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A biological community of interacting organisms and their physical environment. It encompasses the complex relationships between living things and non-living resources.

Lacking in movement, action, or change; in physics, relating to bodies at rest or forces in equilibrium.

An element, feature, or factor that is liable to vary or change; a variable in a scientific or mathematical context.

The magnitude or intensity that must be exceeded for a certain reaction, phenomenon, result, or condition to occur. Often used in biology, psychology, and engineering.

Very rare or seldom seen; unusual. It often describes natural phenomena, rare diseases, or historical events that happen once in a long time.

To move or swing back and forth at a regular speed; regular fluctuation of a value or quantity above and below a mean value.

The act of one animal preying on others for food. This is essential vocabulary for TOEFL lectures describing predator-prey relationships.

The state or process of a species, family, or larger group being or becoming extinct. A core concept in TOEFL passages about dinosaurs or environmental conservation.

Any form of water, such as rain, snow, sleet, or hail, that falls to the ground from the atmosphere.

To produce offspring; to reproduce. A core vocabulary word for TOEFL passages about life cycles and biology.