Ecologists manage prairies and savannas with controlled fire to remove the thick mat of dry plant material left when prairie plants die back at the end of the season and to eliminate invasive species. Prairie plants survive occasional fires because their roots often extend ten to fifteen feet below the ground, unlike turf grass roots that reach only about two inches deep, and those deep roots also help hold water and soil in place to prevent erosion. Fires are best done in the spring, when weeds and invasive plants are just starting to grow, because the fire kills what it reaches above ground without harming the deep roots of prairie plants. Historically, lightning was the first source of fire in prairies, and when prairie fire approached woodland, most trees growing outside the woodland edge did not survive, while certain thick-barked, fire-resistant trees such as hickories and Burr Oaks could. A savanna is the transition ecosystem between prairie and woodland, typically filled with prairie plants and sparsely populated with a few trees, and fire is essential to its survival. Oak savanna plant communities were once very common in Illinois but are now few and far between because of population growth and urban expansion. Deer often graze in savannas, feeding on prairie plants while staying close to woodland shelter, and birds of prey roost in the tree canopy above savannas, where the canopy covers only about 10 to 30 percent of a typical forest canopy and gives them a clear view of prey on the ground below.