predators threaten the existance of other animals & threatening the existence of an animal has a negative impact on that animal ->  predators have a negative impact on an animal's survival  predators have a negative impact on an animal's survival & avoiding something with a negative impact has a positive impact ->  avoiding predators has a positive impact on an organism 's survival  avoiding predators has a positive impact on an organism 's survival & defenses can help animals avoid predators ->  organisms will use defenses as a reseponse to predators  quills are a kind of defense mechanism & quills are part of a porcupine ->  quills are part of a porcupine used for defense  organisms will use defenses as a reseponse to predators & quills are part of a porcupine used for defense -> a porcupine will use quills as a response to predators