-
Rivers are interesting things. They flow inexorably towards the sea, carrying what was once billions of raindrops, in huge web like networks fanning out across the landscape.
They braid and meander, changing their course imperceptibly slowly.
But how have rivers changed over time, and how did this affect the species living within them?
Rivers
In the Cambrian and early Ordovician periods (before around 450 million years ago), the continents were bare. Plants had not yet colonised the land, and without the weathering caused by their roots, there was no soil, only loose rock, gravel, and sand.
Sedimentary records tell us that the rivers at this time were braided: split into smaller streams rapidly shifting across the riverbed, leaving behind characteristic alternating layers of fine grained and coarse grained sediments.
Today, these kinds of rivers are found in the Arctic and high mountains - places where there is no vegetation to stabilise the riverbanks - as well as on beaches. When there is heavy rainfall, the whole channel is submerged and a new pattern of streams appears when the waters subside.
When plants gained a foothold on the land, things began to change. Even the liverworts, diminutive slimy things which you might find on a damp boulder, had an impact on the structure of rivers.
By weathering the top layer of rocks, they created clay. This cohesive mud resulted in more stable riverbanks, which slowed down the sideways migration of the channels. The clay also created soil, which allowed more sophisticated plants to colonise. Their root systems further stabilised the floodplains, forcing braided rivers into more defined and slowly meandering channels.
Shifting mosaic
The next big change came with the origin of trees, which by the Carboniferous period were covering vast areas of river deltas (and, as they fell into the anoxic swamp, forming much of the world’s coal).
Rivers would have often become blocked by fallen trees and piles of debris. Blockages force the current sideways, carving out new channels into the floodplain.
The result would have been a complex shifting mosaic of channels, oxbow lakes, and swamps in various stages of vegetation succession.
It was in similar shallow swamps (although slightly earlier, in the Devonian period), that Acanthostega, thought to resemble the ancestor of terrestrial vertebrates, would have clambered through the waterweed and maybe occasionally wriggled onto land. Today, rivers of this kind can still be found in undisturbed parts of the Amazon.
It’s not just plants that have influenced the changing patterns of rivers, but also animals, and none has had more impact than the beaver (apart from, of course, humans).
Beavers
The first beavers evolved in the Eocene period, around 45 million years ago. By building dams, beavers slow down the flow and promote the creation of new channels as the water spills out sideways. They create mosaics of habitats - ponds, riffles, marshes, wet woodland.
The huge variation in physical environments - speed of flow, the depth and width of channels, size of sediment grains, water temperature - creates niches for many kinds of animals and plants.
Whilst a beaver swamp may appear as one habitat when viewed from our perspective, at a finer scale there are myriad different microhabitats, a huge amount of variation crammed into a small space.
We tend to overlook the effects that living organisms have on their physical world because most of the ecosystems around us have been “downgraded” as we have removed the important species - thus, in these cases it is mainly physical processes that determine how organisms survive.
But there is now an increasing weight of evidence that the interaction works both ways: the earth shapes life, and life shapes the earth.
Freestyle
Beavers’ engineering work benefits many kinds of wildlife: ponds are perfect for frogs and fish larvae, riffles and gravel banks for dippers, swampy areas for water rails and moorhens, dead trees for woodpeckers and owls, and lush coppiced vegetation for songbirds.
The fact that beaver habitat is ideal for so many species should not come as a surprise: beavers were present in our ecosystems for millions of years, so many wetland species may have actually evolved to live in beaver habitats.
Through studying the effects that beavers have on streams, it has become clear that deeply incised river channels disconnected from their floodplain, which we perceive as the norm, are in fact a consequence of the removal of beavers, and other human impacts.
Before we deforested and farmed the land and hunted beavers to extinction for their fur and scent glands, wetlands would have filled the bottoms of valleys, with snaking channels, ponds, wet meadows, and willow scrub.
By bringing back the beaver, and allowing our rivers to freestyle through the landscape, we could revive these incredible ecosystems. Beaver engineered wetlands could fan out into every valley in an interconnected network, like arteries pumping life back into the landscape.
So many other species could flourish in the habitats that beavers create: otters, water voles, marsh tits, spotted flycatchers, lesser spotted woodpeckers, water rails, egrets, lapwings, redshanks. Incredible species which we’ve almost forgotten could return - white tailed eagles, cranes, and even white storks, which last bred in the UK in 1416 but are just starting to make a comeback.
Benefits
Whilst large areas of wild land may always remain distant to most of us, beavers could create pockets of wildness nearby.
The thrill I experienced when squelching through a beaver swamp in Devon was definitely the highlight of the past year. Experiencing these messy, exuberant living landscapes could rewild our own lives and so reconnect us with nature.
This is especially important for younger generations, because people will not care about the living world unless they experience it at a young age. In the words of the lepidopterist Michael Pyle, “What is the extinction of a condor to a child who has never seen a wren?”
A revival of beaver ecosystems would have wider environmental and economic benefits beyond increasing biodiversity and bringing wildness back into our lives. Their leaky dams hold back water in floods, and release it gradually in drought.
By retaining water in the headwaters of catchments where the land is less valuable for farming, they could protect more productive arable land further downstream. As we experience more extreme weather events due to climate change, reintroducing beavers to our river systems could make a valuable contribution to reducing the damage to villages and towns.
Lush swamps
The lush swamps that beavers create have been shown to filter out fertiliser and pesticide runoff, and reduce the washing away of soil to the oceans - something which is currently visible from space whenever heavy rain falls.
As vegetation builds up in the ponds it forms peat, and the carbon that was sequestered by the growing plants is locked away.
We’ve spent thousands of years trashing the complex connections in our living world, and we’ve created ecosystems which are a mere shadow of their former selves.
If there is one animal which we need in Britain right now, it has to be the beaver. The bang for your buck in terms of biodiversity and wider environmental gains is huge.
Beavers transform their world so profoundly that they are like a fully automated tool for ecological restoration. We only have to release them and let them do the work.
This Author
Joshua Harris is a student at Cambridge University, an ecologist, and volunteer with the Beaver Trust.
