Monkey overabundance indirectly affects community seed rain via a disruptive interaction with a keystone palm species

In predator-free fragmented remnants of Brazil's Atlantic Forest, capuchin monkeys (Sapajus nigritus) have become an overabundant consumer of palm heart (Euterpe edulis, palmito), negatively impacting the species local abundance. As palmito fruits are keystone resources for many frugivorous animals, the decline in abundance of reproductive plants may reduce food resources for dispersal agents and alter their foraging patterns. Changes in frugivore foraging patterns might translate into changes in zoochorous seed abundance and seed rain richness. Besides, when palmito is a dominant canopy species in lowland forests, population decline may lead to an increase in the amount of light reaching the understory. In our study, we have shown that in areas where predation by capuchin monkeys have eliminated palmito, the abundance and richness of zoochorous seeds in the seed rain are lower compared with areas with lower or no impact of capuchin-monkeys. The abundance of zoochorous seeds in the seed rain is lower in periods of palmito fruiting in areas where the palm occurs. Canopy openness was found to be higher in the area where capuchin-monkey predation of palm heart was higher. We have concluded that the impact of capuchin monkeys on palmito affects seed rain abundance, most likely due to changes in frugivore foraging patterns. In addition, changes in palmito abundance led to changes in the extent of canopy openness in the area of our study. Therefore, monkey predation on a keystone plant may lead to a series of indirect cascading impacts, potentially affecting plant community and forest regeneration dynamics.