The Human Shield Hypothesis: Does Predator Avoidance of Humans Create Refuges for Prey?

As anthropogenic disturbance restructures ecological communities worldwide, ecologists have developed and tested hypotheses about which species "win" and "lose" in the face of human impacts. One heavily invoked paradigm is that of the human shield, which posits that predators avoid areas of human disturbance due to perceived risk from humans, and prey therefore seek refuge in these areas of perceived safety. Since its introduction in 2007, the human shield hypothesis (HSH) has gained popularity in the ecological literature, although there are more passing mentions of human shields than there are robust tests of the HSH. Here, we systematically review evidence for the HSH and evaluate how it is commonly discussed and tested. While there are several clear-cut cases of human shields, the emergence of human shields is highly context-dependent. By formally outlining the assumptions of the HSH, we derive predictions about what ecological and anthropogenic contexts are most likely to be conducive to human shields. Further robust studies that compete the HSH against alternative hypotheses and account for confounding factors can shed light on the role of human shields in human-modified ecosystems and inform the conservation and management of wildlife in a changing world.