Landscape architect Kim Wilkie has designed the new garden for the museum that replaces much of its existing wildlife garden.
Forum head Steve Head said: "While some aspects of the wide-reaching proposals are sensible and constructive, we on the Forum Board are dismayed that the wildlife garden (which is a unique creation of several important lowland British habitats) will be lost. In the 20 years since its creation, studies by museum scientists have found upwards of three thousand species in its small area, and it has been (and should continue) as a case study in evolving urban biodiversity. It is the only living part of the Museum's display to remind us of the importance and appeal of live biodiversity."
The museum's website states: "Outside the west of the Museum the existing wildlife garden will be expanded to cover a space three times its current size. This dedicated green space will create sustainable new habitats and a place to feel closer to nature in the city.
Head added: "This statement overlooks the destruction of existing functional habitats, which are being replaced by a designer-driven area of isolated architectural terraces and a large circular pond. The area managed for biodiversity will be more than halved and the suggested expanded area will be park-like in form."
Head said: "The latest design has retained a little more of the present garden, indicating there may still be room for some sensible compromise."
There is a petition at http://bit.ly/saveNHMwildlife.