How do plant and animal responses to stimuli increase their fitness for specific environments? Sample Response: An increase in fitness would occur when the organism survives long enough to reproduce . The more the organism reproduces , the higher their fitness. By altering reproductive cycles to stimuli such as day length or temperature, plants and animals can ensure that they are reproducing in optimal conditions where there is increased food supply and where chances increase that their offspring will survive . While animals can respond to stimuli by being mobile , plants cannot . Therefore , their growth responses such as phototropism allow them to seek out the best conditions for survival .