Council workers banded the trees to prevent possums running up and down the trunks, pruned trees to limit movement between upper branches, sealed tree hollows and removed nesting boxes.
Dr van der Ree said the process was undertaken slowly, to give the possums time to relocate, and about 10 mature trees were left at the park.
Amid rapidly declining possum numbers in the park, the City of Melbourne and RMIT University commissioned artist Mikala Dwyer to create public art for the square.
Her work, a hologram possum that appears and disappears from trees, is called Apparition and launched in March.
Dwyer, who says she is interested in the mythology of objects and spaces, was inspired by the possums that lived in the mature elm trees at the square.
“It’s a love story to the possum; I love possums,” she said.
“And it’s also about the problems we have in cohabiting with animals and wildlife and what humans do to the planet.”
Dwyer said the possums’ removal was a complex issue, but her work was intended to provoke thought.
“I’m trying to invoke the possum as a kind of ghost. All ghosts in some ways are messengers from another world, to tell us stories from beyond … this possum is kind of a messenger. Even if it’s now in the digital world, it’s that possum that we might have lost.”
Lobby group Animal Active Australia, which lodged the freedom of information request, said the council’s actions led to “a wildlife tragedy”.
“Intentional starvation of animals and deprivation of shelter basic to survival is cruelty under all jurisdictions but when it comes to urban wildlife, councils are getting away with it,” campaigns director Rheya Linden said.
As part of the park’s redevelopment, which was completed in 2019, council staff removed mature elm trees and planted almost 80 saplings. Ms Linden said the lost trees had provided habitats for microbats, rainbow lorikeets and brushtail possums.
“Forced to endure two years of displacement, food deprivation and consequent distress, the few remaining possums now lack the gender and genetic diversity for survival in the context of a habitat-depleted park encircled by roads, metro tunnel works and dense built structures,” she said.
A City of Melbourne spokesman said the mature elm trees had been in “advanced stages of decline” and the council had planted 79 new trees to replace those lost. The land area of the park was also increased by almost 3500 square metres.
“The park now boasts greater tree biodiversity, helping to ensure the trees’ survival and allowing locals and visitors to enjoy nature well into the future.”
Underscoring the challenges facing urban native animals is the density of Melbourne’s park possum population.
The city’s parks are home to up to 15 possums per square hectare, compared with 1-2 per square hectare in more natural environments.