Burgers' Mangrove (Royal Burgers' Zoo)

Royal Burgers' Zoo in Arnhem has made a speciality of recreating whole ecosystems indoors: after the Bush rainforest, the Desert and the Ocean, the Mangrove is the park's fourth large covered attraction. It has a prehistory. According to Dutch Wikipedia, a first mangrove hall opened in 1992 in a 50 by 20 metre building that had been put up in 1980 as a test hall for the future Bush; its roof, devised by the German engineer Vogler, used plastic cushions three metres by six that were light, insulating and translucent.

Plans for a much larger hall were announced on 6 April 2016, and work began that September, for an investment of around 5 million euros. The setting draws on Belize, in Central America, where Burgers' Zoo and the Swiss butterfly park Papiliorama have jointly protected a 235 km² nature reserve for 28 years; the new hall was meant as an ambassador for the richness of that region.

The official opening took place on Wednesday 12 July 2017, led by director Alex van Hooff and by Omar Figueroa, Belize's Minister of State whose portfolio included agriculture, fisheries, forestry and the environment, and dozens of tropical butterflies were released for the occasion (Wikipedia gives 13 July). The manatees, previously housed in the Bush, gained a pool holding a million litres of water. The zoo chose to let plants and animal populations establish themselves gradually over the years rather than present a finished set on opening day; in its own words, an ecosystem is never complete.

The hall is covered by a double-curved steel dome 60 metres in diameter, rising to 16 metres and enclosing about 3,000 m². Engineered by ABT of Velp, the structure rests on arches of 323 mm steel tube, braced by 193 mm diagonals and carried on HEA 240 columns set 4.5 to 5 metres apart. A tension ring at the foot of the dome balances the forces and avoids costly external buttressing; steel was chosen over timber because it copes far better with the high tensile loads in that ring. Some 250 tonnes of galvanised steel were supplied and erected by Moeskops Staalbouw of Bergeijk.

Between the arches the skin consists of ETFE air cushions supplied by Foiltec, some of them up to 30 metres long, selected to admit as much daylight as possible. To keep the membrane from touching the steelwork, it is fixed to gutters hung beneath the dome on spacers. The climate is set for 80 to 90 per cent humidity, using misting, a heat pump and heat-recovery ventilation. At the pool, a panoramic window 12 metres wide lets visitors watch the manatees below the surface.

The setting reproduces a Belizean mangrove: mudflats cut by saltwater creeks, where a system mimics the ebb and flow of the tide. According to Wikipedia the hall holds Europe's largest collection of mangrove plants; the zoo points out that only about 110 of the world's roughly 380,000 plant species are mangrove plants, and just 40 of them are found nowhere else. Alongside them grow nectar plants for adult butterflies and food plants for caterpillars, typical of the tropical dry forest that fringes Belize's mangroves.

Caribbean manatees are the most imposing residents. They share the water with cichlids and tetras, while fiddler crabs work the mudflats. Free-flying birds (doves, cardinals, tanagers), free-roaming reptiles (basilisks, anoles, geckos) and tropical butterflies, among them morphos, swallowtails and owl butterflies, complete an ecosystem to which the zoo keeps adding new species over time.

The Mangrove stands inside Royal Burgers' Zoo, on the northern side of Arnhem, and is best seen in sequence with the park's three other covered ecosystems: the Bush rainforest, the Desert and the Ocean. Its warm, very humid climate makes it a welcome stop on cold or rainy days, and the planting, left to develop freely since 2017, grows denser year by year. The 12-metre panoramic window remains the best vantage point for the manatees. The Nederlands Openluchtmuseum, the Dutch open-air museum, lies next to the zoo for anyone wishing to extend the day.