Without telling any of its partners, it chose to develop an upmarket housing estate on the land.
Only 4% of Gauteng province is formally conserved, and the global target is a minimum of 10%. Very little of the natural grassland is left, and mansions clutter the ridges.
One of the last remaining unspoilt ridges, home to the Walter Sisulu National Botanical Gardens, forms part of the municipal boundary between Johannesburg and Mogale City. The plan was to extend the gardens and make them part of a 2 000-hectare reserve that would join up with the Cradle of Humankind.
The extended reserve would ensure the protection of several species of animals and an orchid (Brachycorythis conica transvaalensis) found nowhere else on Earth.
The South African National Biodiversity Institute paid R500 000 for a feasibility study and business plan for the proposed Roodekrans Urban Biodiversity Reserve. The city would be responsible for implementation, and pledged 150 hectares of its own land for the reserve.
The Gauteng department of agriculture and rural development also supported the project in documents that the Mail & Guardian has seen.
The first part of the South African National Biodiversity Institute report was completed in 2011. But the public consultation phase ground to a halt after Mogale decided to use its land for a housing estate.
A draft copy of the report said: "The ridge is regarded as largely irreplaceable from a provincial conservation viewpoint."
It allows species to migrate through the area, spreading biodiversity. Without these ridges they have to move through urban areas, said the report.
The department also lists the area as "irreplaceable" in its conservation plans, and the biodiversity institute has recommended "no further loss of habitat" in the area and for it to be formally protected under the National Environment Management: Protected Areas Act.
"Establishing a reserve will not only conserve important biodiversity, it will enhance the growing local tourism economy and create a special place for people to experience nature in an urban environment," said Anthea Stephens, the director of the South African National Biodiversity Institute's grasslands programme.
"There has been a great deal of support, and hopefully one day we will see a biodiversity reserve on the Roodekrans ridge."
The proposed housing estate will have a large office park and nearly 3 000 houses, but the Mogale scoping report says the only problems with this "preferred option" will be a possible "negative visual impact" and more traffic congestion.
No mention is made of the environmental impact. The option to have no development and leave the area wild is dismissed because it would lead to the "loss of valuable income to the municipal authority".
Arthur Albertson, a member of the Proteadal Conservation Association, said the municipality's desire to create a larger ratepayer base was trumping environmental concerns.
"If it goes ahead it will decimate the ecosystem. The remaining wildlife will disappear and the orchid will most likely become extinct."
By authorising this, regardless of the chances of species extinction, a dangerous precedent was also being set for future developers, he said.
Although the reason Mogale chose houses over the reserve is not clear – the municipality did not respond to answers in time – its own environment department has opposed the development, saying the ridge is rare and is needed to continue links between different ecosystems.
"The use of the property should rather be aligned to the holistic vision of the area, and not in isolation thereof," it said in a letter to the developers. It also noted that conservation would create ecotourism opportunities.
The final environmental impact assessment for the development is at an advanced stage, its progress having overtaken the proposal for the reserve. Mogale City has, however, already signed a joint venture agreement with the developer. This will make it responsible for outside infrastructure while the developer does everything inside the estate.
With the area conserved under various environmental laws, it is up to Gauteng's agriculture and rural development department to give the final stamp on any development.
But other developments – Rangeview2, for example, which sits on a ridge next to Proteadal – have gone ahead and been fined afterwards. In that case the department fined the municipality R830 000. Construction there has stalled.
For Tanya Becker, head of the Black Eagle project, the development is just another example of how construction has swept across any nature in the area.
The black eagles in question are confined to a small area in the gardens and are endangered thanks to Johannesburg's westward growth.
They are diverting from their normal behaviour and the new development will cut them off from many of the animals they hunt.