Over the years, human activities have impacted the natural resources of Grand Canyon National Park in many ways. Humans have introduced non-native plant and animal species into the park, which out compete native flora and fauna for space, food and water. Air pollution has routinely drifted into the Canyon from metropolitan areas and nearby coal-fired power plants, affecting visibility from scenic vistas. Water in some streams has been tainted with fecal coliform from trespass cattle and from human waste. The construction of Glen Canyon Dam in 1963, irreversibly altered the riparian and aquatic ecosystems within the park. The natural quiet of Grand Canyon has been disturbed by rumbling aircraft noise, and forest landscapes have been altered by decades of wildland fire suppression.