The fact that animals moved so freely without people present shows how wild species change their behavior in response to human activities. Wild animals that detect humans can experience physiological changes, such as increased heart rates and elevated levels of stress hormones. And humans’ outdoor activities can degrade habitat that wild species depend on for food, shelter and reproduction. Human voices, off-leash dogs and campsite overuse all have harmful effects that make habitat unusable for many wild species. Although some species or individual animals will become used to human presence at close range, many others won’t.