BERKELEY, Calif., Jan. 22 (UPI) -- A U.S. study assessing ecological costs of human activities has found environmental damage caused by wealthy nations disproportionately affects poor nations.

The University of California-Berkeley study led by former UCB research fellow Thara Srinivasan assessed the impact of agricultural intensification and expansion, deforestation, overfishing, loss of mangrove swamps and forests, ozone depletion and climate change from 1961 to 2000.

"At least to some extent, the rich nations have developed at the expense of the poor and, in effect, there is a debt to the poor," said co-author Professor Richard Norgaard. "That, perhaps, is one reason that they are poor."

Srinivasan said the researchers believe their study is the first to examine where nations' ecological footprints are falling.

"In the past half century, humanity has transformed our natural environment at an unprecedented speed and scale," Srinivasan said, noting the Earth's population doubled in the past 50 years. "What we don't know is which nations around the world are really driving the ecological damages and which are paying the price."

Srinivasan, Norgaard and their colleagues report their results in the early online edition of the Proceedings of the National Academy of Sciences.