Bridging the Gap: Influence of Seed Deposition on Seedling Recruitment in a Primate-tree Interaction

Frugivores are thought to influence plant recruitment by creating initial seed deposition templates. However, post-deposition processes (e.g., predation) may obscure these initial patterns. Few studies have examined successive life-history stages of animaldispersed plants to evaluate how initial frugivore seed deposition influences early life stages of plant recruitment. We addressed this issue by quantifying seed removal and seedling recruitment of Monodora myristica (Annonaceae), a tropical, low-fecundity, forest tree in Kibale National Park, Uganda. Fruit morphology suggests that this species is dispersed by only the largest arboreal frugivores. We determined which frugivores removed fruit during focal tree watches and quantified seed fate in experiments designed to mimic natural deposition. We estimated stage-specific transition probabilities of survivorship to seedling establishment and determined expected seedling recruitment from each deposition condition. To evaluate spatial and temporal variation, these methods were conducted at two sites, 15 km apart, over two years. Finally, we compared frugivore abundance between sites to expected seedling and standing seedling, sapling, and pole abundances. We found that large-bodied primates were critical for seed dispersal. They were the only frugivores that opened the hard-husked fruits and were estimated to disperse >85% of mature seeds. Seeds placed away from parents had higher germination and establishment probabilities than those under parents, indicating that seed dispersal is advantageous. Single seeds away from parents (mimicking small-bodied primate seed spitting) had the highest cumulative seedling recruitment probability. However, in three of four site-by-year combinations, 86-94% of estimated recruits came from seeds placed in clumps in dung (mimicking large-bodied primates). High recruitment in the latter condition is due to the large number of seeds estimated to be deposited by large-bodied primates. Despite this concordance, germination and establishment probabilities were highly variable between sites and years. Germination was a limiting step in recruitment, and high seed mortality from beetles and rodents indicates the importance of predation. The site with higher frugivorous primate abundance had higher standing seedling abundance, but lower expected seedling recruitment and lower sapling and pole abundances. Thus, even in a system where frugivores are critical for dispersal, spatial and temporal variation in post-deposition processes reduces predictability of frugivore actions on seedling recruitment.