"Ooh, ooh, ooh, ah, ah," the iconic British conservationist chanted into the microphone, delivering a series of melancholic bursts she said roughly translated as "please help."

"I think that's what chimpanzees would be saying if they could articulate it that way," Goodall told participants at a meeting Thursday of the conservationist umbrella group Avoided Deforestation Partners. The event took place on the margins of the U.N.'s Rio+20 mega-conference on sustainable development, which has drawn an estimated 50,000 diplomats, environmentalists, policy makers and concerned citizens from across the globe to Rio de Janeiro.

The world's forests are among the crucial, life-sustaining environmental systems scientists say are teetering on the brink of a tipping point. The U.N.'s Environment Program warned earlier this month that the planet's systems, which also include air, land and oceans "are being pushed towards their biophysical limits," after which sudden and catastrophic changes could ensue.

Environmentalists had cast Rio+20 as the last, best chance to avert such a scenario, and the event attracted a host of high-profile personalities, including Virgin Group founder Richard Branson and media mogul Ted Turner, who urged policy makers to take action on their pet causes. But the three-day conference was beset by bickering between rich and poor countries, and environmental protection groups have lashed out in chorus against the event's final document, which they say is grossly inadequate.

Goodall, a Cambridge University-trained ethnologist who's among the top advocates for the chimps she has studied for more than half a century, spoke movingly of the deforestation that has encroached on Tanzania's Gombe National Park, where she began studying chimps. The chimpanzee population of equatorial Africa once numbered in the millions, but deforestation and other threats have slashed their numbers to an estimated 170,000-300,000, making the chimp an endangered species.

Goodall said a recent flight over Gombe, a tiny 30-square-mile sliver of a park perched on the shores of Lake Tanganyika, brought the devastation of the surrounding landscape into sharp relief.

"The trees were gone, the hills were bare," she said. Outside the park, trees had been cut down by the impoverished locals for firewood and for plots of land on which to eke out a living.

She said both the kind of "desperate poverty" that surrounds Gombe and, on the other end of the spectrum, the unquenchable appetites for consumer goods in wealthy countries, were to blame for deforestation.

"The unsustainable lifestyles of those not living in poverty is leading to the actions ... of the big mining companies, the big petroleum companies and the big logging companies" -- the enemies of forests worldwide, she said.

Goodall also singled out spiraling population growth as another of the main culprits driving deforestation, which organizers of Thursday's conference say results in the loss of 1 acre of forest every second.

"It's population growth, the sheer numbers of us. It's having a devastating effect on the forests," she said.

Britain's Prince Charles concurred.

In a lengthy address beamed into the Rio meeting via video, the prince said that the burgeoning human population will inevitably lead to the clearing of more forest land for agriculture to feed people's "insatiable and ultimately unsustainable appetite for meat." He cautioned that innovative and region-specific solutions, such as integrating crops on forest floors, will prove necessary if any of the world's forests are going to remain standing.

He also made a compelling argument for just how important it is that they do remain standing: Deforestation itself is a major source of greenhouse gases, spewing out more pollutants annually than the global transportation. Plus, forests help stave off climate change by fixing carbon dioxide and play a crucial role in feeding streams and rivers.

Again and again, the meeting's high-profile speakers emphasized how much previous UN environmental conferences had left undone and how time was running out for the world's forests and the plants, animals and humans that depend on them.

"For me and I'm sure many others, there's a terrible sense of deja vu when I recall those expressions of urgency 20 years ago," said Charles, referring to the Rio+20's predecessor, the Eco 92 conference, which helped put climate change on the world agenda.

The prince stressed that time is running out, a point he made in a previous address, he said Thursday.

"I said that we have less than 100 months left to act to avoid catastrophic change. That was forty months ago," said the prince. "We simply can't wait for the international frameworks to be put into place. If the pace of the negotiations and the speed ... is too slow to arrest the present rate of depletion, then it seems to me we have no option but to forge ahead by taking action now and then linking to the international frameworks when they finally emerge further down the line."

While neither the prince nor chimpanzee champion Goodall appeared to put much faith in the outcome of the Rio+20 conference, they said increasing public awareness of the scope of the problem was heartening.

"More and more of the public have begun to understand what's going on out there," she said, "and people are beginning to vote in the grocery store with what they buy."