Abstract
In healthy forests, vertebrate frugivores move seeds from intact to degraded forests, aiding in the passive regeneration of degraded forests. Yet vertebrate frugivores are declining around the world, and little is known about the impact of this loss on regeneration of degraded areas. Here, we use a unique natural experiment to assess how complete vertebrate frugivore loss affects native seed rain in degraded forest. All native vertebrate frugivores (which were primarily avian frugivores) have been functionally extirpated from the island of Guam by the invasive brown tree snake (Boiga irregularis), whereas the nearby island of Saipan has a relatively intact vertebrate frugivore community. We captured seed rain along transects extending from intact into degraded forest and compared the species richness, density and condition of the seed rain from native bird-dispersed tree species between the two islands. Considering seeds from native bird-dispersed species, approximately 1.66 seeds landed per 26 days in each square meter of degraded forest on Saipan, whereas zero seeds landed per 26 days per square meter in degraded forest on Guam. Additionally, on Saipan, 69% of native bird-dispersed seeds in intact forest and 77% of seeds in degraded forest lacked fleshy fruit pulp, suggesting ingestion by birds, compared to 0% of all seeds on Guam. Our results show an absence of seed rain in degraded forests on Guam, correlated with the absence of birds, whereas on Saipan, frugivorous birds regularly disperse seeds into degraded forests, providing a mechanism for re-colonization by native plants. These results suggest that loss of frugivores will slow regeneration of degraded forests on Guam.
Highlights
Between one-third and one-half of Earth’s land surface has been heavily influenced by humans [1], and 60% of tropical forestland is classified as secondary or degraded [2]
Since approximately 90% of tropical forest tree species have fleshy fruits adapted for vertebrate dispersal [12], and vertebrate dispersers around the globe are under threat from overhunting, habitat fragmentation and invasive species [13], there is a critical need to understand the impact of frugivore loss on both the passive regeneration and active restoration of degraded forest
All seeds from native, bird-dispersed tree species found in traps in degraded forest on Saipan were from three species: Melanolepis multiglandulosa, Premna obtusifolia, and Psychotria mariana
Summary
Between one-third and one-half of Earth’s land surface has been heavily influenced by humans [1], and 60% of tropical forestland (approximately 850 million hectares) is classified as secondary or degraded [2]. Degraded forests tend to harbour lower biological diversity, sequester less carbon, and differ in function and productivity when compared with nearby undisturbed forests [3,4,5]; specific restoration activities by humans in degraded forests can reverse these impacts and result in increased species diversity, ecosystem functioning, and carbon sequestration [5,6,7] Some of this human-driven tropical forest restoration is achieved through active planting of native tree seedlings [8], but the majority of tropical forest regeneration takes place in a passive way, occurring via natural forest succession [9]. Since approximately 90% of tropical forest tree species have fleshy fruits adapted for vertebrate dispersal [12], and vertebrate dispersers around the globe are under threat from overhunting, habitat fragmentation and invasive species [13], there is a critical need to understand the impact of frugivore loss on both the passive regeneration and active restoration of degraded forest
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
