Brownsea is a safe haven for more than just red squirrels

Most people know the stories of how vast numbers of native Americans died in the early days of European exploration as they encountered for the first time, and were defenceless against, diseases brought by the newcomers. We don't hear so much about biological disasters coming the other way across the Atlantic, but they happened. It's how most of England ended without its native squirrel.

The distinctive red creature, with its tufted ears, intelligent eyes and diminutive frame, is today more familiar from Beatrix Potter illustrations than live observation. That's thanks to imported American grey squirrels who, in less than a century, spread from a curiousity in one aristocratic park to the dominant breed scrambling through trees across the country. I'd always assumed it was because the immigrant grays, who are three times the size of the reds, either killed their hosting cousins or stole their food. It turns out there was no rodent genocide. The grays simply carried a disease that doesn't bother them, but quickly wiped out the reds.

My fresh awareness is thanks to a guided squirrel walk on Brownsea Island, one of the last refuges of the native animal. We'd decided on the National Trust property in Poole Harbour as something new and different to do on a gorgeous autumn day before my visiting father headed back to the States. Expecting little, we were blown away ... not just by the squirrels but by the secret garden quality of the whole island and the unexpectedly magnificent sweep of Poole Harbour.

The communities around the harbour host some of Britain's priciest real estate, and one look at the view on a fine day makes it obvious why. This is the second-biggest natural harbour in the world after Sydney, but it's surprisingly devoid of commercial shipping or large buildings. None of the ports in the harbour are big enough to host cruise ships. Instead, this massive stretch of water ... an inland lake but for a narrow mouth opening onto the Solent ... is populated by pleasure craft, yachts and an archipelago of small, mostly privately-owned islands. The shores to the north and east are fringed with charming small towns (Poole foremost amongst them), sleek blocks of modern flats and mansions torn from the pages of architectural magazines. To the west, the Dorset hills fill the horizon, still the wild, green expanses of Thomas Hardy novels. It's spectacular.

Poole is a pleasant place to poke around in, with a higher-than-average number of independently owned boutiques and restaurants thanks, no doubt, to the higher-than-average income of local residents. If you're making for Brownsea, it's notable as being the main departure point for the harbour ferries that will get you there. (You can also depart from Sandbanks, but it's further from the motorways you'll probably use to get here.) There's plenty of room in a multi-story car park to abandon your vehicle for the day, but drivers of larger models should proceed with extreme caution. This is a vintage facility from the era when Brits drove tiny cars; every column and tight ramp entry is marked with the paint of vehicles that didn't judge the tight spaces correctly. Including mine.

Try to get here by noon so you can grab lunch in one of the restaurants along the High Street. There is a National Trust cafe on the island but the selection is very limited.  Alternatively, pack a lunch and take advantage of the island's abundance of gorgeous picnic spots. The ferry costs £11.50 and is not part of island admission, so even National Trust members need to pay up. It goes direct from Poole Harbour to Brownsea, which is a quick 10- to 15-minute hop. On the way back, however, you'll continue on a clockwise circumnavigation of the harbour with some informative commentary, so it's a pleasure cruise as much as practical transport.

A Victorian faux-castle and a range of suitably baronial outbuildings welcomes you as you arrive on the island, but they're some of the only signs of "civilisation". The castle is a private retreat run by the John Lewis partnership for its employees. The National Trust cafe occupies the old stable and some waterside lawns with spectacular views. Other outbuildings house a small gift shop and a couple of holiday cottages. Beyond is 500 acres of woodland, lawns, heath, salt marsh and cliff walks. At just 1.5 miles long and 3/4 mile wide, it's small enough to cover in a day but big enough to offer variety and ... at least on a weekday in October ... to find yourself alone in glorious woodland solitude.

With its salt marshes, Brownsea is a haven for bird watchers. It's also a sacred spot for the Boy Scouts because Baden Powell started the organisation with camping trips here. Scouts still regularly come to stay. But it's the red squirrels that draw most of the crowds. Though visitors should be aware that it's possible, and even common, to spend a day here without seeing any. Red squirrels are small (about seven inches tall without their tail and weighing in at about 10 ounces), fast and reclusive. Fortunately, the National Trust offers daily squirrel safaris with knowledgeable guides who will take you to the spots where you're most likely to catch a glimpse of one of the 200-strong population. By pure circumstance we ended up on the island in peak squirrel-spotting season, when the population is hard at work finding and caching nuts for the winter. We saw five on the group walk, then I returned on my own to commune with a few more.

A stand of beech, hazel and pine forest behind the island's church was the week's hot spot. It's woodland from a fairy tale, dense enough to provide shade and a springy carpet of loam, but well-spread enough to allow glimpses of dappled sunshine and long views through tree trunks to distant clearings. I perched on a log and opened my ears as well as my eyes; you can hear the squirrels scampering through the leaves before you spot them. There was a magical moment when one paused atop a fallen trunk less than 6 feet from me and looked me straight in the eye, but I can't prove it. By the time I raised my camera, he was gone.

We wandered around the island for a couple of hours. With warm summer weather, well provisioned and with a good book, you could lounge here for hours. It's not just a refuge for squirrels. If you want to disconnect from the modern pressures for a bit, this haven feels much further away from the "real world" than a 15-minute boat ride.