The presence of humans changes the way animals behave, and those changes may make them more vulnerable—to poachers, for one, but also in less obvious ways, said Blumstein, who is also a professor in the Institute of the Environment and Sustainability in the UCLA College.
When animals interact in seemingly benign ways with humans, they may let down their guard, Blumstein said. As animals learn to relax in the presence of humans, they may become bolder in other situations; if this transfers to their interactions with predators, they are more likely to be injured or killed.
The presence of humans can also discourage natural predators, creating a kind of safe haven for smaller animals that may make them bolder. For example, when humans are nearby, vervet monkeys have fewer run-ins with predatory leopards. And in Grand Teton National Park, elk and pronghorns in areas with more tourists are less alert and spend more time eating, Blumstein and his colleagues report.