Ecological purists would argue that all 'exotics', or alien trees or plants, should be removed from Johannesburg's public spaces - but the evidence proves that exotics do have a role to play.
Delta Park, some 10 kilometres north-west of the city centre, is a 104-hectare green lung, mostly planted with exotics. "Delta Environmental Centre is living proof that exotics can co-exist happily with indigenous vegetation," says Geoff Lockwood, ornithologist, resident manager and education officer at Delta Park. In fact, if all the exotics were removed from the Bird Sanctuary at the park, there would only be about eight trees left, says Lockwood.
Since its inception in 1886, Johannesburg has been actively planted with exotics like oaks and planes, which still line many of the city's suburban streets, making the suburbs look like an urban forest. These exotics are better suited to pavement planting than indigenous trees, which have thorns that can hurt passers-by and root systems that interfere with drainage. They are also not easily pruned.
There was a period in Delta Park's history when it was believed that indigenous trees would not grow in the veld. So in 1941 a plantation of poplars was started in the park. They have highly invasive root systems, and are very thirsty trees.
But on the other hand, the exotics have created a dense and lush habitat for birds, which will eat whatever fruit or seed the exotics produce.
"The Chinese Elm provides seeds for doves, and the eucalyptus trees attract bees, which attract the bee-eating birds. Chinese Privet bushes produce purple berries which the Rameron Pigeons eat," says Di Beeton, executive officer at the Park.
In the last two decades numerous indigenous trees and shrubs have been planted, particularly around the Delta building.
Delta Park was established in 1934 as a waste water treatment plant, at the time one of the most advanced treatment plants in the world. But as Johannesburg grew, its capacity became too small for the city, and in 1963 the plant was moved to Diepsloot. The main building was constructed during a period when Johannesburg was experiencing a rush of Art Deco construction, and it has a simple elegance in its Art Deco style.
After the plant was relocated, the building and surrounding open spaces became run down and overgrown, and in 1975 the municipality planned to demolish the building. The founder of Delta Park, Norman Bloom, persuaded the city authorities to lease the building to him at a nominal rental. A Section 21 company was registered and the SA Nature Conservation Centre came into being. In 1992 it changed its name to the Delta Environmental Centre.
The first years saw many generous Johannesburgers succumb to Bloom's drive and determination and give of their time and money, and the centre was opened in June 1986.
One of the first projects was the establishment of the 10-hectare Florence Bloom Bird Sanctuary on the eastern boundary of the Park, around 500 metres south of the main building.
Nowadays the centre plays a major role in educating school groups from schools in the broader Johannesburg area, but is also increasingly training teachers, specifically in the environmental aspects of the country's new outcomes-based Curriculum 2005.
"We also run projects up in Limpopo and North West provinces, and train students for an environmental diploma, run by Rhodes University and Rand Afrikaans University," adds Lockwood.
The Centre, says Lockwood, has had a partnership with Rand Water since 1992, which has resulted in the Water Wise Education Team, an environmental education project which has seen 8 000 learners participate in and benefit from the project.
A water wise garden was built in the Park in November 1999, with the aim of capturing and using surface water. A sensory trail garden beyond is zoned with plants with similar water requirements. The point is to demonstrate that a water-wise, indigenous garden can be attractive.
The sensory garden caters for the disabled, with paths that accommodate a wheelchair and braille notice boards. Visitors are encouraged to smell and touch the plants, and listen to the bird calls.
Some 200 species of birds have been sighted at the centre and its surrounding parklands. Several small mammals live in the park - genets, shrews, hedgehogs and mongooses.
Certain species spotted at the centre are traditionally bushveld species - the Gabar Goshawk, the Green Pigeon and the Rameron Pigeon. "The Rameron Pigeon is normally found high up in trees, and is very skittish, but it has been seen here happily feeding on the road," says Lockwood. There are now 500 of these large, blue-black birds with yellow beaks and feet, in the centre.
Lockwood has been resident at the centre for over 20 years, and when he first arrived he noticed a pair of Spotted Eagle Owls living in the park. Over four seasons they failed to raise a single chick as every nest they established failed. He decided to build them a nesting box placed on top of the three-storey building.
Two nights after it was installed the owls came to inspect it, and moved in a short while later and laid three eggs. Lockwood monitored the chicks, eventually winning the trust of the mother owl to the extent that she allowed him to remove a chick to show his school groups before replacing it safely in the nest.
Over the following 13 seasons the pair raised 13 sets of babies, but last August the mother was chased from the nest by a pair of Egyptian Geese, which had been eyeing the nesting box for years.
The centre has environmental and educational exhibits and displays, including an informative display on the Lesotho Highlands Water project, a coal development display, a display of the geology of the central Witwatersrand, and a fig tree exhibit with its root structure visible, positioned inside an old tank. A number of dioramas depicting various ecosystems have clear educational value.
A special exhibit is an elephant bird egg, with a volume equal to seven ostrich eggs. This flightless bird, extinct from the early 1600s, lived in Madagascar and was thought to stand three metres tall and weigh 450 kilograms.
There are various examples of domestic water-conserving technology. A calabash basin consists of a basin that sits above a toilet cistern. The water from hand washing runs down to fill the cistern. There are models of dual-flush toilets with two handles. The small handle flushes four litres of water, the large handle flushes 12 litres of water. These are available in some hardware stores.
The Centre's preparations for the World Summit on Sustainable Development, to be held in Johannesburg in August, include raising R75 000 to re-fence the Bird Sanctuary, which has holes in places. So far R75 000 has been raised for the fencing, and once the full R150 000 has been raised, two donors will re-vamp the two hides in the Bird Sanctuary.
Although most of the trees in the Park are exotic, some of the exotics around the dam in the Bird Sanctuary need to be removed, to encourage water flow.
"It would be wonderful to have the Sanctuary ready for the Summit. We also need to push back the poplar and wattle growth around the Sanctuary, to open areas up to planting indigenous trees," says Lockwood.
Working for Water, a government initiative started by then Water Affairs Minister Kader Asmal in 1995, aims to optimise the potential use of natural resources and eradicate alien species, particularly from river systems, in the process creating thousands of jobs. It has promised that it will start in early May to remove some of the trees close to the water's edge.
"I am optimistic that the fence will be up by August, after all, this is listed as a flagship park by the City of Johannesburg," adds Beeton.
Further action in preparation for the Summit is the removal of exotic hyacinth growth which at present covers the dam in the Sanctuary. The City has promised that it will begin a chemical control programme in early June to remove the hyacinth.
This involves a portion-by-portion programme in which the plants are poisoned in sections, as the poison starves the fish of oxygen as much as it does the plants. Doing it in sections prevents the eradication of the fish.
Johannesburgers have continued to be generous to Delta Environmental Centre - it runs on sponsorship, with no direct funding from the municipality, with the help of the Friends of Delta Park, major corporates, concerned environmentalists and the local community.
"None of us is too small or too weak to make a difference to our environment," says their booklet.
The goose saga
The Egyptian geese which share the nesting box with the owl have a problem getting their goslings to the ground. Normally they nest in trees or other high places and the goslings drop down to the ground like parachutes, with a fluffy landing.
The nesting box is on a side wall of the three-storey section of the Centre and there is a two-storey section five metres below it. One morning the goslings had jumped from the nest and landed on the two-storey section, which is surrounded by a wall too high for them to circumvent. But there is a rainwater gutter on this storey, and the mother shepherded the goslings along the gutter until they reached the six-metre downpipe.
They needed some persuasion to jump into the downpipe, but soon landed unharmed on the lawn below, except for one gosling that landed with its wing outspread and its beak wide open. It soon recovered and joined its mother and the other goslings going down to the Bird Sanctuary.
The next season Lockwood contacted the SABC who filmed "the great exodus". A viewer subsequently phoned the Centre to offer foam mattresses to soften the landing.
This goose saga is repeated every year.