a defensive line or police cordon used to prevent unauthorized access or surprise attacks

Bright colors or patterns on certain animals and plants that serve to warn predators of their toxicity, bad smell, or stings.

(idiom) to act in a certain way to warn others; to provide an example as a deterrent

a formal list identifying endangered species or entities requiring urgent protection or caution

The water level in a river or lake at which flood prevention personnel must begin to be on alert and prepare for flood defense.