In biological systems, a reaction or alteration in behavior or physiology resulting from a stimulus is a fundamental characteristic of life. This reaction can occur at any level of biological organization, from a single cell responding to a chemical gradient to a complex organism exhibiting a behavioral change in response to environmental cues. For example, a plant bending towards sunlight or an animal fleeing from a predator demonstrates this characteristic.
The ability to react to stimuli is crucial for survival, enabling organisms to maintain homeostasis, avoid danger, and secure resources. Historically, understanding these reactions has been central to comprehending how living organisms adapt and interact with their surroundings. Its study has led to advancements in medicine, agriculture, and conservation biology, by informing interventions designed to improve health, productivity, and ecosystem management.