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native species: species that occur naturally in an area or a habitat. Also called indigenous species.

natural enemies: species that have naturally evolved as predators of other nearby species.

natural resource: any aspect of the environment that species depend on for their survival. People depend on natural resources such as land, soil, energy and fresh water.

natural selection: the process by which genetic traits are passed on to each successive generation. Over time, natural selection helps species become better adapted to their environment. Also known as "survival of the fittest," natural selection is the driving force behind the process of evolution.

nitrate: a compound (NO3) that contains nitrogen and oxygen that comes from decomposing organic materials like manure, plants and human waste.

nitrogen: a common typically colorless, odorless, tasteless gas that constitutes 78 percent of Earth's atmosphere, is a part of all living tissues and important for plant growth. Found in many commercial fertilizers, it is often washed through a watershed into streams or rivers where large amounts can act as a pollutant.

non-native species: introduced species not native or endemic to the area in question.

nonpoint source pollution: pollution from numerous widespread locations or sources that have no well-defined points of origin and may originate from land use activities and/or from the atmosphere. Examples include leaching of excess fertilizer from fields and acid rain.

nonrenewable energy: energy obtained from sources that are exhaustible, such as oil, coal and natural gas.

North Atlantic Oscillation (NAO): a large-scale fluctuation in atmospheric pressure in the Atlantic Ocean between the high-pressure system near the Azores and the low pressure system near Iceland, quantified in the NAO Index. The surface pressure drives surface winds and wintertime storms from west to east across the North Atlantic affecting climate from New England to Western Europe as far eastward as central Siberia and eastern Mediterranean and southward to West Africa.

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