Trophic cascades are textbook examples of predator indirect effects on ecological systems. Yet there is considerable debate about their nature, strength and overall importance. This debate stems in part from continued uncertainty about the ultimate mechanisms driving cascading effects. We present a synthesis of empirical evidence in support of one possible ultimate mechanism: the foraging-predation risk trade-offs undertaken by intermediary species. We show that simple trade-off behaviour can lead to both positive and negative indirect effects of predators on plant resources and hence can explain considerable contingency on the nature and strength of cascading effects among systems. Thus, predicting the sign and strength of indirect effect simply requires knowledge of habitat and resource use by prey with regard to predatorsÕ presence, habitat use and hunting mode. The synthesis allows us to postulate a hypothesis for new conceptualization of trophic cascades which is to be viewed as an ultimate trade-off between intervening species. In this context, different predators apply different rules of engagement based on their hunting mode and habitat use. These different rules then determine whether behavioural effects persist or attenuate at the level of the food chain.

There is a large body of evidence indicating that predator behavior may strongly influence patterns and processes

Foraging theory was first developed to predict the behaviour of widely-foraging animals that actively search for prey. Although the behaviour of sit-and-wait predators often follows predictions derived from foraging theory, the similarity between these two distinct groups of predators is not always obvious. In this review, we compare foraging activities of trap-building predators (mainly pit-building antlions and web-building spiders), a specific group of sit-and-wait predators that construct traps as a foraging device, with those of widely-foraging predators. We refer to modifications of the trap characteristics as analogous to changes in foraging intensity. Our review illustrates that the responses of trap-building and widely-foraging predators to different internal and external factors, such as hunger level, conspecific density and predation threat are quite similar, calling for additional studies of foraging theory using trap-building predators. In each chapter of this review, we summarize the response of trap-building predators to a different factor, while contrasting it with the equivalent response characterizing widely-foraging predators. We provide here evidence that the behaviour of trap-building predators is not stereotypic or fixed as was once commonly accepted, rather it can vary greatly, depending on the individual's internal state and its interactions with external environmental factors.

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Ecosystems are complex owing to the fact that emergent properties like trophic structure and productivity depend on details related to lower-scale interactions among individuals. A key challenge is identifying how much individual-level detail is needed to predict patterns at the ecosystem level. We tested for the effect of individual herbivore body size on trophic interactions and consequent abundances of plant and herbivore trophic levels in a New England meadow ecosystem. Body size is an important determinant of vulnerability to predation and thus should influence the way individuals tradeoff time spent foraging against time spent avoiding contact with predators. Such tradeoffs can then influence the degree of damage herbivores inflict on their plant resources. We experimentally assigned field-caught grasshoppers to three distinct body size treatment groups (small, normal, and large) and crossed them with two spider predator treatments (spider present and absent) in a fully replicated design. We observed size-dependent differences in grasshopper survival and development. Moreover, predators caused grasshoppers to inflict greater damage to herbs and lesser damage to grasses relative to treatments without predators. However, there were no size-dependent differences in net damage level on grasses and herbs in either predator or no predator treatments owing to size-dependent compensation in grasshopper foraging effort. We thus conclude that in this ecosystem the foraging-predation risk tradeoff displayed by typical or average-sized herbivore is a sufficient amount of individual-level detail needed to explain ecosystem patterns.size-dependent predation risk ͉ herbivore-mediated trophic effects ͉ old-field ecosystem ͉ grasshoppers ͉ trait-mediated indirect effects E cosystems are paradigmatically complex. They contain many different components that interact directly and indirectly in integrated networks (1, 2). In such complex networks, higher scale system properties like trophic structure, nutrient fluxes, and productivity emerge from lower scale interactions and selection among components (1, 2). Furthermore, feedback loops in which higher scale properties modify lower-scale interactions cause new emergent properties to arise over time (1, 2). A central problem is identifying which lower scale processes should be included in theory aiming to predict higher-scale properties of ecosystems.Classical ecology (e.g., refs. 3-6) has approached this problem by assuming that it is sufficient to abstract lower scale details, such as interactions among individuals in populations, and characterize ecosystem function simply in terms of net changes in numbers or densities of individuals at the level of whole populations. Abstracting such individual-scale detail is reasonable if the effects of individual-level interactions attenuate on the time scale of changes in population density. However, the assumption that individual-scale detail can be safely abstracted is increasingly being called into question (7). Populations are effec...