20Seed Dispersal Ch 18

Seed dispersal patterns affect the rate at which offspring encounter specialized natural enemies, and thereby the strength of the associated interspecific interaction (Howe and Smallwood, 1982). Relatively specialized natural enemies such as pathogens and insect herbivores are expected to be found in higher densities near plants of their host species (Janzen, 1970). Because dispersing offspring are less likely to land near a conspecific than those remaining near their maternal parent, they are presumed to be less likely to encounter specialized enemies, and thus to have a higher than average survival and growth (Ridley, 1930; Janzen, 1970; Howe and Smallwood, 1982). In line with these predictions, empirical studies have generally found that seed predation by insects declines with increasing distance from conspecific adults, while overall survival increases (Hammond and Brown, 1998; Hubbell et al., 2001). Thus, both offspring and parent gain a fitness advantage from dispersal; an advantage that may be an important factor in the evolution of dispersal (Muller-Landau et al., 2003). Interactions with specialized natural enemies can contribute powerfully to the maintenance of plant species diversity (Janzen, 1970; Connell, 1971; Armstrong, 1989; Pacala and Crawley, 1992). This contribution depends fundamentally on the population-level density dependence induced by natural enemies: as a species becomes more common, its natural enemies become more common and reduce its reproductive rate; as a species becomes rarer, its natural enemies become rarer and its reproductive rate increases (Ridley, 1930; Gillett, 1962). Such population-level density dependence is the signature of a stabilizing contribution to the maintenance of diversity, sensu Chesson (2000). While the tropical forest literature on natural enemy effects has focused mainly on the spatial context of local seed dispersal and local enemy attack since Janzen’s seminal paper (but see Schupp, 1992), theoretical