While Mexican immigrants blanket all corners of Houston, expansion for other large migrant groups hasn't been as clear-cut -- even if Spanish is their native language -- based on a Houston Chronicle analysis.

Dot-density maps created by the Chronicle using 2013 Census tract data released this year show that of the top 10 foreign-born groups, many are still largely confined and densely clustered in Houston’s western and southwestern communities.

The contrasting settlement patterns of Mexicans compared with other groups are in part lingering reflections of the political and economic landscape that defined Houston in 1970s and 1980s.

Several other factors come into play, of course, including Texas’ proximity to Mexico, which grants Mexicans easier access to the U.S. For other groups, war, political or religious conflict were often reasons to flee to the U.S, according to Census studies.

Map by John D. Harden/Houston Chronicle

Mexicans migrated to Houston heavily during record-setting growth in the area’s petrochemical and construction industries in the 1970s, said Nestor Rodriguez, a professor and Ph.D. at the University of Texas at Austin who has studied Houston migrant settlement patterns for nearly 40 years.

At that time, housing and jobs were plentiful, he said.

Mexicans established three distinct immigrant zones in north, west and south Houston.

Other groups, however, flocked to Houston between the late 1970s and 1980s as federal immigration reform from the 1960s began to take hold.

However, it was during that period that Houston faced an economic downtown. And it was during that downturn that many apartment complex owners in Houston saw occupancy numbers plummet as the energy industry struggled.

Those apartment owners, who had vacancies to fill, turned to immigrants in need of cheap housing as they sought to find their footing in a new city. Many migrants took root in the several large apartment complexes in west and southwest Houston, creating new ethnically diverse, dense communities, Rodriguez said.

Diverse migrant zones like those in southwest Houston aren’t unique to the city, but Houston’s lack of zoning provided a unique opportunity for migrants.

No zoning meant that developers could build and place those large, cheap complexes in close proximity of each other in middle class to wealthier neighborhoods. The complexes became the migrants’ refuge and moved them into predominately white neighborhoods – uncommon of migrant districts in other large cities.

"If anyone benefited from the lack of zoning, it was the migrants," Rodriguez said, explaining that the dense housing placed thousands of migrants near resources such as schools, health care and jobs.

The new settlements led to development of country-specific shops, restaurants and retail, which further led newcomers from that country to cluster.

“It makes assimilation easier,” Rodriguez said. “It’s easier to settle and grow when everything feels familiar. This allows new migrants to find their footing."

The new settlements of Central Americans in rows of apartment complexes found in southwest Houston produced "a striking contrast with the areas' original commercial design" of rows and rows of restaurants, Rodriguez said.

Communities of people from similar backgrounds or with shared ethnic and cultural identities can offer a sense of identity and security, he said. It’s something Rodriguez refers to as “social currency."

For instance, about 40 percent of Vietnamese immigrants don’t speak English well, and they don’t share the luxury of having their homeland nearby like Mexicans.

“They have that language in common and they really stick together. They have that common bond of losing a country. They mourn it, to this day,” , said Jannette Diep executive director of Boat People SOS-Houston, which serves about 25,000 mostly low-income Vietnamese in the Houston area. “So they just resume living that Vietnamese culture ...”

The isolation faced by the Vietnamese group also holds true for many immigrants from African countries.

Other Asian groups -- Chinese, Japanese and Indian -- aren’t as isolated because they tend to have higher education levels and need less support to establish themselves in a new country, Rodriguez said.

Smaller immigrant groups also appear to cluster, based on the dot-density maps, but the data show that those groups aren’t tied to a single area. Croatians, who immigrated to the U.S. to escape war, have found homes throughout the Harris County’s western half and the French and English have manage to spread out more than most European countries.

Though the immigrant population continues to grow and spread, Rodriguez said the settlement patterns established decades ago will continue to influence future groups.

“It’s the one constant we can always predict,” he said. “Even as the immigrants become more established and find better jobs with better pay, they’ll continue to cluster with those that feel most familiar.”

Houston Chronicle reporter Lomi Kriel contributed to this report.

Diversity reigns Houston is the most diverse metropolitan area in the U.S., according to the Migration Policy Institute. In 2013, it was home to 6.3 million people. About 1.4 million were foreign born – an increase of almost 60 percent from 2000. Houston's immigrant population was the fifth-largest in the U.S., with Mexican, Vietnamese, El Salvadoran and Honduran making up a large portion of the groups. Read more about the city's diversity in our special section, The Million.