"We've got to talk about these issues -- population, birth rates, immigration," said Paul Watson, founder of the Sea Shepherd Conservation Society in Washington state.
Watson was a Sierra Club board member, but he resigned in protest because he thinks the organization ignores immigration as a major factor in population growth, the Christian Science Monitor reported.
The split is political. Environmentalists generally see themselves as political progressives; they don't want to be bedfellows with anti-immigrant activists, sometimes labeled as xenophobic or racist, the newspaper said.
U.S. Census Bureau statistics released this week show that Hispanics -- the largest U.S. minority at 42.7 million -- are the nation's fastest-growing group. Between July 2004 and July 2005, they accounted for 49 percent of U.S. population growth.
"If you're going to be an environmentalist, you have to be concerned about the numbers as well as the usual issues -- public lands, energy, pollution, and so forth -- because the numbers will just wipe you out," said Alan Kuper, a retired physicist in Cleveland and founder of Comprehensive US Sustainable Population.
2014: The Year in Music [PHOTOS]