Background/Question/Methods Traditionally, studies of predation have focused on the influence of predators on prey communities, and how predation might explain natural prey population oscillations via mortality. Studies carried out for nearly 40 years in one place have enabled us to evaluate the generality of traditional views of the influence of predators. In open systems where high prey mobility can swamp potential effects of predation, the overriding influence of predators on prey distribution and abundance may result from adaptive changes in prey behaviors and life histories, rather than from predator-induced mortality. Results/Conclusions Early observations in one high-altitude, rocky-bottom stream in western Colorado fit the traditional model of negative correlation between predator and prey abundance, suggesting that predation by salmonid fishes may explain spatial variation in the abundance of stream insects. In contrast, results of a whole-stream experiment showed that predation by salmonids did not explain variation in abundance of primary consumers (mayflies) or large predatory insects (stoneflies). Expanding the domain of “place” to include more streams and more years forced traditional thinking to give way under a progression of evidence supporting the under-appreciated importance of predator-induced changes in prey behavior and life history traits. Our story follows a sequence of studies implementing a “follow your nose” research approach, which allowed the place to motivate the questions, and resulted in the evolution of a conceptual model to explain how the place works. Consequently, natural history observations combined with many years of phenomenological and mechanistic experiments have contributed to the following generalizations about the importance of non-consumptive effects of predators, which are relevant not only to other streams, but also to other open systems. First, mechanisms of trophic cascades may be predominantly non-consumptive. Second, the effects of predation may be obscured or exaggerated by prey immigration or emigration. Third, if top and intermediate predators have opposite effects on emigration of shared prey, counterintuitive patterns of prey abundance may result when both predators are present. Finally, predator-induced changes in prey behavior and development can reduce prey fecundity, which may have stronger effects on rates of prey population growth than does predator-induced mortality. Thus, intuition gained by studying one set of organisms in one place supports the general conceptual model that in open systems with high levels of prey dispersal, the predominant influence of predators may be non-consumptive.
