Oostvaardersplassen is an area of marshland that was the last to dry out when the vast Flevoland polder to the east of Amsterdam was slowly drained over the 1950s and 1960s.

Waterbirds moved in and migrating birds found a haven here, and the original decision to create an industrial site over the 5,600 hectares between the new towns of Almere and Lelystad was reversed.

There are now around 100 bird species that breed here and 25 more that have been sighted by the many birdwatchers that gather at vantage points on the outskirts of the wetland with their telescopes.

Access is restricted. Much of the area is open to rangers only, but guided tours of small parties along defined walks are permitted.

The habitat is maintained in its current condition - grassland interspersed with reed beds and wetland - by hardy horses and cattle turned out to graze the vegetation.

Frans Vera, whose submission to the relevant authorities was crucial to the establishment of Oostvaardersplassen as a conservation area, is proud of his achievement.

“Through Oostvaardersplassen the greylag goose has recolonized the Netherlands after vanishing completely in the 1920s,” Vera says. The geese moult here - an indication that they feel safe - as for around a month they are unable to fly.

The wetland has the largest colony of spoonbills in Europe - some 300 breeding pairs - and thousands of cormorants and barnacle geese.

“There are now around 100 breeding pairs of great white egret, which disappeared in the 19th century, and they are having an impact on populations throughout this part of Europe,” Vera says.

The “boom” from 50 male bitterns can be heard, and a similar number of rare marsh harriers have set up home.

And another iconic bird - the osprey - has put in an appearance in recent summers.

“There is a pair trying to breed,” Vera says. “There is a pair of white-tailed eagles displaying, so they may breed. They have been back over the past four years for the first time since records began.”

Among the more unexpected species sighted - vagrants to the birdwatching fraternity - are black vulture, common crane, red kite, snowy owl, flamingo, raven, penduline tit, common rosefinch and honey buzzard.

Disturbed balance

But in crowded northwestern Europe, conservation - even in this man-made form - leads inevitably to controversy.

Animal lovers are worried about the 31,000 large mammals kept to graze the vegetation. There are three species: Red deer, which are naturally wild, Konik horses and Heck cattle, also called aurochs.

The latter two are “re-created” - bred back to recover the original wild species from which our cattle and horses are descended.

The Dutch national conservation authority, the Staatsbosbeheer, which manages Oostvaardersplassen, does not provide supplementary feeding when grazing becomes scarce at the end of the winter.

There are good reasons for this. Feeding disturbs the natural dynamic of the ecosystem, which has developed a balance of its own.

During winter the animals slow down and tend not to breed. Feeding them would reverse this, creating a vicious cycle, in which feeding leads to more animals, which leads in turn to more feeding.

And artificial feeding generates stress within the herds, which have developed a social structure akin to that of wild animals.

Vera, an expert on grazing animals, says the Konik behave much like zebra, each stallion maintaining a harem, while the Heck cattle have formed herds with a structure like that of the Cape buffalo.

International significance

He describes how rangers spot animals that will soon die. “They are separate from the herds, on their own, and they hardly react when you approach them - resigned,” he says.

The rangers shoot these animals in line with what they call a ”predator model.”

Last winter 787 animals died, with around 65 per cent being shot. This year the dying will go on into early April.

Vera laughs at the idea of introducing wolves to control the grazers. “Oostvaardersplassen could in theory support 150 wolves. If I were to suggest that, the politicians would send for the men in white coats,” he says.

“The politics is not ready for it. This has to do with people’s mental maps,” he says.

The ecologist notes that a recent study conducted over the vast plains of the Serengeti in East Africa shows that large mammal populations are controlled, not by predators as is popularly believed, but rather by food availability.

“Large herbivores tend to starve rather than get preyed on,” he says. In March the Dutch courts agreed with this approach, throwing out a case brought by the animal protection society, Dierenbescherming, which said that the non-feeding policy was cruel.

Policy in this unusual conservation area is under constant review.

A committee of experts from all over the world is to report to Agriculture Minister Cees Veerman in June on how best to manage the large mammals.

But Oostvaardersplassen is now a fixture in this man-made landscape. It is a Dutch natural monument and it is also listed as a wetland of international significance under the Ramsar Convention.