You have all probably heard of the Firth of Forth. It is an estuary on the East coast of Scotland where Edenburg is located. The Firth of Tay is smaller and is the next Firth to the north.
The Tay is the longest river system in Scotland an has a huge catchment extending three quarters of the way across to the West coast. The catchment includes, amongst other areas, much of the Cairngorm Mountains to the North.
Catchment area of the River Tay
Scotland is a very green country. Areas which are not in crops are in pasture or forest and along the major rivers, reasonably wide riparian zones* are fenced off from grazing animals.
* Riparian zone - The strip of land along both sides of a river. Keeping this area fenced off from domestic animals and well vegetated preserves water quality and protects adjacent land from flooding. It shades the river, reducing temperature extremes and provides habitat for a variety of wild life and path ways through which they can migrate.
The Scots have a wonderful attitude toward their beautiful country with wind turbines everywhere and a goal of having 100% renewable energy within a decade or two. Despite a virtually 100% vegetation cover in its catchment, the tributary rivers of the Tay are prone to quite serious floods. So what is the status of beavers in the Tay catchment at present.
The beaver population, Castor fiber is increasing by one estimation at at about 26% per year. I gather that this is an estimate based on counting lodges (beaver houses). From the same statistic, it is estimated that as of 2014 before the spring birth of kits, there are about 250 beavers in total. At present, with a couple of exception, the beavers are still in the process of occupying rivers which are too large for them to dam. In such situations, they build bank lodges by burrowing into the bank, usually starting under water and often under a tree.
| A typical bank burrow. Often there is an air hole in the top of the breeding chamber covered with branches |
They burrow upwards on a slope and often cover the location of the burrow with a large pile of cut logs and branches. Typically in both bank lodges and pond lodges there is a wide area just above the water level where the beavers dry off and groom and a higher wide area where the beavers sleep and the kits are born. In a pond lodge, this is a more or less stable situation since the beavers are determining, to a large extent, the height of the water in their pond. River levels, by contrast, go up and down considerably with floods and droughts.
Pile of logs and branches above a bank burrow. The upper chamber of the burrow can extend up into the pile of branches.
Besides lodges, you can detect that beavers are in an area by the felled trees along the shore. Often the tree that was felled is gone and what you see is the stump with teeth marks. You may also see branches with some or all of the bark nibbled off and if you look closely, you can see nibble marks on the underlying wood.
Beaver felled tree. Pond created by dam in background.
Beavers usually cut all around a tree and have no idea which way it will fall.
The origin of the beavers
There is some controversy about the origin of the Tay beavers and I would like to put one such legend to rest. Besides, dealing with it, introduces some information about beavers and their populations.
Some people feel that they were accidentally or purposefully released from Bamff. This perception is enhanced because Paul and Louise Ramsay are two of the staunchest advocates for beavers in Scotland.
When you bring any animal or plant into a new area the odds are very high that the organism will escape its confines. If the 'outside' is suitable it will start to spread. So it is not possible to say that no beavers escaped from Bamff. However, we can say with a very high degree of probability that most of the beavers in the Tay catchment are not from Bamff. There are a number of reasons for this.
Firstly, after many tries, the first breeding success at Bamff was in 2007. Since beaver kits stay with their parents for two years and then look for their own territory in the third year, the earliest any beavers could have left Bamff would have been in 2010. However there are many suitable sites in Bamff and much evidence of colonies which spread from the original colony so let's be very conservative and add one more year. Beavers were most unlikely to have left Bamff before 2011.
Then you have to look at the fencing around Paul's beaver compound. Beavers burrow and would eventually get out of any compound but they would have their work cut out for them to get through this level of fencing. Again let's be conservative and add one more year. We are up to 2012 before the first beavers could have left Bamff. Myself, I would put the time a couple of years later but let's stick with 2012. Here is the clincher.
The first reliable, confirmed reports of Tay catchment beavers were in 1999 on the lower Earn. Since initially beavers are virtually invisible* in a catchment when you don't know they are there and difficult to spot even when you are looking for them, it is likely that some previous reports from well before 1999 were correct but let's just use the later, confirmed 1999 date. This means that the beavers already in the Tay catchment had at least 13 years to multiply and be fruitful before the first Bamff beavers could have entered the catchment.
* It's a little like buying a new car. Suddenly you notice cars of the same type everywhere. Once your eyes are tuned in to the different appearance of a swimming otter and a swimming beaver and once you know what a beaver nibbled or cut branch looks like, you spot them easily.
Let's further assume that in 1999, when there was the first iron clad sighting of beavers, there was only one colony of 5 beavers in the Earn. If beavers were already being seen, it is likely that there were more but again let's be conservative and assume there was only one colony. We will use the figure I have heard for the growth rate of the population of 26%pa. That seems pretty conservative* considering the almost complete lack of bears and wolves which are the main beaver predators, and the fantastic beaver habitat along the Tay and its tributaries. To find the population after 13 years, starting with 5 beavers at 26%, raise 1.26 to the 13th power and multiply by 5. The result is just over 100 beavers in the catchment before the first Bamff beavers could have been recruited to the Tay catchment. Clearly most of the beavers came from elsewhere.
*A colony of beavers consists of two adults, some two year olds, some one year olds and in the spring the kits. Let's assume two surviving kits from each year. Each spring we have two additions to a colony of 6 beavers or an increase rate of 33%. Hence 26% seems a reasonable increase rate. Andrew Kitchener in his excellent book Beavers, records that when introduced to virgin territory, beaver populations increase at from 20 to 34%. As the population increases, the rate slows as beavers come into conflict with each other. Beavers are tied to aquatic territories and repel strange beavers. Populations increase until all the available territories are filled, have a slight overshoot and then settle back to a stable population.
As for beaver escapes from Bamff, I walked the area downstream of Bamff, and despite it being, to all appearances, excellent beaver territory, I couldn't find any beaver trace.
Above, I mentioned that most of the beavers are still in rivers and living in bank burrows. This is not so in Bamff. There are no large rivers running through Bamff. The beavers were introduced to a small lake which was completely fenced in. Now the beavers have started building dams in the little burns and ditches on the property. Here we get a hint of the tremendous benefits beaver will bring when they spread to the minor burns (streams) and seeps of the Tay catchment but first of all, what good are they doing at present when they are largely confined to bank burrows in the major rivers.
Benefits from beavers at present.
All over Scotland we saw uprooted trees. True, these were mainly evergreens (conifers) and very seldom in the riparian zone. In the riparian zone most of the trees are deciduous and beavers cut these trees down. They can easily fell a tree that is a foot in diameter and even larger. At first glance, it seems strange to cite cutting down trees in the riparian zone as a good thing, especially in light of my former comment about keeping the riparian zone vegetated so let's examine this.
When a deciduous tree is felled, it coppices. That is to say it sends out a plethora of branches from the stump. The roots remain and continue growing and holding the banks of the river together during floods. With the top of the tree gone, it is no longer in danger of being tipped over by a high wind or heavy snow fall and is less prone to been torn loose by a flood. A tree that is tipped over by wind, snow or flood is uprooted, exposing the bank to erosion. Our weather is already extreme enough and said to be getting more so. Any move that stabilizes river banks has to be good.
When there is a heavy canopy, along the riparian zone, the understory is suppressed from a lack of sunshine. By cutting down trees, the beavers open the understory to light which grows better and has a stronger, more extensive root system, further reinforcing the banks. I would go so far as to say, if you have a heavy canopy on the banks of a flood prone river and beavers have not yet settled in your area, start a program of thinning the trees. Cut them at waist level or so to leave lots of buds for coppicing. This is even more important if the riparian vegetation is conifers which "chemically" suppress the understory in addition to shading it.
One farmer, who shall remain nameless, apparently thought he would stop the beaver from settling in his area by ripping out all the vegetation on the bank along side his farm. He was attempting to deny food to the beavers and thus stop them from burrowing and thus preserve the stop banks. Nice move!!!
Going back to the evergreens, common cause has it that beavers very very rarely cut down evergreens. To the contrary, we saw many evergreens felled by beavers. Below is a picture of the needles of one of these trees. It was tentatively identified as a Norwegian Pine. This brings me to one of my favorite hobby horses.
Evergreen felled by beavers. Possibly a Norwegian pine.
Beaver Detractors
It is an odd fact that some anglers and some fisheries biologists are against the introduction of beavers. I actually met a fisheries biologist some years ago in Prince Rupert on the coast of British Columbia who was on his way to destroy a beaver dam. He thought that it was blocking the migration of salmon upstream. I posed a question to him. I asked him, "How come in the days before the European trapped the beavers to near extinction and when there was a beaver dam in every possible location where one could be squeezed, the salmon runs were orders of magnitude greater than the present runs". He still went out to break up the beaver dam but hopefully it got him thinking.
Actually, the modern biologist could be forgiven for thinking that beaver dams retard the upstream migration of salmon. Not only does our modern biologist spend an inordinate amount of time writing grant proposals, if he gets the grant, he has to write numerous reports on his work. And he is unlikely to get a grant just to study the behaviour of beavers. He has to measure something. When he gets out in the field, he has to quickly collect data.
Suppose he is studying the flora or fauna in a beaver pond and in the stream above and below the pond. He only has a day or two in the field because of the expense of staying somewhere which is coming out of his grant and all his time is spent collecting samples and preserving them for examination back in his lab. As he is working, he observes salmon in the plunge pool below a beaver dam and jumps to the wholly understandable conclusion that the dam is stopping the migration of salmon. He hasn't got time to just sit quietly beside the beaver dam and observe.
How much less he is inclined to sit beside the dam during and after a period of Scottish sunshine which tends to fall from the sky in buckets or at night when the beavers are active. And following a rain event is the time when salmon tend to migrate upstream. Rather sensible adaptation on their part when you think about it. When you have a rain, the stream swells. If the beaver dam is not quite full, it fills and the water trickles or flows over the top. This triggers the migration urge of the salmon and points the way to the rest of the stream. From a salmon eye point of view, it must be a bit puzzling to know which bank of the plunge pool to jump over until a trickle or a stream of water points the way.
I'm surprised at the assumption by biologists, who have undoubtedly seen movies on National Geographic of salmon vaulting great water falls, of thinking that a sex crazed salmon on his once in a life time act of procreation will be stopped by a wee beaver dam. Besides as mentioned, before European hunting, beaver dams were ubiquitous in streams and salmon runs were huge.
Likewise, anglers could be forgiven for jumping to the same conclusion. They are out there fishing and see beavers and their works from time to time. If they aren't sitting beside a beaver dam when the salmon pop over them, they could be forgiven for thinking that the fish they see in the plunge pool are stuck there. Besides, anglers tend to be sensitive to the word Dam, not differentiating between a hydo dam with no fish ladder and a beaver dam. It is not surprising they are suspicious. They have spent a huge amount of time in the past removing old tires, car bodies and so forth from their streams. With that mind set, it is hard to conceive that anything in a stream is beneficial for fish. For instance, when a tree falls into a stream, the automatic response will be to get out the chain saw and remove it. How many anglers realize that "Big Wood" in a stream is actually beneficial to the salmonoid family.
Biological science has changed since the days of the old time naturalist and not entirely for the better. The old style naturalist was often a parson of some little church in the countryside with much of his time to do with as he pleased. Or he was member of the landed gentry, also with much time to spare. They developed interests. Some recorded the date of first bud burst in a variety of plants in their environment or the date a migratory bird first appeared. This is now giving us invaluable data on climate change as we observe the present day situation. Others would record everything they could observe about the behaviour of a species of bird in their garden. Even Darwin, himself, studied the earthworm for much of his life with special attention to how much soil they brought up to the surface each year.
There are still people who spend a lot of their time observing nature but they are few and far between and rarely have doctorates. Their observations are often dismissed by main stream biologist as not being "scientific". I met three of them in Scotland and here are a couple of odd observations they told me about.
In one location in the Tay Catchment, there was an otter den close to a beaver den. The beavers took exception to the proximity of the otters and walled up the otter den. I've never heard of this behaviour.
In another incident, an otter was attacking beaver kits. The mother beaver grabbed one of the kits in her mouth and the other one climbed on her back. She she took off underwater, taking them to safety. An otter has to be suicidal to tackle an enraged mother buzz saw with her razor sharp chisel teeth that can cut down large trees. Predators tend to tackle, in so far as possible, prey that isn't likely to damage them. After all, they don't have the National Health system to patch them up.
An otter would have to be suicidal to challenge an enraged mother beaver that can cut chips like this out of a tree with each bite.
I mentioned that beaver dams don't impede the upstream migration of salmon but this is not entirely true. If a beaver dam has just been built in certain types of location and a plunge pool has not yet developed and especially if the flow of water in the stream is particularly low, salmon my find it hard to pass a dam. Fortunately the beavers themselves help to solve the problem of low flows. When they have populated a catchment, they hold water during high flow events and release it during droughts. Streams that used to be intermittent, flow all year round, One has to have the foresight and patience to wait until beavers have populated a catchment to see the full benefits. If you jump to conclusions when the first beaver dam is built on a stream and destroy it, you will never see the benefits beavers bring.
Benefits of Beavers to Fish
However, it would be hard to justify beaver dams just on the fact that they don't impede the migration of salmon and trout. The really important reason for beaver ponds vis a vis fish is that they are fantastic nurseries for fish. Rather than typing the whole story again, have a look at this site. In point form, though, Beaver ponds:
* catch twigs, wood chips leaves and so forth which powers a cellulose based detritus cycle which feeds juvenile salmon
* catch spent adult salmon in the fall and incorporate their nutrients into the pond ecology and ecology of the surrounding area - also feeding juvenile salmon when they hatch out in the spring.
* increase the total amount of salmon habitat by turning seasonal streams into perennial streams and providing perennial ponds.
* clear the water of silt making the habitat more acceptable for salmon and trout and allowing light down to the bottom of the stream so that water plants can root and grow.
* provide deep water where predatory wading birds can not operate
* provide many nooks and crannies around the lodge and dam where small fish can hide.
* provide quiet water so that the energy the fish takes in with its food is used for growth instead of for fighting currents.
* evens out stream temperature.
The Atlantic Salmon
Now a word on the magnificent Atlantic salmon Salmo salar. His biology is extremely variable within and between catchments so anything I say here will have exceptions.
The Atlantic salmon reminds me in many respects of the Sockeye of the Pacific North West where I hale from. His Latin name, Salmo salar, means 'the leaper'. He is aptly named. Atlantic salmon are known to swim at least 200 miles upstream to spawn and for the most part they die after spawning. Some, however return to the sea and are called Kelts. Juveniles generally spend two years in fresh water but have been known to remain for as much as 8 years before returning to the sea. Some even remain land locked and never go back to the sea. Just like Sockeye, some populations use a lake as if it was the ocean and migrate upstream to spawn. The young return to the lake to grow. Landlocked Sockeye are called Kokane. In North America, land locked Atlantic salmon are called Quananiche. I don't know if they have a special name in Europe. Atlantic salmon tend to stay at sea from one to three years. Atlantic salmon have been caught weighing as much as 45kg. If there was ever a fish that would benefit from extensive beaver dams in the catchment, this is it.
Atlantic Salmon leap great water falls in a single bound.
As mentioned, beavers at present are mostly confined to the rivers but with a growth rate of 26%, soon they will start to migrate up smaller and smaller streams. Much of Scotland has high vertical relief so their ponds will tend to be narrow with their long axis along the flow of the stream. Many of the smaller streams do not have a vegetated fenced off riparian zone making it sub-optimal habitat for beavers. If after consideration, you want to encourage beavers to take up residence in your area, fence off a 5 or 10 meter wide zone on either side of your stream and truncheon* in a wee forest of willows.
Beaver also eat grasses and sedges.
Truncheoning
* Cut up a deciduous tree into piece a couple of hand spans long. Larger logs you can split into 4, small branches cut up with pruning shears. Sharpen the bottom end of the larger logs and simple pound them into the ground along side the stream. For the smaller twigs, use a steel bar to punch a hole and drop them in and heel. You can do this even by little steams that only flow when it rains. You will be amazed where beavers can create a pond and change a seasonal stream to a perennial one.
A beaver dam in high relief country creating a long narrow pond. Dam is vegetated with the roots further strengthening the dam. Plunge pool on the right gives the salmon purchase to leap the dam.
Long Term Benefits from Beavers
We now get to the long term benefits from beavers as they create dams throughout the catchment. We have already touched on their effect on fish populations. They also mitigate flood and droughts in downstream rivers. They store water , not only in their dams, but in the raised water table around their dams. They increase the wetted area of a stream, increasing infiltration into the ground and by holding the water on the land, give it time to infiltrate. All this water, held on the land during high steam flow events is released slowly over time. Water that is stored underground is not prone to evaporation and flows slowly back into the streams. Over the years as a deeper and deeper sponge of material is produced by the beaver dams and as some become wetlands, the flood mitigation effect increases.
Five of 12 dams that the beavers built in this little ditch. Each dam will fill up and overflow in a rainfall event, holding water on the land and both delaying and decreasing the flood peak. More water will be directed into the ground to later appear during times of drought.
This is a man-made dam which holds back a very large pond/small lake. All the beavers had to do was to dam up this little gulch to raise the water level in the pond. All done for free with no need to bring in a back hoe.
Streams that have never flowed continually, now flow all year round increasing the total area of the catchment available for fish. Floods which used to occur with rain events of a given magnitude now don't occur. Rivers flow clearer because beaver ponds settle silt out of the water and catch bed load. in addition to decreasing flood peaks, beaver populated streams have delayed flood peaks. When the flood peaks are fed into the downstream river in a staggered fashion, the flood peak in the river is further decreased.
Over the long term, with the dams collecting silt and inter-layering it with organic material caught in the dam, they create rich bottom land.
As the beaver ponds fill up with silt and organic material, and as sedges and grasses grow in from the edges of the pond, the beavers may continue to raise the dam or, if there is no wood close by, abandon the site. Later when the trees have recolonized the area, a new colony of beavers will take up residence and often build their dam over the old dams. Thus rich bottom land is accumulated and the stream takes on a terraced appearance.
Note that fish that spawn and die above the dam will be caught by the dam and add to the nutrient stock of the area. This may explain the observation by one biologists that he found fish tangled up in a beaver dam. He thought they had got tangled on their way upstream, a mistake the old time naturalist would never have made!!! The creation of bottom land is a long term process and won't be apparent until your children or grandchildren take over the farm. beavers abandon sites from time to time and a beaver pond then becomes a wetland with all the known benefits they bring. Later, another colony of beavers will take over the area and another layer of rich organic sponge will be added, further mitigating down stream floods and droughts.
Beavers in the catchment of a hydro electric dam even increase the amount of electricity the dam can produce and the amount of water available for irrigation. Water which has to be let out over the spill-way during high rain fall events, simply flows down to the sea. By reducing flood events and increasing water flow during low rainfall, a hydro dam become more effective. Beavers also lengthen the life of a dam by catching the sediment that eventually fills up all dams and the bed load which makes deltas into the hydro dam, further decreasing its water holding capacity.The increased water flow beavers create tends to be in the summer when rainfall is lowest and when irrigation water is most needed. The floods they mitigate tend to be in the spring when rain and melting snow pack swells the streams.
And here is an odd thought. Beavers can help us charge our electric cars as they begin to penetrate the car market. Most hydro-electric dams have generating capacity that far exceeds the amount of water available from the upstream catchment. This is because hydro-electric dams are used for supply balancing. They must be able to draw down the water in their lakes when there is a high demand for electricity and let the lakes fill up again when demand is low. Any water you can divert from the spillway through the generators for some useful purpose is money earned by the dam operators and the cheapest form of electricity supplied to the customer. By evening out the flow of water into the hydro lakes, more of the water can be used to generate electricity. This is part of the answer to people who say electric cars are not practical because of all the extra generating capacity we would need. In hydro-electric dams the generating capacity already exists. We just have to make more efficient use of the water flowing into the hydro lake. Beavers facilitate this.
Beavers also greatly increase the diversity of life in an area. Instead of just a forest or pasture with a stream running through it, you now have some ponds as well and a meadow that may extend as much as 50 meters from the pond in which, with the shade removed, shrubs and forage plants grow. A wide variety of beneficial birds and insects grow in ponds which don't prosper in streams. Sedges and grasses grow in the shallows, food for deer and other wild life and these terrestrial animals spread nutrients, which were caught by the beaver pond, up slope in their dung. The dam itself grows many varieties of water plant that the beavers themselves use as food, reducing the pressure on adjacent trees. The whole area is enriched. The following link is of a beaver eating lilly pads.
https://www.youtube.com/watch?v=eT9BdZeYZPA
Beaver Damage
One of the problems with beavers vis a vis people is that the so called damage they do is visible, immediate and in your face. The good they do is long term and may not even be perceived by humans. You notice when a flood washes a bridge or your house away. You don't notice when a similar rainfall event doesn't wash away your assets. Only a hydrologist with records of rainfall events vs floods would realize the same or greater rainfall event didn't cause a flood.
Beavers do fell trees but as was mentioned at the beginning, this is not always a bad thing. And if you have some "must-be-potected" trees within 50 or so meters of a beaver colony, put a little chicken wire around them. Cheap and effective.
Tree protected by chain mail mesh. Chicken wire can also be used.
Beavers can also cause flooding. Again the solution is simple. Take away some logs and branches from the middle of the dam, Lay a piece of pipe of a suitable diameter in the cut, extending the pipe into the pond. Drive in a U shaped frame of wood to hold the pipe down . Problem solved. The pond will remain at whatever level you have determined. The beavers will repair the damage that night. Once more cheap and effective.
A beaver deceiver. Simply a pipe inserted into a dam to limit the level to a given depth. Used when there is the chance that the pond might flood something that one doesn't want flooded. Note the beginning of a plunge pool below the dam.
The inner end of the pipe is protected by a frame like this.
.
This is one way to protect a culvert. A beaver deceiver can be added if necessary. You might ask, if such a big culvert is needed to take water during flood event, how can the pipe of a beaver deceiver carry this water. The point is that when the water courses upstream of this culvert are populated by beavers, floods will be a thing of the past.
The benefits from beavers are so great that these minor inconveniences are worth solving and pretty inexpensive.
PS
Lastly a word about rewilding, not from me but from George Monbiot. I put this in because many people in Scotland, amongst them beaver enthusiasts, are determined, in so far as possible, to bring back the exact variety of Castor fiber that existed in Scotland before it was extirpated. The Norwegian variety seems to be the favoured one. I can see where they are coming from but I would be inclined to bring some beavers from all over Europe, introduce them into various catchments and see which variety does best in an ecology which is nothing like it was 400 years ago, never mind a thousand years ago. Further more, let them breed together when they meet and with this greater available genetic pool, develop a beaver by natural selection which is most suited to Scotland. George Monbiot, it his book Feral, p8 expresses it much better than I could.
"So young a word , yet so many meanings. By the time 'rewilding' entered the dictionary, in 2011, it was already hotly contested. When it was first formulated, it meant releasing captive animals into the wild. Soon the definition expanded to describe the reintroduction of animals and plants species to habitats from which they had been excised. Some people began using it to mean the rehabilitation, not just of particular species, but of entire ecosystems; a restoration of wilderness. Anarcho-primitivists then applied the word to human life, proposing a wilding of people and their cultures. The two definitions of interest to me, however, differ slightly from all of these.
The rewilding of natural ecosystems that fascinates me is not an attempt to restore them to any prior state, but to permit ecological processes to resume. In countries such as my own [UK], the conservation movement, while well intentioned, has sought to freeze living systems in time. It attempts to prevent animals and plants from either leaving or - if they do not live there already - entering. It seeks to manage nature as if tending a garden. Many of the ecosystems, such as heaths and moorland, blanket bog and rough grass, that it tries to preserve, are dominated by the low, scrubby vegetation which remains after forests have been repeatedly cleared and burnt. This vegetation is cherished by wildlife groups, and they prevent it from reverting to wood-land through intensive grazing by sheep, cattle and horses. It is as if conservationists in the Amazon had decided to protect the cattle ranches, rather than the rainforest."
by the by, have a look at George's TED talk on rewilding.
PPS
To get an idea of what Scotland can expect as beavers spread, get a book by a fellow Brit, Eric Collier titled Three Against the Wilderness. It talks about the state of the High Chilcoten in British Columba before the beavers were brought back and the effect they had. Incidentally, I was asked in Scotland why beavers cut down more trees than they need at the moment. Or to put it in Darwineze, what is the adaptive advantage to a beaver of felling more trees than it needs for it's dam, lodge and food cashe. Eric found the answer during the floods of 1948. It's in his book.
1 comment:
Beaver activity sighted on Kingjamesv1 golf course which is situated on Moncrieffe Island , Perth,Scotland. After recent flooding a number of trees have been felled by what clearly looks like beaver. A bit of a surprise to locals.
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