Why might the brown tree snake or the peregrine falcon serve as "poster species" for causes of the sixth mass extinction?

The sixth mass extinction unlike the previous mass extinctions, it is due to human actions. For example exotic species introduced by humans into new habitats. like the brown tree snake or peregrine falcon; They may carry disease, prey on native species, and disrupt food webs. Often they can out-compete native species because they lack predators. For instance the brown trees snakes; hitch-hiked from their native Australia on ships and planes to Pacific Islands such as Guam. Lacking local Islands predators, the snakes multiplied quickly. They have already caused the extinction of many birds and mammals they preyed upon in their new island ecosystems. 

Transpiration is the answer