Why is deforestation considered a double whammy?

Deforestation is a double whammy because wooded areas are a necessary environment for thousands of species, both plant and animal. When forests are removed, these species have no place to survive, they die out of existence. The second whammy is the erosion of soil. Forest trees and the other vegetation that depend on the forest environment have root systems that hold the soil in place. When soil washes away, new trees can't take root.

There are numerous other detrimental results of deforestation, but loss of habitat and soil erosion are the most critical.