Nearly 2,000 invertebrate species – from predatory wasps to explosive beetles – flourish in this unique nature reserve in the ruins of an oil refinery on the Thames

Around a roundabout in the Thames estuary, beyond a Morrisons supermarket and just off an old concrete road, stand great hummocks of bramble, scrubby hawthorn and silver birch. An old car seat is dumped in tangled grass amid the ruins of an abandoned oil refinery.

These desolate flatlands, behind where the old Thames does flow, do not look like a biodiversity hotspot. But Canvey Wick is a “brownfield rainforest”: one of Britain’s most precious nature reserves, it is an atmospheric, accidental landscape that teems with some of the country’s rarest insects.

Nearly 2,000 invertebrate species have been recorded here and more are being discovered every year. There’s the shrill carder bee, one of Britain’s rarest bumblebees, and the declining brown-banded carder bee. Three insects previously thought to be extinct in Britain have been rediscovered here. There are fearsome predators (the bee wolf, a brutal-looking wasp), exploding bombardier beetles and ingenious parasites such as Hedychrum niemalei, a wasp that lays its eggs in the burrow of the rare five-banded weevil wasp, so that its larvae can steal the wasp grubs’ food.

Facebook Twitter Pinterest Summer visitor ... the marbled white butterfly.

It is a cool, grey autumn day and I’m hoping to see some of the rare treasures of Britain’s first and only bug reserve with the help of entomologist – insect expert – Rosie Earwaker. She works for nature conservation charities Buglife and the RSPB, which manage this unique place. (It is owned by another charity, the Land Trust, which bought the site in 2012.)

“It’s a bit of a mecca for entomologists,” says Earwaker as we walk through strong metal gates, designed to deter the motorcyclists who break in and tear around this wilderness. Inside, there’s a profusion of what almost resemble rooms: rough meadows of long grass, glades sheltered by sallow clumps, reed-filled ditches and huge, dark circles of asphalt.

Canvey Wick was once flat, green grazing marshes, rather like West Canvey marsh, a conventional RSPB reserve up the road. Then humans wrecked the place – and in doing so turned it into something much more marvellous. It was used as a dumping ground for sediment extracted from the Thames, so the meadows were covered with gravel, sand, chalk and fragments of old shells. In the 1970s, the ground was prepared for an oil refinery; 13 circular pads of asphalt are the foundations for huge, cylindrical tanks. But the oil price crash of 1973 meant the refinery was abandoned before it opened.

This history has created a wide variety of soil types and habitats in a small space. It is more like a savannah than a rainforest: hot and sunny, not swamped by dark trees. Ecologists call this “open mosaic habitat”. I put my hand close to the crumbling asphalt; even on a cool day, the surface is warm. Reptiles – such as slowworms and common lizards – thrive in such conditions, as do rare insects. Newly arriving species from the continent enjoy the estuary’s low rainfall, too.

Facebook Twitter Pinterest ‘It was used as a dumping ground for sediment extracted from the Thames’ ... an asphalt pad built for the ill-fated oil refinery at Canvey Wick. Photograph: Alamy

The first insect to land on me is a mosquito. Not a good start. Next, we stroll through the meadows and carefully lift a small square of black roofing felt that has been (deliberately) left on the ground. “Bombardier beetles hide under things like this,” explains Earwaker. These striking beetles, which have metallic, blue-green wing cases, detonate an explosion inside their abdomen if disturbed, spraying hot liquid at attackers, whether insect or human.

We don’t find any bombardiers, but Earwaker detects a flying insect and, with a decisive swish of the net she’s carrying, catches one of Britain’s 5,000-plus species of parasitic wasp. “I wouldn’t be surprised if this is a new species,” she says. It is black and spindly, with four bright-orange front legs and two black rear legs. Often rare and beautiful, parasitic wasps do no harm to humans, but skilfully parasitise other insects.

Autumnal Canvey Wick is surprisingly floral. There’s a great deal of narrow-leaved bird’s foot trefoil underfoot, plus clumps of sea aster, a mauve, daisy-like plant that is the sole food of the sea aster mining bee. This rare bee builds a waterproof lining around its underground cell, so it can survive an unexpectedly high tide.

The site has a random mix of exotic plants, non-natives that have arrived here as ship stowaways or through other unfathomable accidents. Fennel and pale-yellow evening primrose stand tall beneath a broken lamp post. There’s even a succulent (a house plant) sprouting on an old bridge.

Facebook Twitter Pinterest ‘They really do have a high-pitched buzz’ ... the shrill carder bee.

Particularly useful for rare bees are the lurid-pink flowers of everlasting pea. We scour a patch and spot a dainty bumblebee. It has a dusting of primrose-yellow colour and a distinctive square of black on its thorax: it is the shrill carder bee. This extremely rare bee doesn’t forage far from the nest, so it needs nearby flowers and, crucially, late-flowering plants. Unusually for a bumblebee, it can still be seen in mid-October, when its queens require nectar to survive. Earwaker beckons me to listen to their buzzing. “They really do have a high-pitched buzz, which you can get a feel for, which explains their name,” she says.

A green woodpecker cackles in the trees and a ship – larger than I expected – slips up the Thames, just the other side of the sea wall. A sand hill is covered with miniature volcanoes, made by the hairy-legged mining bee, also called the pantaloon bee. Hairs on the females’ hind legs help collect pollen and really do make them look like they are wearing baggy trousers.

Many insects are disappearing by autumn, hunkering down for the winter, but we still see the last meadow browns of the year, a silver Y moth and a brown-banded carder bee. There is even more to find at Canvey Wick in June, a time of orchids, and July, when marbled white and wall brown butterflies jink over the reserve. Summer days are also when the bee wolf strikes, a wasp that pounces like a raptor on honeybees.

Canvey Wick is at its most atmospheric in autumn and winter, though. This hybrid edgeland, which blends the gorgeous bubbling call of curlew with the beeping of a reversing lorry, is replete with surprises. It is a hopeful place, too, showing how nature bounces back – and how we sometimes possess the vision to protect it.

You can find Canvey Wick next to the cemetery at Northwick Road, Canvey Island, SS8 0PT. The car park is open from 9am until 5pm/dusk; the nearest railway station is Benfleet (from there, take the 21 bus to Morrisons)

• This article was amended on 1 November 2017. An earlier version said slowworms and common lizards were invertebrates. This has been corrected to reptiles.

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