If all of the immigrants in the Church in the US left, how many stakes would the US Church contract by? 

First of all, while I am in general pro-immigration, this isn’t an argument for immigration, just an interesting thought experiment. If they weren’t in the US they would be somewhere else, so their loss is our gain, and a member practicing in Guatemala is not worth any more or less than a member practicing in San Antonio. 

The Cooperative Election Study has a fairly large Latter-day Saint sample and asks respondents about their immigration status, so it’s as good a source as any for this question. While Latter-day Saints aren’t as immigrant heavy as other faiths in the US it’s still not insignificant. 

In terms of stakes, it’s not bleeding edge but as of last June there were 1,709 stakes.

According to the CES: 

4% of Latter-day Saints indicate “I am an immigrant to the USA and a naturalized citizen.” 

4% indicate “I am an immigrant to the USA but not a citizen.”

6 % indicate “I was born in the USA but at least one of my parents is an immigrant.”

13% indicate “My parents and I were born in the USA but at least one of my grandparents was an immigrant.”

So, our non-citizen immigrants comprise the equivalent of 68 stakes, as do our citizen-immigrants, for a total of 136 stakes worth of immigrants overall.  So if we were to remove all of the immigrants in the US Church it would be roughly equivalent to getting rid of the entire Church in Arizona or California. 

For 2nd generation Americans it’s 103 stakes, and for third generation Americans it’s 222 stakes. So if we remove all members with at least one foreign-born grandparent that’s equivalent to removing 70% of the Utah Church, or three Idahos.