title: Distance-responsive predation is not necessary for the Janzen-Connell hypothesis.
abstract: The Janzen-Connell hypothesis states that tree diversity in tropical forests is maintained by specialist predators that are distance- or density-responsive (i.e. predators that reduce seed or seedling survival near adults of their hosts). Many empirical studies have investigated whether predators are distance-responsive; however, few studies have examined whether distance-responsiveness matters for how predators maintain tree diversity. Using a site-occupancy model, we show analytically that distance-responsive predators are actually less able to maintain diversity than specialist predators that are not distance-responsive. Generally, specialist predators maintain diversity because they become rare when their host's densities are low, reducing predation risk. However, if predators are distance-responsive, and most seeds cannot disperse away from these predators, then seed predation rates will remain high, even if predator density is low across the landscape. Consequently, a reduction in a host's population density may not lead to a significant reduction in seed and seedling predation. We show that habitat partitioning can cause recruitment to be highest near conspecific adults, even in the presence of distance-responsive predators, without any change in the effect that the predators have on coexistence (a result contrary to predictions of the Janzen-Connell hypothesis). Rather, specialist predators and habitat partitioning have additive effects on species coexistence in our model, i.e., neither mechanism alters the effect of the other one.
authors.affiliation: University of Arizona, United States. Electronic address: stumpsim@msu.edu.
authors.forename: Simon Maccracken
authors.initials: SM
authors.lastname: Stump
authors.affiliation: University of Arizona, United States.
authors.forename: Peter
authors.initials: P
authors.lastname: Chesson
mesh_terms: D020387:Food Chain; D016468:Forestry; D008954:Models, Biological; D011156:Population Density; D012639:Seeds; D014197:Trees / Q000502:physiology*
keywords: Coexistence; Fitness-density covariance; Habitat partitioning; Janzen–Connell hypothesis; Predator partitioning; Spatial storage effect