The curious case of Britain's wildlife revival

It's been one of the few "good news" stories of the summer: otters can now be found in every English county.

This elusive mammal has made an extraordinary comeback since the dark days of the 1970s, when it almost went extinct in England and Wales and could only be found in a few remote corners of Scotland. Nowadays otters can be seen on rivers all over Britain, including such unexpected places as the River Tyne, in the very centre of Newcastle.

The reason for the otter's post-war decline was simple: pollution, coming on top of decades of persecution. The Tyne was the worst offender, with millions of gallons of untreated sewage pouring into the river every single day, turning it into a vast septic tank.

The solution to the problem was equally straightforward. Once a sewage treatment scheme had been installed, the clean-up could begin. Within decades the Tyne had become one of the best salmon fishing rivers in Britain.

And with the returning salmon came one of their most effective predators, the otter. The resurgence of these two iconic creatures is not the only success story for British wildlife. Birds of prey such as the buzzard and sparrowhawk have also seen a boom in numbers in the past couple of decades. Their cousin, the peregrine falcon, has seen an even more spectacular upturn in its fortunes.

After almost going extinct as a British breeding bird in the 1960s, the fastest creature on the planet can now be seen in the middle of London, with nesting birds on Tate Modern regularly amazing passers-by with their aerobatic hunting displays. City-centre peregrines can also be seen in Manchester, Derby, Bristol, Bath and Exeter – indeed most British cities now have at least one breeding pair.

As well as these comebacks, there are new arrivals to our shores. Some, such as the red kite and beaver, have been given a helping hand through reintroduction schemes; others, like the little egret, have colonised from the south as a result of climate change; and a few, notably the wild boar, are here by accident – the animals escaping from farms and now thriving in what was once their native land.

The three main drivers of these comebacks and colonisations are legal action, habitat creation and climate change. Changes in the law have reduced both deliberate and unwitting pollution; with the banning of the agricultural chemical DDT in the early 1980s coming just in time to save the peregrine which, as a predator at the very top of the food chain, was especially vulnerable.

Persecution has also been reduced, both by legal means and by changes in attitudes. A handful of misguided landowners and gamekeepers continue to target birds of prey, mistakenly blaming them for everything from the reduction in numbers of red grouse to the decline in our songbirds. But as the regular presence of buzzards, kites and sparrowhawks in our skies shows, the shotgun has fallen out of favour in much of Britain. Meanwhile, my own part of the country, the Somerset Levels, is rapidly turning into the best place in Britain to see a range of long-legged waterbirds.

Three species of exotic-looking egrets – little, cattle and great white – are now regular visitors to this watery wonderland, having colonised Britain from continental Europe in the past two decades.

Their arrival is a result of the two other factors driving the rise in fortunes of many of our wild creatures. These are climate change, which has allowed these birds to extend their ranges across the Channel in the first place; and habitat creation, which provides them with a place to breed when they arrive.

The ambitious new policy of landscape-scale conservation – also known as "rewilding" – is currently transforming a landscape once devastated by intensive agriculture and peat digging into one of the best places to see wildlife in Britain.

As well as egrets, there are also bitterns and a small population of cranes, reintroduced to their ancestral home by a partnership between the Royal Society for the Protection of Birds, Wildfowl & Wetlands Trust and the Pensthorpe Conservation Trust.

It would be tempting, given this litany of success stories, to take the Panglossian view that all is for the best, in the best of all possible worlds. But climate change is itself, of course, a double-edged sword. Even as southerners welcome continental arrivals such as cattle egrets, the large tortoiseshell and the Queen of Spain fritillary, so in the north of Britain wildlife on the southern edge of its global range is beginning to retreat northwards. Rising temperatures are bad news for specialist mountain creatures such as the ptarmigan, mountain hare and Britain's only alpine butterfly, the mountain ringlet, because as temperatures rise they simply have nowhere to go. Meanwhile, on our offshore islands, once vast seabird colonies are being devastated by a shortage of their staple food of sand eels, which are also retreating northwards at a frighteningly rapid rate. These amazing seabird spectacles – memorably described by conservationist Roy Dennis as "Britain's Serengeti" – may well disappear in the next decade or so.

Even when climate change does offer opportunities, not every species is able to take advantage of them. So while some butterflies, such as the comma and peacock, are currently spreading north into Scotland, others, such as the wood white and high brown fritillary, have been unable to do so. This is because they are confined to ever decreasing fragments of specialised habitat and cannot make the leap across what to them is hostile territory.

Indeed, vast swathes of the British countryside are now virtually a wildlife-free zone. Chris Baines, the man who invented wildlife gardening partly as a way of providing an urban refuge for our declining rural wildlife, has wryly observed that the best way to improve the biodiversity of an arable field is to build a housing estate on it.

Post-war farming's quest to squeeze every last ounce of yield per acre, encouraged by our own insatiable desire for cheap food, has meant that the wildlife that used to thrive on farmland is now disappearing.

In my own lifetime we have lost over two million breeding pairs of skylarks, grey partridges have vanished from much of lowland Britain and last year there was just a single sighting of that classic farmland bird, the corn bunting, in my home county of Somerset.

If things are bad for these resident creatures, what hope is there for those that run the risks of the twice-yearly journey to and from their African winter quarters? Migrants such as the cuckoo, spotted flycatcher and turtle dove have disappeared from many of their former haunts, as documented in Say Goodbye to the Cuckoo, a poignant epitaph written by The Independent's environment editor Mike McCarthy.

The sound of the cuckoo is now just a memory to many of my generation and may never be heard by our children and grandchildren.

So the picture is, clearly, rather more complex than the good news stories might lead us to believe. And yet there is still cause for optimism.

Organisations such as the RSPB, Wildlife Trusts and National Trust – along with smaller groups like Butterfly Conservation, Buglife and the BTO – are doing all they can to give a helping hand to Britain's wildlife at this time of unprecedented change.

They, and TV programmes such as Springwatch, are also encouraging even more people to get out into their local neighbourhood and discover the wonders of the wildlife on their doorstep. Localism, first championed by the 18th century Hampshire vicar Gilbert White in his celebrated book The Natural History of Selborne, is well and truly back in fashion.

As one of White's modern-day disciples, author and naturalist Mark Cocker, has noted: "He redeemed the word 'parochial' from its sense of narrowness and limitation; he exalts the parish as a place where all life exists and we can follow in his footsteps."

I have spent the last year or so writing an account of the natural history of my own country parish, deep in the heart of the Somerset Levels.

During this time I have encountered a wonderful range of wild creatures: some common, some rare, but all fascinating. Two images have stayed longest in my mind: the leaping of wild hares on a frosty March morning; and the unexpected appearance of a hummingbird hawkmoth – a scarce visitor from the shores of the Mediterranean – on my buddleia bush in July.

For me, each of these encounters summed up the wonder of Britain's wildlife: one long-established creature of the countryside, the other a newcomer, but both equally special.

And they reminded me that, whether you take an optimistic or a pessimistic view of the future of our natural heritage, there is always something to surprise us, and to enjoy.

Stephen Moss's book, Wild Hares and Hummingbirds: The Natural History of an English Village, is published in September by Square Peg, price £14.99.