Top-level carnivores comprise a diverse array of taxa and occur in most natural ecosystems. This chapter considers three main questions: What are the ecological and evolutionary effects of top-level carnivores, how general are they, and under what conditions do they occur? To answer these questions one first must determine whether a food web is under top-down control, and ultimately, the species, systems, and circumstances for which top-down control exists. For systems affected by top-down forces, the cascading effect of carnivory on autotrophs is expected to vary with the order of trophic complexity. Given this conceptual framework, three more specific questions are explored. First, how general are the direct and indirect effects of top-level carnivores? Second, what is the breadth of trophic influence from the direct impact of carnivory? Broadly ranging effects are expected for systems in which (1) top-level carnivores occupy odd-numbered trophic levels and (2) the food web is strongly linked. (3) what are the evolutionary consequences of trophic cascades on species lower in the food web? These questions can best be answered via manipulative or natural experiments in which the presence or absence of carnivores varies in space or time. Natural experiments are advocated as the only practical means of addressing these questions for many species and systems, in particular those in which the key players are large and highly mobile, or those in which the manipulation of predators has legal, political, or social ramifications.
