Butler County has grown by more than 8,800 people since 2000 and can expect similar growth in the next 20 years, according to the U.S. Census Bureau.

On Wednesday, the census bureau released population estimates for Pennsylvania cities, boroughs and townships.

The 5.1 percent population increase in Butler County can be directly tied to development in its western half.

Adams, Connoquenessing, Cranberry, Franklin, Slippery Rock and Worth townships each have jumped by greater than 10 percent in population since 2000.

In contrast, townships such as Concord, Summit and Washington recorded little to no change.

Only eight townships lost population, each between 1.5 percent and 5 percent: Brady, Butler, Fairview, Marion, Middlesex, Oakland, Parker and Venango.

Butler lost more than 5 percent of its 2000 population, according to the census bureau. The city is down to about 14,400 people.

However, the county as a whole is bucking the overall trend of population loss in the western half of the state versus population gain in the eastern half.

Counties surrounding Butler all had population losses. Armstrong County lost more than 3,600 people, about 5 percent of its population, while Lawrence, down 4,367; Mercer, down 3,640; and Clarion, down 1,776, all lost between 3 percent and 4.6 percent of their populations.

Butler County's male-to-female percentage measured fairly even, with men making up 49 percent of the population and women accounting for 51 percent.

The county's mix of ethnicity is dominated by Caucasians, who account for 97 percent of the population. Black, Asian and mixed-race citizens account for about 1 percent of the county's population each, while individuals of Inuit, Hawaiian or Pacific Islander descent make up only a minuscule part.

The census bureau estimates that the county's population will grow by another 10 percent in the next two years, up to 191,000 people, then growth will continue at a slower rate through 2030. The bureau estimates the county's population at 206,000 by 2020 and more than 220,000 by 2030, an increase of 26.7 percent during a 30-year period.

Clarion and Lawrence counties are expected to lose more than 10 percent of their populations during that same span, while Mercer and Westmoreland counties should have minimal population gains.

According to the Pennsylvania State Data Center, 94 Pennsylvania municipalities experienced population growth of 25 percent or more since 2000, while 29 declined by 10 percent or more.

Populations in the state's two major cities continue to decline.

Pittsburgh, with a population of 310,037 in 2007, has lost 24,526 people since 2000, a loss of 7.3 percent.

Philadelphia's 2007 population of 1.45 million people is 70,155 fewer than in 2000, or about 4.6 percent.