A former marine biologist with the U.S. Fish and Wildlife Service, Carson drew attention to the unintended and often disastrous consequences of the mass use of insecticides, such as the death of species (especially birds) higher in the food chain through loss of food, concentration of toxic chemicals, the emergence of resistant strains of insects, and, paradoxically, increased devastation of some crops as natural insect predators turned out to be more vulnerable than the targeted pests. Silent Spring sold over a half million copies and crystallized the environmental movement in the United States, which has since become global. It led directly to the U.S. ban on domestic use of DDT in 1972 and of other chemical insecticides. More generally, it explained, in comprehensible and occasionally poetic prose, the complex dynamics of ecological systems, and the fact that heavy human or other external pressure at any one point can have ramifications throughout the system. In addition, few other informed scientists had the courage to state the dangers and limitations of insecticides. Much of the scientific work on insect control in the 1940s and 1950s had been undertaken by the U.S. Department of Agriculture, devoted to improving crop and livestock yields, and by the major chemical companies, devoted to selling insecticides.