Seed dispersal by frugivores from forest remnants promotes the regeneration of adjacent invaded forests in an oceanic island

Abstract

Forest remnants often act as refuges for native plant species within a degraded and highly fragmented forest matrix. Understanding whether these native patches can function as feeding grounds for frugivores and seed sources for native plant dispersal into the surrounding forest can provide critical information on ecosystem functions on a landscape scale and guidance on forest restoration. We used a large-scale natural system of eight granitic inselbergs in the Seychelles and recorded the identity and transport direction of seeds retrieved from the droppings of mist-netted birds across an invasion gradient. We found that inselberg forest remnants are important feeding areas for frugivores, acting as a source of native propagules to the surrounding invaded forests and potentially limiting the progression of non-native plant invasion. Two dominant non-native plant species (Cinnamomum verum and Clidemia hirta) were highly integrated into the frugivores' diets, competing with native plants for dispersal services. Despite the high non-native propagule pressure, the spill-over effect of native seeds into the invaded forest seemed to have a more durable positive effect on native plant recruitment fading out with distance to the inselberg edge. Our findings illustrate that remnant forest patches can generate positive spill-over of native seeds into degraded surrounding forests through directed seed transport by frugivores. This cross-boundary transport may slow down plant invasion and contribute to the recovery of adjacent degraded ecosystems. Forest remnants and avian frugivores therefore play a key role in the maintenance of native biodiversity and act as insurance for future restoration efforts.