A simple mechanistic model of seed dispersal, predation and plant establishment: Janzen-Connell and beyond

1. Although, in nature, seed dispersal usually declines with distance from the source, seedling establishment patterns are highly variable. An increase in seed survival can lead to either hump-shaped (Janzen-Connell (J-C) pattern) or declining (Hubbell pattern) establishment with distance from seed source, but declining establishment can also be generated if survival decreases with distance (McCanny pattern). Pathogens and seed predators are considered to be major mortality agents structuring recruitment patterns, but it is unclear how well predation alone can explain variation in these patterns. 2. We introduce a simple mechanistic model showing that distance and density-dependent seed predation can generate all of the observed recruitment patterns. Our approach provides the first mathematical reconstruction of conceptual models previously considered to be based on contrasting underlying mechanisms. Three easily measurable quantities (the proportion of seeds escaping predation at the source, and the mean distance from the source of dispersed seeds and of predators’ activity) can be used to test for consistency with the J-C pattern. The association between recruitment patterns and plant (dispersal) and animal (predation) characteristics is robust with respect to parameter values and various functional forms. 3. The model shows that the J-C pattern can occur only if the mean distance over which predators are active is lower than that over which seeds are dispersed, corresponding to a system with host-specific, or immobile, seed predators (often invertebrates) that are restricted to areas of high seed density near adult plants, and therefore selecting for longer dispersal distances of seeds. 4.The Hubbell pattern is generated by the model when dispersal and predation distances are of equivalent magnitudes. The McCanny pattern emerges if more generalized, or more mobile, seed predators (often vertebrates) are attracted to the adult trees but also tend to forage farther away, thereby selecting for short dispersal distances that generate high densities needed to satiate seed predators. 5.The model also predicts that the total number of seeds surviving predation is lowest at intermediate distances, suggesting that distance-dependent predation promotes either short or long dispersal distances, or both (dimorphism)