A charity said plants have been flattened and materials dumped, damaging green vegetation as the multi-use games area at the Canons Leisure Centre takes shape.
Complaints have been lodged with Merton Council by Friends of the Canons group and Mitcham Cricket Green Community and Heritage group about the way contractors are carrying out work at the controversial Mitcham site.
The groups argue the conservation area, used as a feeding area for bats, could be irreparably damaged as photographs show machinery is parked on plants, plastic covered rolls of material were dumped beyond the fencing into the copse and a woodchip path was removed and the ground used to house a skip.
However the council said it is carrying out work sensitively and to a high standard which meets planning conditions, and the area will be landscaped once work is finished next month.
Two disused tennis courts are being converted into multi-use games areas with six metre high floodlighting masts and four and half metre high mesh fencing to the rear of the leisure centre in Madeira Road.
Permission was granted by the council in December last year and was opposed by the Friends of the Canons group and residents concerned about issues including the impact of floodlighting on wildlife and neighbours.
Tony Burton of the Mitcham Cricket Green Community and Heritage group, said: "They could manage it so it doesn’t damage vegetation.
"The council want to get rid of the nettles without realising the importance of them as food for butterflies and other animals. "It’s mindless and un-thinking damage."
Chris Lee, the council’s director for environment and regeneration, said: "We appreciate residents’ concerns about the Canons and would like to reassure them that we regularly meet the contractors on site to ensure work is carried out sensitively, to a high standard and meets the planning conditions.
"They are making every effort to keep the disruption to a minimum and, in order to keep the access road open, they have had to temporarily use the path area to store building materials.
"Also, at this stage in the works they also need temporary storage for machinery while they lay the surfaces."
Work began on the site in April and is set to continue until next month.