The interacting effects of clumped seed dispersal and distance‐ and density‐dependent mortality on seedling recruitment patterns

Abstract

Summary1. Seed dispersal and natural enemies both influence spatial patterns of seedlings, which in turn influence future abiotic and biotic interactions, with consequences for plant populations, distributions and diversity. Clumped seed deposition is common, especially for vertebrate‐dispersed seeds, and has the potential to significantly affect interactions with density‐responsive enemies, yet has received relatively little attention.2. We used spatially explicit simulation models to examine how different patterns of seed dispersal and natural enemy attack structure seedling spatial patterns. We simulated clumped seed dispersal by combining a two‐dimensional Student’s T dispersal kernel for expected seed rain with a negative binomial distribution for seed deposition. We based our models for seed mortality on published data reflecting differing life histories of insect seed predators and soil‐borne pathogens. We varied dispersal distance, degree of clumping, type of enemy, enemy dispersal distance and fecundity among simulations.3. Under insect seed predation, seeds escaped predation by dispersing longer distances than insects, resulting in ‘Janzen–Connell’ patterns in which seedling recruitment peaks at intermediate distances. When insects dispersed longer distances than seeds, higher seed densities near the tree satiated insects, resulting in ‘McCanny’ patterns in which seed deposition, survivorship and seedling establishment all decrease with distance from the parent tree. Total seedling establishment was lowest when insects and seeds dispersed similar distances.4. Under pathogen attack, Janzen–Connell patterns predominated except when seedling survival was virtually zero or one everywhere, or in the case where pathogen dispersal distances exceeded seed dispersal distances, producing ‘Hubbell’ patterns in which seed deposition and seedling establishment decrease with distance, though survivorship increases.5. Clumped seed deposition increased the probability of seedling establishment under both insect seed predation and pathogen attack as it led to local satiation of insect seed predators and made it harder for pathogen distributions to track seeds.6. Synthesis. Our modelling study suggests that the relative dispersal distances of seeds and natural enemies are crucial to determining establishment rates and spatial patterns of seedlings. Better characterization of the movement and natural histories of natural enemies is critical to improving our understanding of seedling distributions and plant–enemy interactions.