Introduction

Marine communities have served as productive laboratories for the discovery of fundamental processes and mechanisms driving community structure and function (e.g., Paine 1966; Connell 1961; Dayton 1975). Within these communities, inducible responses are ubiquitous; extending from the seafloor to the sea-surface, and from microscopic plankton to charismatic marine megafauna (e.g., Harvell 1990; Toth and Pavia 2007; Hay 2009; Vaughn and Allen 2010). These changes may influence energy and nutrient cycling within and among ecosystems (Hay and Kubanek 2002; Long et al. 2007b). Over the past decade, marine ecologists have begun to demonstrate the often dramatic, cascading consequences of these trait modifications on co-occurring species and thus community structure and function. These trait-mediated indirect interactions (TMIIs) influence trophic cascades (Trussell et al. 2002; Grabowski and Kimbro 2005), competition between herbivores (Denno et al. 2000; Long et al. 2007a, 2011), apparent competition (Schmitt 1987), herbivore population dynamics (Denno et al. 2000), linkage across ecosystems (Nevitt et al. 1995) and energy flow and food chain length (Trussell et al. 2006b). In this chapter we discuss how TMIIs affect marine populations, communities and sometimes ecosystems, and consider the insights that can be gained from understanding and contrasting marine patterns and processes with those occurring in terrestrial or freshwater systems.

Before the role of TMIIs was broadly recognized in marine systems, marine ecologists recognized and documented the importance of indirect interactions in structuring communities such as rocky shores (Paine 1966; Lubchenco 1978; Menge 1978) and kelp forests (Estes et al. 1998). Recent evidence highlights that many of these indirect interactions may be trait-mediated. In a recent field manipulation, predation risk alone was enough to drive the cascade from predatory crabs, through grazing snails, to seaweeds on New England rocky shores (Trussell et al. 2004). It appears that considerable portions of the total indirect effects seen in previous marine experiments may be trait-mediated, rather than density-mediated (density-mediated indirect interactions, DMII). This hypothesis seems possible because most studies of marine trophic cascades either (1) assumed that changes in the abundance of intermediate consumers (e.g., snails) were the result of predators eating prey or (2) restricted herbivore movements and predator avoidance strategies in ways that prevented expression of some important TMIIs. Thus, some influential field studies could have confounded density-mediated effects with trait-mediated effects.