If cities were businesses, Tucson would be losing market share.

While the city and its suburbs both kept growing during the past decade, the suburban growth outpaced the city's so much that it further chipped away at Tucson's share of the overall population.

The percentage of all Pima County residents living inside Tucson is now only 53 percent, new census figures show. As recently as 1990, it was 60 percent.

If this trend continues, Tucson could have less than half of Pima County's population in 10 to 20 years, two local planners predict.

"The city's share has dropped 4.6 percent in the past decade," said Eric Kramer, of the Pima Association of Governments. How much that trend will continue, he said, "depends on how attractive it is to drive to distant suburbs."

Suburbanites have long flocked to those areas in search of cheaper housing, better schools and more spacious living conditions. But many planners and environmentalists criticize sprawl on the grounds that it hurts the tax base of the main city, runs up big taxpayer bills to build new roads, and destroys open space near important natural lands.

How the city-suburban split evolves over the coming decades could well depend on which areas develop most quickly and intensively: vacant land within city limits on Tucson's far south and southeast sides, or vacant land outside city limits in those areas and on the northwest and southwest sides, planners say.

Some questions and answers:

Q. What's the difference in growth rates in Tucson and the suburbs in the 2010 census numbers announced Thursday?

A. The city's population, while vaulting above 500,000 between 2000 and 2010, grew only about 7 percent. Sahuarita, Oro Valley, Marana and unincorporated Pima County - the traditional suburban strongholds here - grew at rates ranging from 14.9 percent in the unincorporated areas to 679 percent in Sahuarita.

Q. Why?

A. Kramer, a senior researcher for the Pima Association of Governments, points to more available land and cheaper land, which leads to less expensive housing. It's relatively easy to get around here on Interstates 10 and 19, which also tends to attract people to farther-flung areas, he said. While he only moved here from Florida in November, Kramer added, "My guess is that growth regulations in smaller jurisdictions are less restrictive than in Tucson."

David Taylor, a retired PAG and Tucson planner, said, "What's behind the growth in the suburbs is people searching for the house they can qualify for. It's nothing unique about Tucson. ... But it is notable to us that the city's growth has slowed dramatically."

Q. What's the reaction from suburban officials?

A. Sarah More, Sahuarita's planning and building director, credited the town's rapid growth to its three big master-planned communities with a lot of services and amenities. They offer new, large housing for relatively affordable costs, she said.

"It's so much easier to develop in these large master-planned developments," More said. "You don't have to assemble different ownerships. You're not doing any demolition. You don't have to deal with the issues in the city about infill."

Josh Wright, Marana's strategic initiatives director, said the main reason for Marana's growth is that people want to live there.

"It's a community we worked very hard to invest in. We've invested a lot in parks and trails, and we're also working a lot in developing a stable job base."

Q. What's the outlook once the building slowdown ends as predicted will occur in a few years?

A. Marana's Wright and Sahuarita's More both expect solid growth over the next decade. In Sahuarita, for instance, that would mean 3 percent annually for the next five years and 5 percent annually over the following five years, More said.

"We are nowhere near buildout," Wright said. "We have a lot of land for future growth of many types."

Q. What factors could deflect growth away from the suburbs?

A. If gasoline prices keep rising toward or above $4 a gallon, Kramer, Taylor and More said.

Another factor: Taylor points out that Pima County's Sonoran Desert Conservation Plan has earmarked the south- and southeast-side lands within Tucson as good future growth targets. Their desert is not as ecologically rich as in other areas, County Administrator Chuck Huckelberry says.

Growth rates

City vs. suburban growth, 2000-2010

City of Tucson: 6.9 percent.

Town of Marana: 150 percent.

Town of Oro Valley: 38 percent.

Town of Sahuarita: 679 percent.

City of South Tucson: 2.9 percent.

All unincorporated Pima County: 14.9 percent.

Catalina, unincorporated: 7.7 percent.

Corona de Tucson, unincorporated: 598 percent

Drexel Heights, unincorporated: 1.8 percent.

Flowing Wells, unincorporated: 7 percent.

Green Valley, unincorporated: 23.7 percent.

Picture Rocks, unincorporated: 17.3 percent.

Tanque Verde, unincorporated: 4.3 percent.

Tucson Estates, unincorporated: 24.9 percent.

Vail, unincorporated: 310 percent.

Source: U.S. census

Contact reporter Tony Davis at tdavis@azstarnet.com or 806-7746.