Limited directed seed dispersal in the canopy as one of the determinants of the low hemi-epiphytic figs’ recruitments in Bornean rainforests

Ficus species are keystone plants in tropical rainforests, and hemi-epiphytic figs play a notably important role in forest ecosystems. Because hemi-epiphytic figs have strict germination requirements, germination and establishment stages regulate their populations. Despite the ecological importance of hemi-epiphytic figs in the rainforests, seed dispersal systems by fig-eating animals under natural conditions remain unknown because of the difficulty in tracing the destiny of dispersed seeds in the canopy. Therefore, seed dispersal effectiveness (SDE) has never been evaluated for hemi-epiphytic figs. We evaluated the SDE of hemi-epiphytic figs using qualitative and quantitative components by three relatively large-sized (> 3 kg) arboreal and volant animals in Bornean rainforests that largely depend on fig fruits in their diets: binturongs Arctictis binturong, Mueller’s gibbons Hylobates muelleri, and helmeted hornbills Rhinoplax vigil. The SDE values of binturongs was by far the highest among the three study animals. Meanwhile, successful seed dispersal of hemi-epiphytic figs by gibbons and helmeted hornbills is aleatory and rare. Given that seed deposition determines the fate of hemi-epiphytic figs, the defecatory habits of binturongs, depositing feces on specific microsites in the canopy, is the most reliable dispersal method, compared to scattering feces from the air or upper canopy. We showed that reliable directed dispersal of hemi-epiphytic figs occurs in high and uneven canopy of Bornean rainforests. This type of dispersal is limited to specific animal species, and therefore it may become one of the main factors regulating low-success hemi-epiphytic fig recruitment in Bornean rainforests.

榕属植物（Ficus）是热带雨林中的关键种植物，而半附生榕（hemi-epiphytic figs）在森林生态系统中发挥着尤为重要的作用。由于半附生榕对萌发条件有着严苛的要求，其种群动态受萌发与定植阶段调控。尽管半附生榕在热带雨林中具有重要的生态价值，但由于难以追踪冠层中扩散后种子的命运，自然条件下依赖食果动物的种子扩散机制至今仍未明确。因此，学界尚未对半附生榕的种子扩散有效性（seed dispersal effectiveness, SDE）开展评估。本研究针对婆罗洲热带雨林中三种主要依赖榕果为食的大型（体重>3千克）树栖与飞行类动物，通过定性与定量维度评估了半附生榕的种子扩散有效性：分别为熊狸（Arctictis binturong）、穆氏长臂猿（Hylobates muelleri）以及盔犀鸟（Rhinoplax vigil）。在三种研究动物中，熊狸的种子扩散有效性得分遥遥领先。与此同时，长臂猿与盔犀鸟对半附生榕实现的成功种子扩散则具有偶然性且发生频次极低。鉴于种子沉积位置决定了半附生榕的存活命运，相较于从空中或林冠上层散落粪便的传播方式，熊狸将粪便排泄于冠层特定微生境的排便习性，是最为可靠的种子扩散策略。本研究证实，在婆罗洲热带雨林高耸且结构不均的冠层中，半附生榕可实现稳定的定向种子扩散。这种定向扩散模式仅局限于特定动物类群，因此其可能是调控婆罗洲热带雨林中半附生榕低成功率更新的核心因素之一。