More white Americans are dying than being born for the first time in modern history—suggesting minorities and newcomers will play an important role in fueling the population growth America's recovering economy needs to thrive.

The number of non-Hispanic white Americans who died in the year ended June 2012 exceeded the number who were born during that period by about 12,400, the first "natural decrease" for this group. That's according to a U.S. Census Bureau report released Thursday and an analysis of separate data from the National Center for Health Statistics by demographer Kenneth Johnson at the University of New Hampshire. The white population edged up in absolute terms last year thanks to immigration, Census figures show.

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The Census data show that as the non-Hispanic white population grows at a slower pace, the share of young Americans who are minorities is increasing, said demographer William Frey of the Brookings Institution, a left-leaning think tank in Washington. While most American children under 5 years old are still white—50.1%—that proportion is expected to fall because the majority of births have been minority children for two years in a row, a trend driven mostly by Hispanic and Asian births.

The growth rate of America's white population has been slowing for years, but demographers had expected white births to continue exceeding deaths for some time. Census researchers had projected the "natural decrease," or white Americans' deaths exceeding births, would begin around 2020 and the overall white population would start falling outright several years later.

"Even during the great influenza epidemic of 1919, there was no white 'natural decrease,' " said Mr. Johnson, calling the new numbers "stunning." The epidemic of 1918-19 killed more than 600,000 Americans. The big driver of the recent numbers, Mr. Johnson said, was a drop in white births, which fell about 13% last year from 2007 levels.

The findings illustrate how rapidly the U.S. is becoming more diverse and how much of the nation's population growth is likely to be driven by minorities and new immigrants in the years ahead.

The recent trends are partly due to the nation's weak economic recovery, which has encouraged many young women to delay having children—a trend that demographers say could now reverse. Fertility rates dropped to low levels during the Great Depression and the 1970s malaise before rising again, according to Mark Mather of the Population Reference Bureau.

But the Census numbers underscore a more powerful, long-term demographic shift. As the nation's white population ages, there are fewer white women of childbearing age—a trend unlikely to change—and that results in fewer white children. At the same time, young adults, whether white, black or Hispanic, are having fewer children. America's replacement level—how many children it takes to keep population constant as people die—is about 2.1 births per woman. The fertility rate of U.S. women is about 1.9 births.

That means U.S. population growth will depend significantly on immigration. There are now 14 states where the majority of children under 5 are nonwhite, Mr. Frey said; in 2000, only five states, including the District of Columbia, had "minority majority" toddler populations.

The U.S. population is "browning from the bottom up," Mr. Frey said.

Write to Neil Shah at neil.shah@wsj.com