Deadwood is extremely important both in terrestrial and aquatic ecosystems (see, e.g., Wood Wise: Life in Deadwood by The Woodland Trust, 2019).
In riparian systems, deadwood (log jams and natural or artificial beaver dams) has been used as a river health restoration tool (Wheaton, J.M. et al., 2019).
I have been considering another aspect of deadwood potential in riparian systems which is that of providing a travel corridor for individuals of various species that either seek to cross the river on their daily routes or that are attempting dispersal.
For such purposes, fallen trees (connecting both banks) but also deadwood jams or structures like beaver dams might be of importance.
Removal of such elements or reduction in their efficiency (for example, river channel erosion can result in banks that are too high for the trees to effectively fall across the stream connecting the banks completely or, on other occasions, the tree might get stuck too high above the water which might be perceived as a risk factor for some species that would otherwise use the trunk to cross the river).
My deadwood-connectivity theory is related to biodiversity and population (including metapopulation) maintenance in riparian systems where deadwood might serve as a path of dispersal or as an opportunity to exploit larger home ranges (or to exploit them more often – for example, avoiding becoming wet in colder seasons or avoiding the risk of having to cross ice that might be too thin due to warmer winters etc.) when the trees have fallen over the river or when they have become jammed, dammed in some segments of the river.
Truthfully speaking, I was inspired by this Twitter account which often posts trail cam videos of species crossing an oak tree which has fallen over a river (e.g., this footage).
In the specific video, three species can be observed (river otter, red fox and badger).
These are species that can cross the river swimming, as well, but the use of the fallen tree is not limited to these species (many species using the oak might be microscopic or very small).
Additionally, even if an animal can cross the river by swimming, some species are unwilling to swim if it can be avoided and many species would prefer remaining dry to save energy, for example, in autumn or winter.
As mentioned before, there might be many species that cannot swim and that could possibly benefit from logs that have fallen over rivers thus connecting both shores.
This could expand their home range (enabling them to use both banks of the river or to use them more often) and it could be even more beneficial during their dispersal and colonization processes.
Rivers can pose obstacles and there are species (e.g. invertebrates) that cannot really cross rivers or that cannot cross them readily.
Deadwood can offer bridges (in form of fallen trees or jams/dams) but it can also create shallows, for example, where the water is pushed out to the sides widening the river width etc.
It is possible that, historically, when deadwood (and stones and other natural obstructions) was not removed from the rivers, such elements might have enabled dispersal in even larger species (e.g., read about gray wolf range expansion limitations in Spain due to the presence of River Duero Artery which possibly halted recolonization to the south of the river for 15 years – Blanco, J.C. et al., 2005).
While these are wide rivers, large deadwood accumulations that compile downstream, might result in safer passages or at least slow down some stream segments enabling the individuals to walk over the formation or to swim in shorter bouts or in slower water.
I have seen many videos where animals cross, for example, rather long beaver dams (e.g., this video by Voyageurs Wolf Project).
Retention of riparian deadwood might assist the colonization of suitable patches on both banks in species big and small.
Removal of deadwood might also affect the species home range by reducing facilitation of movement and on some occasions, animals might even come into conflict with humans because they cannot expand their home range on both banks and thus, rather than exploiting more adequate wild or semi-wild habitats, they are forced to move closer to human settlements where problems ensue.
This could be a seasonal issue, for example, when individuals can use the entire range during summer when the water level drops but are limited in winter when the water is deep, cold and when the ice does not form due to the increasingly warm weather.
Additionally, non-swimming or poorly swimming species might not be able to disperse across rivers in winter if these species spend the winter in hibernation or torpor (e.g., badgers, hedgehogs).
There might be even some plant species or fungi, or (micro- or macro-) invertebrates that colonize the deadwood first, and the opposite bank as a consequence.
Not only mammals, invertebrates etc. could benefit from deadwood-provided connectivity routes, but also plants, fungi, bryophytes, lichen etc.
I cannot find any research dedicated to riparian deadwood as habitat connector.
If these aspects of deadwood functionality have not been researched, this could be an interesting topic to explore.
Notes by Ieva 