Abstract
From blood-sucking lice and food-stealing gulls to pandemic-inducing viruses and egg-eating snakes: parasites and predators are ubiquitous in shaping ecology and evolution. Fundamental to these interactions is the way in which parasites and predators choose their victim. Here, I argue that a trade-off between host quality and vulnerability can be generalised across systems to understand parasites' choice of hosts. This principle defines quality as the value of resources a host has, and vulnerability as the ease with which a parasite can obtain those resources. A parasite can choose a low-quality host, which is easier to attack but offers limited resources, or a high-quality host, which is more challenging to attack but offers more resources if the parasite is successful. The optimal decision for a parasite will depend on its ecology and the shape of the trade-off in a given system. The trade-off applies equally to predator-prey systems. Many studies of different types of parasitism and predation across taxa have investigated traits pertaining to quality or vulnerability, but their findings have not previously been integrated. Doing so makes it possible to draw out broad principles that determine whether quality or vulnerability has the greater impact on victim choice. It can also help explain contradictory findings, such as why the same antagonists choose high-quality victims in some studies, and low-quality victims in others. Further applications include predicting the effects of global change on host-parasite and predator-prey dynamics, and providing an integrated perspective on coevolutionary adaptations.