Conservation biology is the study of biodiversity in relation to maintaining species and protecting their habitats in order to prevent extinction. It's aim is for sustainability of resources that are needed by living things and uses principles of science, resource management as well as economics to make sure their goals are reached. The Endangered Species Act in the 1970's helped to protect hundreds of species in danger of becoming extinct. It is obvious as the human population continues to grow worldwide, this will impact the available food and habitat for other living things as well as for people. As we clear forests and construct new housing, habitats and resources may be lost from the ecosystem for other living things. Therefore, conservation biologists aim to protect the environment as well as endangered species to preserve biodiversity. This can be done ex-situ in zoos, aquaria, hatcheries, etc, or in-situ in the natural habitat by creating wildlife preserves or by creating laws to protect various species. It is generally accepted that a loss of even a single species can have far-reaching effects throughout an ecosystem. Some species are bell-weather species and their absence from the ecosystem can spell disaster for the other species as they play a critical role in the maintenance of that ecosystem.