Introduction
Understanding the relative roles of resource availability and natural enemies (i.e., predators, parasites, pathogens) as determinants of species abundance and trait variation has been a research area of fundamental interest to ecologists and evolutionary biologists for decades. Essentially every organism copes with the dual concerns of bottom-up and top-down trophic pressure. Primary producers are positioned between the acquisition of nutrients, water, space, and light versus herbivory and disease; herbivores between plants versus predators, parasitoids, and disease; and predators between prey acquisition versus other predators, parasites, and disease. Consequently, the conceptual framework of bottom-up versus top-down control can be applied uniformly across all trophic levels.
In this chapter, we consider how species evolve in response to pressures imposed by resources and consumers, with a focus on species responses to the so-called “trophic sandwich,” where selective pressures are imposed simultaneously from trophic levels both above and below. A consideration of evolutionary processes within the context of trophic dynamics is critical, as it is the evolved traits of the species consuming and being consumed that determine the nature of those interactions (Mooney et al., 2010). We first provide background on the concepts and theories pertaining to the ecological and evolutionary consequences of top-down and bottom-up dynamics. Second, we review work that has addressed (implicitly or explicitly) evolution in the context of bottom-up and top-down trophic dynamics. Third, we explicitly compare aquatic and terrestrial systems. Fourth, we present a framework outlining the mechanisms that determine the combined selective effects of resources and consumers and, based on this framework, consider how common such dynamics might be. Fifth, we present a case study from our research on the interactions between the perennial herb Ruellia nudiflora Engelm. and Gray Urban (Acanthaceae), a seed predator (caterpillars of a noctuid moth), and parasitic wasps attacking the latter. Finally, we outline our perspective on future directions. Because of the long history of studies of plant–herbivore interactions, we center our review of the literature within this setting.