Road building can have catastrophic effects on the rainforest's integrity as an ecosystem. One effect is to disprupt natural migration of animals. Some birds, for example, will not cross open spaces. A road is an effective barrier, hemming them in and preventing them moving to new food sources or nesting areas. Roads also assist the spread of non-native weeds and pest animals, which can hitch a ride on cars and trucks. These species can out-compete local organisms. However, the worst effect of road building is to provide access to people for settlement. Once a road is built, people follow: The stage is set for the degradation of forest due to settlement and subsequent urbanization.