How do organisms react to a stimulus?

Answer 1

Practically anything can be a stimulus, so organisms can respond to them in just about any way. The definition of 'stimulus' and 'response' are a little paradoxical in this context in that a stimulus is anything that provokes a response and a response is anything done to react to a stimulus.

Perhaps some examples will make it clearer: if a predator grasps a lizard's tail (stimulus) it will detach the tail and escape (response). If the weather is excessively hot (stimulus), a plant will close the stomata in its leaves to conserve water (response). If a laboratory mouse notices a new object in its cage (stimulus), it will go investigate it (response).

Answer 2

Organisms can react to a stimulus through various mechanisms, such as moving towards or away from the stimulus, changing their behavior or physiological processes, or by initiating specific responses to counteract the stimulus. These reactions are typically adaptive and help the organism in responding to changes in their environment.