UNITED NATIONS -- Biodiversity experts are warning that climate change is making it harder to halt that rapid extinction of species under way in much of the world, and hundreds of scientists and government officials have gathered in South Africa to discuss ways to attack the problem.

And though more direct human factors like overfishing, illegal hunting and landscape alterations are having a much more significant impact on species loss, climate change is poised to become the No. 1 factor over the next few decades, say officials with DIVERSITAS, an international nongovernmental organization sponsoring this week's gathering.

Among the most vulnerable species are those that live in freshwater lakes, rivers and streams, according to the organizers of a weeklong global biodiversity conference kicking off tomorrow in Cape Town. Research showing rapid declines of freshwater species diversity is especially alarming. Though constituting less than 1 percent of the Earth's surface, freshwater systems are home to 35 percent of the world's vertebrates and 10 percent of all animals, they say.

"All these impacts, like climate change, like overexploitation, like invasion of non-native species and so on, they are all combining to lead to this major erosion of biodiversity and the ecosystems services related [to freshwater environments]," said Klement Tockner, a scientist at Germany's Leibniz-Institute of Freshwater Ecology and Inland Fisheries and one of the forces behind the conference.

In April 2003, at a meeting under the auspices of the U.N. Convention on Biological Diversity, more than 100 government environment ministers committed the countries to the goal of achieving "significant reduction of the current rate of biodiversity loss" by 2010, when nations are scheduled to meet in Nagoya, Japan, to review the state of the convention. But many ecologists say the world is not even close to meeting this modest goal, as little action has been taken to follow up on the 2003 commitment.

Six hundred officials working in the fields of biodiversity, conservation and the emerging "ecosystems services" movement will hold a series of workshops until Friday in the hope of drawing greater attention to the mass extinction problem.

Evidence of 'dramatic effects' on ecosystems

The 2010 biodiversity commitment has been overshadowed by the raucous debate over a new international climate change treaty, which is supposed to be completed in Copenhagen in December of this year. But biodiversity campaigners say that their own studies show that climate change is becoming the prime culprit behind species loss.

"We're seeing evidence of climate change having some dramatic effects on ecosystems," said Harold Mooney, a professor at Stanford University who chairs DIVERSITAS's scientific community. "We're seeing, as you might imagine, changes in distribution of species, species moving up to higher elevations or moving up to more northerly latitudes, and it's an unequal sensitivity to temperature change, so communities are being torn about a little bit. Some are moving, and some aren't."

A recent study shows that "land-use changes right now ... are still the main driver of change, but that slowly, over the next 40 years and by about 2050 -- and of course that's a model prediction -- that climate change should become the main factor for biodiversity change and gain over land-use change," said DIVERSITAS Executive Director Anne Larigauderie in an interview.

Organizers of the Cape Town event are calling freshwater species "the most threatened on Earth." Climate change is expected to only make the situation worse as environments are affected by droughts or as climatic changes push human populations to put more stress on existing water resources.

Tockner says that already, 25 species of freshwater urchins could be wiped out. "These are species 300 million years on Earth, and now, within 50 years, they are almost all close to extinction," he said.

More than 50 percent of native plants and animals found in the wetlands surrounding the Mediterranean Sea are already gone, and populations of amphibians throughout the world are becoming endangered, Tockner said.

Next year, the United Nations will issue the latest "Global Biodiversity Outlook" report, which should form the basis of discussions between scientists and policymakers at the November 2010 Nagoya talks. Larigauderie says that two keys to stemming the rate of extinction are better enforcement of conservation laws and a greater understanding by regulators of local and regional factors contributing to extinctions.