Native grassland areas are among some of the most threatened habitats in the country. Arizona has its own habitats also facing threats.

Greg Barron-Gafford, Ph.D., a biogeographer who works as an assistant professor and associate research scientist at the University of Arizona's Biosphere 2, said grassland ecosystems have been changing significantly for more than 100 years.

These changes began when human habitants increased their impact on the land, such as by adding cattle and other non-native species, as well as suppressing fires.

Also, nowadays there are many more woody plants in the landscape than there were traditionally, as documented by staff in the U.S. Geological Survey, UA photos of the Santa Rita Experimental Range, and other agencies, Barron-Gafford explained.

While some people may like the look of the trees and other plants, there are some consequences for people and the habitats.

"The challenge there is the water side of things," Barron-Gafford said. "Mesquites have really deep roots that are tapping into ground water, and sub-surface pools of water...from a geography perspective, the challenge is that this water is also what would have been available for human consumption."

Barron-Gafford said modern society has become much better at managing range lands and other habitats. However, mesquite trees and other plants are continuing to spread where they did not live historically.