The mesquite originally came from Russia. When they came with their livestock, they brought mesquite beans to feed them on the boat. When the livestock came ashore, they passed most of the whole seeds and the southwest is now covered with the mesquite trees. At least that is the story I have always heard.

I stand corrected, google is my frind

Historians once believed that mesquite was originally limited to extreme South Texas and spread north only after the Civil War when cattle drives became frequent. Cattle eat mesquite beans when grass is not plentiful. The bean’s husks are so hard that about 50 percent of them travel through cattle’s digestive systems unscathed, to be deposited on the ground with a large helping of natural fertilizer. The historians figured that cattle distributed seeds along the trails as they went north.

But well before the heyday of cattle drives, mesquite was growing in the same areas where it is found today. Mesquite trees were part of Texas’ landscape long before Spanish explorers, in the early 1500s, first recorded finding them, mainly along Texas’ rivers, creeks and draws, but also completely covering some prairies. What has increased since then is not the range, but the density.