It’s a warm summer evening. The sun is low, firing the tops of the birches around the loch to burning gold. There’s no wind, and gnats dance in three-dimensional mobiles in front of the hide we’re sitting in at the edge of the water. It’s a Monet scene. Brown trout nose lazily through a maze of lily leaves lying like large dinner plates on the surface of the water. Every few seconds they snatch at a fly; a pointillist flank curves and vanishes.

“There!” A ranger is pointing. Suddenly, everyone is peering intently through binoculars. “Where? I can’t see it.” A woman sounds desperate. “Look for the bow-wave. The long line – there! – under the overhanging branches, heading this way. Got it now?”

We’ve been showing people wild beavers for nine years now. Wild, yes, but captive. Our beavers are in a large, fenced wetland enclosure, a whole eight-acre loch and its associated marshes and deciduous woodland in Inverness-shire – perfect beaver habitat. They don’t know they’re captive, and they don’t need to, since they have everything they want: plenty of food; birch and willow trees to fell for bark – they are strictly herbivorous – and to build their lodges and dams; streams and ditches to dam; canals to dig; lagoons to create. That’s what beavers do: beavering away, they adjust their habitat to suit themselves, and just about everything else as well.

This week, Natural England took the sensible decision to let the free- living beavers on the River Otter at Ottery St Mary in Devon stay where they are, to breed and disperse naturally, while being monitored for the next five years – effectively a beaver trial similar to what has happened in Scotland, officially at Knapdale in Argyllshire and unofficially on the River Tay in Perthshire.

Our setup is different. It’s a demonstration project, designed to show people what beavers are and what they do. Our rangers monitor the habitat through every season with photographs from set positions, and by that delightful every-child-should-do-it business of pond-dipping – messing about with nets and buckets, in and out of boats, shrieks of glee and endless wet feet.

We systematically analyse the catches, count the bugs and beasties: dragonfly and caddis larvae; great diving beetles, tiny fish, water boatmen, worms, flukes and leeches; tadpoles, water spiders and snails; even right down to some semi-microscopic stuff like midge larvae, water fleas, hydra and rotifers. We do the plants, too – some in the water, some out – and the fungi, and the lichens and ferns, and great green hassocks of wetland mosses. Then there’s the wild flowers, such as northern marsh orchids and the insectivorous butterwort – the lovely, lethal, sky-blue flower on a single stem that lurks in acid bogs and tricks flying insects into landing on its sticky leaves. And every spring we do a bird count.

Beavers lower the canopy around a water body by felling trees and digging canals – opening it up. Solar energy piles in to places that have been in shade for decades. They stir life into action, kicking up nutrients as they beaver about their daily doings. Nature loves change; it frees up opportunities. Species of every size and shape wade in and snatch their chances. Beavers shift everything, tirelessly, instinctively, creatively. That’s why ecologists call them a “keystone species”. By doing their own thing, they create habitats and opportunities for just about everything else.

Our nine years of soggy monitoring have demonstrated precisely what the scientific literature predicts. Measured against adjacent wetland the beavers have not utilised, we find that biodiversity has expanded by a factor of four.

That’s a 300% increase on the initial pre-beaver presence: more aquatic bugs for fish to hoover up; more fish for herons, diving ducks, grebes, otters and ospreys; more newts, frogs and toads; more insects for small birds to snatch; more small mammals, including water voles and water shrews, for owls and other predators. And so it continues up the chain: more food for pine martens, stoats, weasels, foxes, badgers; more and a wider variety of wild flowers and wetland plants. Altogether more of just about everything, in a happier, fatter, richer, healthier and more diverse ecosystem bubbling with life and energy.

That, you might think, would be justification enough to reintroduce the beaver, but it’s only half the story. The incidental benefits beavers deliver for free include flood retention, thanks to their small dams; preventing flash-flood siltation, which is a real hazard to young fish; and creating fish nurseries in their lagoons and ponds behind the dams.

When the BBC’s Autumnwatch came to our loch a few years ago, their sneaky night-vision cameras caught an otter in one beaver lagoon hunting for fish and frogs. Round and round it went, up and down, as lithe as an eel, every few seconds surfacing with a munchy, crunchy mouthful. Cameras don’t lie. That lagoon, entirely created by beavers, contained an otter smorgasbord.

And then there is the animal itself. Our project has shown wild beavers to 5,000 people on week-long wildlife programmes over nine years. If you include food and accommodation, petrol, postcards and all the other usual tourist spin-offs at around a grand a week, that’s an ecotourism benefit to the local rural economy of £5m.

‘Beavers have been quietly cohabiting with many species of fish for millennia.’ Photograph: Gareth Fuller/PA

All wetland and riverine ecosystems in Britain should include beavers. That sounds a sweeping statement, until you examine it a bit more closely. First of all, they did – for thousands of years. After the ice cap retreated from northern Europe 12,000 years ago, the Eurasian beaver (Castor fiber) was a permanent member of our mammalian fauna, present in just about every wetland habitat that suited it. Then we put a price on its head and hunted it for its seductively soft fur (excellent for making felt), its castoreum (glandular oil used for medicinal purposes) and its meat. By the reign of Henry VIII, beavers were extinct in England and reduced to a few remote locations in the Highlands of Scotland. We think the last wild beavers were trapped near Loch Ness in the late 17th century.

Now they are back. There could easily be 300 wild beavers in Britain today. Most of them are on the Tay and its eight tributaries, stretching more than 100 miles from Loch Rannoch in the west to Forfar on the Firth. They arose from an initial escape of two females and a male from a wildlife park in 2001, later supplemented by accidental escapees from one or two private collections.

The River Tay is ideal beaver habitat, particularly the upper, less agricultural reaches of its tributaries. It is also prime salmon- and trout-fishing territory. Despite reams of scientific evidence from the 24 European countries that have successfully reintroduced beavers, and the fact that beavers have been quietly cohabiting with many species of fish for millennia, some anglers – and now fishermen in Devon – worry that beaver dams will prevent access by migratory fish, especially salmonids, to some tributary streams.

We await a decision on both the Knapdale and the Tay populations from Scottish ministers: to cull or to chill – or, most likely, to fudge the issue and hope to be moved to another department as quickly as possible. The Tay beavers have already had one stay of execution, when Scottish Natural Heritage dreamed up a spectacularly dotty policy to trap them all, then discovered – oops! – that perhaps there were a few more than they had bargained for, met a barrage of fury from the public and the press and retreated with muddy faces and tails between legs. The only animal they caught promptly died, and the public indignation quotient shot off the scale. It will be a brave minister who risks another fiasco.

Years ago, I knew the River Otter very well. I have fished it and canoed its dreamy Devon reaches. It is right for beavers. They are mammals of optimum habitat. That means they will cruise upstream and downstream until they find the place they like best. Then they will stay, dig burrows in banks and build impressive lodges of mud, sticks and logs dragged and packed into a dome, with underwater entrances to protect their young from predators (they are hard-wired to think there is a wolf, a lynx, a bear or a wolverine behind every bush). Here they will breed, producing two to three kits every year.

When they have eaten all the food easily reached from the safety of water in that location, they will move on. They will disperse, often for miles, until they find the next most appealing place. They may not return to the former site for decades. A beaver very rarely ventures more than 50m from a watercourse, so the only trees likely to be affected are those on riverbanks and around lakes. If you don’t want the cherry trees on your water frontage felled, all you have to do is wrap a 3ft length of wire netting around the trunk – beavers won’t touch it. If they do cause a problem, they are not difficult to remove.

In Norway, I have asked farmers what they think about the beavers on their land. They are both very accepting and very practical. “Oh, the beaver,” they say, “he is there, and this is his home.” And if he is a nuisance? “Oh, then we shoot him and eat him and make a hat for the wife from his fur.” But Britain has signed up to European protection for wildlife, which could block unlicensed hunting. Whatever English or Scottish ministers would like to do, they will have to dream up damage mitigation measures that are legal, publicly palatable and acknowledge the beaver as a native species.

Suddenly, someone shouts. “Look!” Another beaver has popped up among the lilies, right in front of us. Only its head is visible, and its prehensile hands that are delicately plucking lily blooms and stuffing them greedily into its mouth. “Oh my God,” someone mutters, “that’s bloody brilliant!”

John Lister-Kaye is director of Aigas Field Centre (aigas.co.uk). His new book, Gods of the Morning – A Bird’s Eye View of a Highland Year, will be published by Canongate in March.