Immigrants accounted for almost half of all population growth in the United States between 2017 and 2018, according to newly released Census Bureau data.

Expand chart Data: U.S. Census Bureau; Chart: Axios Visuals

Why it matters: It's a reminder of how rapidly the demographics of the country are changing — and how the bitter political fights over immigration aren't changing the broader trends.

The high levels of immigration could also help mitigate the negative impact of falling birth rates — which could leave the U.S. with a large dependent population of children and retired people and a much smaller workforce, slowing economic growth.

The big picture: Nine percent of the nation's counties grew due to immigration rather than more births than deaths — including counties that contain most of San Francisco, Houston and Boston, according to a Wall Street Journal analysis.

More than half of the population growth in the District of Columbia, Florida, Kansas, Michigan, Ohio, Pennsylvania, Virginia and seven other states was due to immigration.

of the population growth in the District of Columbia, Florida, Kansas, Michigan, Ohio, Pennsylvania, Virginia and seven other states was due to immigration. In other cases, immigration has helped stymie falling populations. In New York state, for example, the overall population fell by 48,500 people between 2017 and 2018 — largely due to people moving to other states. International immigration was a greater source of population growth there than natural increase (caused by more births than deaths.)

Between the lines: If it weren't for immigration, 44% of Americans would be living in shrinking counties, the New York Times notes.

Two other notable trends:

1. Big cities are shrinking. There's a lot of talk about the concentration of wealth and opportunity in the nation's biggest cities. Yet, New York City, Los Angeles and Chicago all saw their populations decline in the past year as people move out to surrounding suburbs.

Meanwhile, small metro areas and suburbs outside of urban cores are booming, according to a Brookings Institution analysis given to Axios.

and suburbs outside of urban cores are booming, according to a Brookings Institution analysis given to Axios. Phoenix, Dallas, Houston and Atlanta, for example, had the highest numeric population growth from 2010 to 2018 for metro areas.

for example, had the highest numeric population growth from 2010 to 2018 for metro areas. Of the 10 counties that saw the highest percent increase in people over the past year, four were in Texas.

2. A small rural revival. For the second year in a row, non-metro areas — which include rural areas as well as towns with between 10-50,000 people— grew at a low rate.