Arizona has seen the sharpest decline in undocumented population of any state, losing 18 percent, or more than 100,000, of its illegal immigrants in 2008, according to a new government report.

Experts agree that the decrease in Arizona's undocumented immigrant population was fueled by the staggering loss of jobs the state experienced during the recession, and that to lesser extent immigration crackdowns also contributed to the decline.

But the situation does not simply mean a certain number of people have left the country. In any given year, some new illegal immigrants arrive in the U.S. and others leave.

Experts agree that the number of new illegal immigrants entering the country is in decline. But there is disagreement over whether the number of people leaving has increased.

The question is important as the Obama administration and Congress prepare to tackle the divisive issue of comprehensive immigration reform.

If more immigrants than normal have departed, that could boost the argument of those who support more immigration enforcement, such as employer crackdowns. The statistics show illegal immigrants will leave the country, they say, if government makes it harder for them to get work.

But if departures have not increased, the decline is simply because new immigrants aren't coming to replace those who leave. This could aid those who support immigration reform. They say the country's large illegal-immigrant population will remain and must be addressed, because when the economy returns, so will more migrants.

The numbers

Arizona's undocumented immigrant population fell from 560,000 in January 2008 to 460,000 in January 2009, according to newly released estimates by the Department of Homeland Security.

The undocumented population is difficult to count. Most illegal immigrants either entered the country illegally, or stayed after their visas expired. DHS relied on immigration, visa and other data on foreign-born residents from several government agencies to come up with its estimates, including the Census Bureau's American Community Survey, Citizenship and Immigration Services, and the State Department.

No other state in the nation experienced such a large percentage decrease in its undocumented population, according to the report. In Florida and New York, the size of the undocumented population fell by 14 percent, while California's fell by nearly 9 percent.

Nationally, the undocumented immigrant population decreased 7 percent during the same period, from 11.6 million to 10.8 million.

The decline in the undocumented population, both in Arizona and nationally, comes following a huge surge in the illegal immigrant population that had continued for most of a decade.

Between January 2000 and January 2008, Arizona's undocumented population grew 70 percent, according to the DHS report. Nationally, it grew 37 percent.

The reasons

Experts point to two factors for the huge decrease in Arizona's undocumented population: the economy and stepped-up immigration enforcement.

"I don't think it's any big mystery. It's the economy dropping off more than anything else, but I think enforcement also played a role," said Erik Lee, associate director of the North American Center for Transborder Studies, a think tank at Arizona State University.

Arizona had a net loss of 261,000 jobs during the recession, which started in December 2007, according to the U.S. Bureau of Labor Statistics. That is 9.8 percent of the state's workforce, the second-largest percentage in the U.S. behind Nevada.

Immigrants, both legal and illegal, are largely concentrated in the construction, manufacturing and hospitality industries, which were particularly hard hit. The state's construction industry lost more than 78,000 jobs during the recession, a 37 percent drop, according to data from the Arizona Department of Commerce.

Arizona also has cracked down heavily on illegal immigration. The crackdowns include crime sweeps and jail immigration screenings by Maricopa County Sheriff Joe Arpaio, and a statewide employer sanctions law, which took effect in January 2008. The law requires employers to use a federal program to electronically verify whether new employees are legally eligible to work in the U.S. It also has the power to suspend or revoke the business licenses of employers caught knowingly or intentionally hiring illegal workers.

Arizona "has had a very bad job market and it has had a very robust enforcement scene, with the hiring law, and Sheriff Joe," said Steven Camarota, research director at the Center for Immigration Studies, a think tank in Washington, D.C., that favors reductions in immigration.

At the same time, the federal government has deployed thousands of additional Border Patrol agents along the U.S.-Mexico border, including hundreds more in Arizona, erected hundreds of miles of fencing and barriers, and deported thousands of illegal immigrants from the U.S.

Coming or going

Jeffrey Passel, a senior demographer at the Pew Hispanic Center, a nonpartisan research center in Washington, D.C, said the flow of illegal immigrants goes up and down depending on the availability of jobs. Tighter border security and stepped-up immigration enforcement also have made it more difficult, dangerous and expensive to cross the border illegally.

But Passel said there is no evidence that either the economic downturn or stepped-up immigration enforcement has spurred an increase in the number of Mexicans returning to their country from the U.S. Mexicans make up about 60 percent of the nation's undocumented population, and the majority of the illegal immigrants in Arizona.

There is strong evidence, however, that the number of immigrants coming to the U.S from Mexico, has plummeted, Passel said.

During 2008 and 2009, about 636,000 Mexicans came to the United States, a 38 percent decrease from 2006 and 2007, according to a July Pew Hispanic Center report co-authored by Passel. During the same period, the number of Mexicans returning to their country remained roughly the same, from 470,000 to 433,000, a 9 percent decrease the report said.

"The current recession has had a harsh impact on employment of Latino immigrants, raising the question of whether an increased number of Mexican-born residents are choosing to return home. This new Hispanic Center analysis finds no support for that hypothesis in government data from the United States or Mexico," the report said.

Carlos Velez-Ibanez, an ASU anthropology professor and chair of the transborder studies department, said based on anecdotal evidence most undocumented immigrants who left Arizona moved to other states, not back to Mexico or other countries.

Camarota agrees that the number of illegal immigrants from Mexico coming to the U.S. has plummeted. But he believes the number of illegal immigrants returning home is also growing, because of the economy and immigration enforcement.

According to a report Camarota co-authored in July, the number of "likely" illegal immigrants returning home skyrocketed from about 210,000 a year in the middle of the decade to about 550,000 a year toward the end.

"They couldn't just be moving" to other states, Camarota said. "There has been a very substantial increase in out migration" coupled with falling in-migration.

Economics

It is difficult to gauge how the loss of 100,000 undocumented immigrants in 2008 affected the state's economy, said Lee, at ASU's North American Center for Transborder Studies.

On one hand, the loss of so many people so quickly hurts the economy because it means fewer consumers buying goods and paying taxes. But it also means fewer people using government and public services. "When you have an exodus, you are going to have a whole set of complex consequences," Lee said.

Doris Meissner, an immigration policy expert at the Migration Policy Institute in Washington, D.C., and the former head of the Immigration and Naturalization Service, said despite the recent decline in the undocumented immigrant population "we certainly still have a very large immigration problem, a large number of people without status, and a broken immigration system."

Congress is expected to address immigration reform later this year. The decline in the undocumented immigrant population could cut both ways during the debate, she said.

"On one hand, the rate has slowed, so theoretically it provides an opportunity to fix the system before the economy starts to pick up again so that immigration reforms can be part of the recovery," she said.

On the other hand, a legalization program for the undocumented and a temporary worker program, two provisions viewed as key to comprehensive immigration reform, will be hard to sell with so many Americans still out of work, she said.

Staff reporter Ronald J. Hansen contributed to this article.