Abiotic Nonliving; often used to refer to the nonliving components of an ecosystem (such as climate and temperature).

Abundance The number of a given species within an ecosystem; an abundant species is strongly represented within the wider population.

Acid rain Any form of precipitation with high levels of acidity, causing damage to the environment; may occur naturally or as a result of human activity.

Anthropogenic Originating in, or influenced by, human activity.

Apex predator A predator that is not prey for any other species.

Atmosphere The layer of gases surrounding Earth. It also protects organisms from ultraviolet radition.

Autotroph A producer; an organism that makes its own food from sources such as light, water, and ...

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