Secondary dispersal by ants promotes forest regeneration after deforestation

Abstract

Many remnants of tropical forests are surrounded by deforested areas dominated by successional vegetation. Although secondary dispersal by scatter-hoarding rodents and ants may influence seed predation and seedling recruitment, very little is known about its importance in degraded forest ecosystems. We studied the effects of secondary dispersal on seed predation, germination and seedling recruitment of the primarily bird-dispersed tree Clusia trochiformis in a tropical montane forest in Bolivia. We carried out exclosure experiments that allowed or excluded access to seeds by vertebrates in three habitat types (forest interior, degraded habitat close to and degraded habitat far from the forest margin) in a spatial block design at six sites. We offered a total of 1440 seeds (both with and without an aril) and marked half of them with a thread to follow their fate after 48 h and after 1 month. We found that secondary dispersal by ants was highest in the forest interior but was also frequent in degraded habitats close to and far from the forest edge. Secondary dispersal significantly increased seedling recruitment, particularly in the degraded habitats, probably because seeds were often dispersed to sites beneath the leaf litter. Recruitment success increased significantly with dispersal distance. High recruitment of secondarily dispersed seeds in the degraded habitat was due to the combined effect of reduced predation and increased germination of seeds that had been moved by ants. Synthesis. In the absence of secondary dispersal, seed germination and seedling recruitment were very low in degraded habitats. Seed dispersal by ants substantially increased natural regeneration in the deforested habitats. Our experiments thus demonstrate that the effects of biotic interactions on plant demography can vary across habitat boundaries at small spatial scales and that secondary dispersal is a crucial and overlooked process that can aid the regeneration of deforested habitats in the tropics. In the absence of secondary dispersal, seed germination and seedling recruitment were very low in degraded habitats. Secondary seed dispersal by ants substantially increased natural regeneration in the deforested habitats. Our experiments thus demonstrate that secondary dispersal is a crucial and overlooked process that can aid the regeneration of deforested habitats in the tropics. © 2014 British Ecological Society.