Functional importance of seed dispersal by vertebrates in forest edges in a diverse tropical tree community

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Onja H. Razafindratsima, Integrative Biology, University of California, Berkeley, Berkeley, CA, Nancia N. Raoelinjanakolona, Mention Zoologie et Biodiversité Animale, Université d’Antananarivo, Antananarivo, Madagascar, Rio R. Heriniaina, Mention Sciences Expérimentales et Mathématiques, Ecole Normale Supérieure, Université d’Antananarivo, Antananarivo, Madagascar, Rindra H. Nantenaina, Mention Biologie et Ecologie Végétale, Université d’Antananarivo, Antananarivo, Madagascar, Tianasoa H. Ratolojanahary, Association Mitsinjo, Andasibe, Madagascar and Amy E. Dunham, BioSciences, Rice University, Houston, TX

Background/Question/Methods
Edge effects, driven by human modification of landscapes, can have critical impacts on ecological processes such as species interactions, with cascading impacts on biodiversity as a whole. Characterizing how edges affect vital biotic interactions such as seed dispersal by animal frugivores is important for better understanding potential mechanisms that drive species coexistence and diversity within a plant community. Here, we investigated how differences between frugivore communities at the forest edge and interior habitats of a diverse tropical rainforest relate to patterns of animal-mediated seed dispersal and early seedling recruitment. We collected a year-long field survey of animal frugivores, measured seed dispersal rates, and monitored seedling recruitment in forest edge and interior habitats in a Malagasy rainforest bordered by a small expanse of successional old fields, which adjoined small-scale agricultural fields.
Results/Conclusions
We found that large frugivores, especially lemurs, tended to avoid the disturbed habitat in the forest edge. Also, the forest-edge habitats had lower species richness and density of frugivores than the forest interior. These patterns did not translate into different overall rates of seed dispersal by animals into these habitats. However, seeds that were actively dispersed by animals in forest edge habitats were smaller in size than seeds dispersed in the forest interior, reflecting the prevalence of smaller-sized frugivores in the forest edge. This pattern was found despite a similarity in seed size of seasonally fruiting adult trees and shrubs between the two habitats. Altered dispersal patterns did not translate into any observed differences in the rates of seedling recruitment or seed-size distribution of successful recruits. Our results suggest that frugivores may act as a potential biotic filter, acting on seed size, for the arrival of certain plant species to edge habitat. However, further research is needed to better understand the potential long-term impacts of altered dispersal regimes on the successful dynamics of edge communities. These findings are important for understanding potential ecological drivers of tree community changes on forest edge and have implications for conservation management and restoration of large-seeded tree species in disturbed habitats.