Shadow competition: its definition, prevalence, causes and measurement

Competition is a fundamental ecological process and an important mediating mechanism to natural selection and evolution. One form of competition, shadow competition, is evident when an approaching moving prey item is captured by a competing predator, earlier in the prey's trajectory, preventing it from reaching the attack range of a focal predator. The necessary definitional involvements of space and movement direction differentiate shadow competition from the more classical competition types of interference and exploitation, even though competition, in general, intensifies with spatial proximity. Compared to the latter two, shadow competition is understudied. Differentiating distinct competition types is important, because they may distinctively affect animal behaviour and higher levels of organisation. Although shadow competition is probably common in nature, there are only sporadic reports applying this terminology in systems of ambush predators, such as web-building spiders, pit-building antlions and sit-and-wait predatory fish, and their moving prey. Here, we summarise clear cases of shadow competition in the published literature, cases in which we believe it to be present but not explicitly described as such, and potential scenarios in which shadow competition seems likely to be present but is currently unreported. We end with potential research directions for enhancing our still-fledgling understanding of shadow competition.