Seed mass and mast seeding enhance dispersal by a neotropical scatter-hoarding rodent

Many tree species that depend on scatter-hoarding animals for seed dispersal produce massive crops of large seeds at irregular intervals. Mast seeding and large seed size in these species have been explained as adaptations to increase animal dispersal and reduce predation. We studied how seed size and seed abundance simultaneously influenced seed dispersal and predation by scatter-hoarding rodents in the large-seeded rain forest tree Carapa procera (Meliaceae) in French Guiana. We individually tracked the fates of 3000 seeds, using remote video monitoring and thread-marking. Seed size was manipulated by broadly varying intraspecific seed mass, whereas effects of seed abundance were examined by tracking seeds in three seed-rich years and two seed-poor years. The hypotheses, that seed mass and seed abundance both enhance dispersal success and that seed abundance reinforces the effect of seed mass, were supported by the results. Most seeds were removed by the scatter-hoarding rodent red acouchy (Myoprocta acouchy) and subsequently were buried in scattered, single-seeded caches up to distances >100 m. Seeds that were not removed failed to establish seedlings. Seed removal was slower, pre-removal seed predation was greater, and seed dispersal was less far in seed-rich years than in seed-poor years, suggesting poorer dispersal under seed abundance. However, this was more than counterbalanced by a disproportionally greater survival of cached seeds in seed-rich years. The per capita probability of seed survival and seedling establishment was at least 4(1/2) times greater under seed abundance. Large seeds were removed faster, were more likely to be scatter-hoarded, and were dispersed farther away than smaller ones, resulting in a higher probability of seedling establishment for larger seeds. Size discrimination was greater under seed abundance, albeit only during seed removal. Overall, large seeds shed in rich years had the highest probability of seedling establishment. Hence, both larger seed size and greater seed abundance stimulate rodents to act more as dispersers and less as predators of seeds. We conclude that scatter-hoarding rodents can select for both large seed crops and large seeds, which may reinforce mast seeding.