The international community is spending barely three-quarters of the amount needed to maintain the world's protected areas, researchers say.
By Alex Kirby
BBC News Online environment correspondent
Most of the shortfall occurs in developing countries, the ones richest in wildlife.
To expand protection to all the areas that need it, they say, would mean a threefold spending increase for a decade.
But they say protecting nature usually pays for itself in benefits to society.
The estimate comes from an international panel of economists, scientists, governments and protected area managers.
It is published by the US-based Conservation International's Center for Applied Biodiversity Science, the University of Cambridge, UK, and BirdLife International, a consortium of wildlife groups working in more than 100 countries.
The findings were released at the World Parks Congress in Durban, South Africa, organised by IUCN-The World Conservation Union.
Erosion in Madagascar, caused by forest loss
The authors say global funding for parks and other protected areas is $7bn annually, less than $1bn of which is spent in the developing world.
They estimate the shortfall in maintaining the existing protected area network at $2.5bn a year.
But to expand the network so that it could conserve many of the most threatened but currently unprotected species would cost about $23bn annually for the next 10 years, they say.
They describe the plight of many protected areas today: "Tens of thousands, most dramatically those in the developing world, suffer from a chronic lack of funding, resulting in a shortage of staff, ranger stations, communications equipment, vehicles and other basic infrastructure."
Conservation International says the spending shortfall often has "catastrophic" consequences: "In West Africa, for example, funding of many parks is so poor that areas once rich with elephants, hippos and monkeys are now empty.
"In Latin America, protected areas have been cleared for agriculture, and in Asia the last individuals of some of the world's most amazing species - tigers, monkeys and crocodiles - are poached for illegal sale."
Golden-crowned sifaka clings to life
John Hanks of Conservation International said: "The acceleration of human encroachment is transforming vast natural areas, and species are still teetering on the brink of extinction - in the very places designed to provide them with safe refuge."
Aaron Bruner, manager of conservation economics for the organisation, said: "For $23bn, significantly less than Americans spend on soft drinks alone each year, we can save a large number of the places that house the greatest diversity of life on Earth.
"And for a fraction of that, just $1.5bn a year, we could take the vital step of making sure that basic management of all existing protected areas in developing countries is well funded."
A study published in 2002 in the journal Science said the long-term economic benefit from healthy ecosystems was far greater than the cost of protecting them.
But development in wild areas jeopardised flood and storm protection, the protection of watersheds, and the uptake by vegetation of carbon dioxide, thought to contribute significantly to climate change.
The study put the value of these services at $33 trillion a year worldwide. Conservation International says establishing and managing protected areas often provides direct benefits to poor communities as well.
Madagascar images copyright and courtesy of Haroldo Castro/Conservation International