During my beaver observations I have noticed that beavers most gladly forage along the river channel which is, of course, logical because it is close to safety and trees/shrubs growing near the water (unless heavily waterlogged for extended periods) might be lush and easily regenerating due to the influx of light, water and nutrients.
However, sometimes beavers can venture further from the river channel.
This can happen when beavers have exhausted their riparian resources.
In several beaver colonies, the habitat has been altered (destroyed) by humans and beavers cannot sustain themselves on the remaining resources if shrubs and trees have been removed to grow grass or crops or to build over.
In these colonies (but also in others, for example, in a colony which has appropriated a ditch as its ‘second channel’ to access riparian slope further from the stream – Beaver observations – an interesting family home arrangement), beavers sometimes seek to obtain resources farther from the river channel.
I have observed foraging marks near trails (leading through a riparian forest, approximately 40 metres from water), on tops riparian slopes (> 50 metres from water) etc.
In these situations it appears to me that beavers have sometimes (when they have felt it was safe enough) crossed through the riparian forest to access edge (micro)habitats (e.g., the areas near trails or even near the riparian forest edge).
Henceforth I will use the term edge habitat although at times it might refer to microhabitats such as trails or other openings in the riparian forest that do not constitute a major ecotone but that still alter the amount of light that passes through canopies.
As, historically, beavers might have utilized edge microhabitats created by storms, ungulates, tree disease etc., I feel that these microhabitats are essential to understanding beaver foraging behaviour in natural context even if nowadays they are often the result of human activity.
The trees that the beavers have passed by within the denser riparian woodland are not necessarily unfavoured species (ash, alder, birch, hazel), nor the beavers have always travelled the distance to reach highly favoured species (for example, I do not believe that hazels are better preferred than alders but beavers have let the alders in the denser grove be in order to access hazels growing along the edge of the riparian forest).
I have begun wondering whether beavers might have developed selectivity for edge trees and shrubs.
These woody plants often grow in more open conditions where at least from one side they receive more light than the woody plants deeper in the riparian forest.
Thus, photosynthetic rates might be higher in these trees (perhaps the accumulation of sugars in phloem might reach greater levels, as well).
These trees could also have a higher proportion of palatable organs.
Trees in the riparian forest sometimes tend to grow very tall in order to reach the light in the upper canopy while edge woody plants do not have to thin themselves out to access sunlight.
Therefore there might be a greater number of branches per trunk (because they begin growing at a lower point along the trunk while the tall trees in the denser groves do not have many lower branches).
There could also be more leaves per branch or per trunk volume.
Perhaps trunk volume is not an adequate measurement because beavers, unlike humans, do not attempt to gather timber.
Thus, it might be better suited to assume there could be more leaves and branches per tree diameter category (i.e., thinner trees in edge habitats might have more branches and leaves than thicker trees deeper in the woodland).
Leaf (and twig) quality could be different, as well, due to the enhanced photosynthetic rates.
Additionally, it would be interesting to discover whether sun leaves and shade leaves have the same nutritional value (according to their respective chemical constitution).
Perhaps there are other differences between the woody plants growing in the riparian forest vs. woody plants in edge habitats that make one or the other more palatable.
These differences could be related to the ease of acquisition, as well.
Edge woody plants can be smaller in diameter and more easily transported without making the effort to fell a large tree which might not offer as much nutritional benefit.
On the other hand, large trees that have been felled closer to the river could be foraged on (phloem) for a longer time if their trunks are bigger and there is more to eat.
It probably depends on the location of the felled tree.
A large tree that has been felled near the water does not have to be transported in order to forage on its trunk while smaller trees/shrubs (or branches thereof) that have been felled in edge habitats far from the river have to be dragged to safety before consumption and the frequency of travel respective to energy benefits gained could determine whether the edge plant is preferred to deeper woodland plant.
Perhaps the preferences change seasonally.
For example, edge habitats might be more attractive when the woody plants growing there are in full leaf while during the cold season when bark is consumed, the choice made by the beaver between larger trees closer to river vs. branchier trees farther from river could be based on other considerations.
Woody plant community composition in edge vs. denser riparian forest habitats could also be of significance.
Perhaps tree/(but especially) shrub species favoured by beavers are likelier to grow in sunnier conditions.
As beavers mainly prefer smaller diameter trees, the proportion if this size category could also be higher in edge habitats.
However, while beavers mainly forage on woody plants < 15 – 20 cm in diameter, they also tend to avoid very small stems (e.g., < 5 cm in diameter).
It is quite likely that edge habitats provide a greater proportion of these beaver-preferred woody plant size categories because mature forests typically consist of large trees + saplings which would befall outside of beaver’s favoured range (too small or too large).
I have made my observations mostly in a rather mature riparian forest habitat which might affect decisions made by beavers because such woodlands predominantly consist of trees that have already established many decades ago + undergrowth that can survive in their shade.
Overall, if there is selectivity for edge woody plants, it could have evolved not merely due to better quality of the forage but also due to beaver’s adaptations to forage digestion and predation.
Namely, beavers prefer to forage near the water and this riparian habitat is as such an edge habitat.
Thus, beavers mostly consume edge habitat vegetation and their digestion might be better adapted to processing these plants (vs. woody plants obtained deeper in forest).
It might not be the superiority of edge forage itself but its compatibility with the beaver’s digestive potential that drives any selection as beavers have evolved alongside edge vegetation.
This evolution has been at least partly determined by predation.
It is dangerous to forage far from the water and thereby beavers have restricted their foraging choices to the water edge riparian species with their respective chemical composition and growth patterns.
Without predation pressure, beavers might still not venture too far in order to conserve energy but they might have ventured father and selected more of the dense grove trees and shrubs.
Thereby, any adaptations that beavers might have for edge woody plants could be the combined result of semi-aquatic lifestyle, energy conservation considerations and predation that disallows to more extensively explore and exploit areas far from water.
I am not trying to state that beavers will forage by the water and on the other side of the riparian forest, namely, that they will readily travel great distances to access edge habitats far from the waterline.
However, under circumstances where the beaver is either suffering resource scarcity or there is some ease of access to edge habitats further from water (e.g., low predation pressure, relatively safe travel routes, proximity of temporary shelter such as ditches or smaller ponds), beavers might pass by the woody plants in the denser groves to access edge species/specimens.
Notes by Ieva 