Forest wildlife is vital for maintaining healthy woodlands and jungles which act as a major carbon store and help curb climate change, a report by conservation charity WWF said.
That is because creatures such as primates and large birds perform an important role in the regeneration of forests through dispersing tree seeds, eating plants and other activities - helping woodland thrive and absorb carbon.
But a global assessment by WWF and the Zoological Society of London (ZSL), of 455 populations of 268 species of wildlife that live only in forests, reveals they have declined by 53% on average since 1970.
The biggest threat to forest-dwelling wildlife is the loss and damage to their habitat, mostly caused by humans clearing land for producing commodities, unsustainable logging, shifting agriculture and wildfires.
But species of mammals, amphibians, reptiles and birds are also at risk from hunting, climate change, invasive species and disease, the report said.
Declines have been greatest in the world's tropical forests, such as the Amazon - a key carbon store and habitat for nature but one which scientists warn is close to a "tipping point" where much could dry out and become savanna.
The report calls for efforts to halt and reverse the loss of tree cover, but warned keeping forests standing is not enough on its own to protect the wildlife below the canopy.
Other threats such as the illegal wildlife trade and climate change also need to be tackled or there is an increasing risk of "empty forests" in which the trees still stand but much of the rich array of life is lost.
WWF is calling on world leaders to declare a planetary emergency and draw up a new global "deal for nature and people in 2020" which the green group says is needed to stop climate breakdown, restore nature and fix the food system.
Protecting and restoring forests, which support more than half the world's land-based species and provide food, medicine, clean water, healthy soils and livelihoods to people, should be at the heart of the deal, WWF said.
Will Baldwin-Cantello, global lead on forests at WWF, said: "Forests are complex systems that depend on the wildlife that live in them to keep them healthy and the rapid decrease in forest wildlife in recent decades is an urgent warning sign.
"Forests are home to more than half of the world's land-based animals, but they're also our greatest natural ally in the fight against climate breakdown.
"We lose them at our peril," he warned.
"We need global leaders to declare a planetary emergency and kickstart a global programme of recovery to keep our forests standing to protect our planet."
The report pointed to success stories such as the return to forest of ranchland in Costa Rica and the increase in numbers of mountain gorillas in East Africa as ways in which the loss of nature can be reversed.