Trees can grow in this area, but most were stopped by fires that would sweep through. Some oaks and hickories could survive a prairie fire. If there are scattered trees, it is called a savanna, and an area in a prairie where trees grew closer together is called a grove. In the last century, fires have been stopped, and trees and brush that once did not grow in prairies are taking over. Some of these invasive species were accidentally introduced from Europe or elsewhere. In Illinois, less than 0.01% of the original prairie still exists. For this reason, conservationists in this area practice prairie restoration to bring the prairie back.