seed production.
The tree might experience productivity release upon the conditions of protection provided by the localized predator that these small species attempt to avoid.
However, small prey species also ensure important regulatory services such as pest control by consuming the invertebrate populations.
If the tree hosts predators naturally, this might already attest to its losses in structural integrity (the forming of cavities).
Injured trees would likely be subject to invasions of pests and pathogens that require regulation.
Trees inhabited by small birds/rodents might exhibit greater longevity due to reduction in the decomposition/consumption rates by much smaller herbivores.
Meanwhile, trees hosting predators of the small prey might turn into deadwood at a faster rate and demonstrate increased levels of invertebrate herbivory on the canopy level, as well (alongside the associated defense mechanisms that the tree must invest in at the cost of vegetative/reproductive growth).
However, tree roots might be spared by small rodents resulting in greater nutrient and water cycling efficiency (with the additional benefit of significant fecal/urine depositions by the large predators as well as remains of prey species that decompose and reach the root level).
Deadwood and decaying trees are as important as regeneration.
Thereby, these effects are not really contrasted as positive vs. negative.
I would rather be curious to follow these proceedings and to see how the presence of the predator might lead the tree toward health, producing new generations or deterioration.
For example, the age of the veteran trees could be estimated in a local tree population in order to determine if predator den/nest site trees deteriorate at a younger age (smaller diameter at breast height relative to the tree species etc.).
Else, the soil seed bank or sapling bank could be quantified to determine whether the host tree is represented at a rate that exceeds the average values for the respective tree species.
Nutrient assimilation and defense metabolite concentration rates could be analyzed.
Other factors could be studied that currently do not come to mind.
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It might also be interesting to study how an active predator den/nest affects prey species habitat use surrounding the den area and the respective changes in plant/fungal communities (e.g., on the tree and on the ground level).
Notes by Ieva 