Undergraduate Highly Commended: Living rivers

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?

A deep history of 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 diagnostic 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 and decaying organic matter created soils, which allowed more sophisticated plants to colonise. Their root systems further stabilised the floodplains, forcing braided rivers into more defined and slowly meandering channels.

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 swamps, forming much of the world’s coal. Rivers would have often become blocked by fallen trees and piles of debris, which 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 in shallow pools. Today, rivers of a similar kind can still be found in undisturbed parts of the Amazon.

It is 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). The first beavers evolved in the Eocene period, around 45 million years ago.

Beaver ecosystems

Beavers are herbivores and mostly eat the young shoots of trees, leaves, bark, and aquatic vegetation. In small streams, they build dams to raise the water level and so enable easy access to their food without leaving the water. Flooding the surrounding land encourages the growth of willow trees, one of their favourite foods, and by coppicing these trees the beavers maintain a supply of new shoots. Their ponds also serve as a food store: in summer, they cache branches at the bottom of the pond to eat in the winter. Because the engineering of their environment is so crucial to the beavers’ own survival and reproduction, it is even argued that beaver dams are part of the beaver’s extended phenotype.

Not only are beavers’ engineering works fascinating in their own right, but they create huge benefits to other species. Sequences of dams create deep, slow flowing sections interspersed with faster riffles. Dams force water sideways onto the floodplain, creating braided multi-channel systems, and this is sometimes helped by the beavers digging canals to get to nearby trees. Some trees behind the dams are killed by the rising water levels, whilst other species regrow after being coppiced by the beavers. The huge diversity of physical environments at different spatial scales – speed of flow, shape of channels, sizes of sediment grains, water temperature, air temperature, sunlight penetration, density of vegetation – creates many different niches which can support a huge number of species.

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, owls, and bats; and coppiced vegetation for understory plants. In fact, many of these species may have evolved to exploit the habitats created by beavers, so it is no surprise that wetlands with beavers support more life than those without.

More generally, we tend to overlook the effects that living organisms have on their world because most of the ecosystems around us have been “downgraded”, and we perceive what we know as the norm. In many places, most of the highly influential species, which are often the largest, have been lost. Now, physical processes and simple food chains are the main factors at play, but in systems which retain a huge diversity and abundance of lifeforms, and species which engineer their environment, living organisms have profound effects not only on other species, but also the flows of energy and nutrients, and even the physical geography. Even some scientists use almost poetic phrases like “ecosystem metabolism” to talk about the functioning of the biosphere.

Through studying the sedimentary record in floodplains it has become clear that the river structure which we perceive as the norm – a single channel and dry floodplain – is in fact a recent anthropogenic feature. Before we hunted beavers to extinction (for their fur and scent glands) and canalised rivers to protect agriculture and enable navigation, river systems would have filled the bottoms of valleys, with snaking channels, ponds, wet meadows, and willow scrub.

By bringing back beavers and setting aside un-farmed corridors along rivers, we could revive these incredible ecosystems. Beaver ecosystems could fan out into every valley in an interconnected network, like blood vessels pumping life back into the landscape. So many other species could flourish in the habitats that beavers create: otters, water voles, lesser spotted woodpeckers, spotted flycatchers, frogs, water rails, egrets. Long forgotten species could return, like black storks, which thrive on the abundance of amphibians in beaver wetlands.

Optimism and a functional future

Although seriously conserving biodiversity will ultimately require a reduction in the amount of land which we use for agriculture, reintroducing beavers could have huge ecological benefits with relatively little impact on farming, if an appropriate management system is in place. Furthermore, their wetlands can filter out some of the runoff from agriculture, and attenuate flash flooding which damages both farmland and people’s homes.

Beaver wetlands could also rekindle our sense of wonder at the natural world. Squelching through a beaver wetland in Devon, I found it humbling that this entire ecosystem had been created by another species. Experiencing these living systems would create hope and optimism at a time when the state of the natural world seems to be always in decline.

For thousands of years, humans have been untangling the complex web of life, yet now, armed with an unprecedented understanding of the living world, we can start to put things back together. Reintroducing beavers could create wonderful living corridors along our rivers, with huge benefits to both nature and society. They are like a fully automated tool for ecological restoration, and we only have to release them and let them do their work.


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Cite:

Harris, Joshua “Undergraduate Highly Commended: Living rivers” ECOS vol. 42(1), ECOS 2021, British Association of Nature Conservationists, www.ecos.org.uk/undergraduate-highly-commended-living-rivers/.

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