Hauling field gear up Gothic Mountain…
Studies of predation traditionally focus on how predators influence prey communities and prey population oscillations via mortality. Studies carried out for nearly 40 years in one place have enabled us to evaluate the gener- ality of this focus. Early observations in one high-altitude, rocky-bottom stream in western Colorado fit the traditional model of negative correla- tion between predator and prey abundance, suggesting that predation by 186 chapter nine salmonid fishes may explain spatial variation in the abundance of stream insects. In contrast, a large-scale experiment showed that predation did not explain variation in abundance of primary consumers (mayflies) or large insect predators (stoneflies). We describe a series of studies conducted in more streams over many years that forced traditional thinking to give way to a new appreciation for predator-induced changes in prey behavior and life history. Our research used a “follow your nose” approach, which allowed the place to motivate the questions and resulted in the evolution of a conceptual model to explain how the place works. Natural history observations, com- bined with many years of phenomenological and mechanistic experiments, have contributed to the following generalizations about the importance of nonconsumptive effects of predators. First, mechanisms of trophic cascades may be predominantly nonconsumptive. 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 kinds of predators are present. Finally, predator-induced changes in prey behavior and development can reduce prey fecundity, which may af- fect rates of prey population growth more than predator-induced mortality. Thus, intuition gained from studying one set of organisms in one place sup- ports a general conceptual model according to which in open systems with high levels of prey dispersal, the predominant influence of predators may be nonconsumptive.
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