Reduced hornbill abundance associated with low seed arrival and altered recruitment in a hunted and logged tropical forest

Logging and hunting are two key direct threats to the survival of wildlife in the tropics, and also disrupt important ecosystem processes. We investigated the impacts of these two factors on the different stages of the seed dispersal cycle, including abundance of plants and their dispersers and dispersal of seeds and recruitment, in a tropical forest in north-east India. We focused on hornbills, which are important seed dispersers in these forests, and their food tree species. We compared abundances of hornbill food tree species in a site with high logging and hunting pressures (heavily disturbed) with a site that had no logging and relatively low levels of hunting (less disturbed) to understand logging impacts on hornbill food tree abundance. We compared hornbill abundances across these two sites. We, then, compared the scatter-dispersed seed arrival of five large-seeded tree species and the recruitment of four of those species. Abundances of hornbill food trees that are preferentially targeted by logging were two times higher in the less disturbed site as compared to the heavily disturbed site while that of hornbills was 22 times higher. The arrival of scatter-dispersed seeds was seven times higher in the less disturbed site. Abundances of recruits of two tree species were significantly higher in the less disturbed site. For another species, abundances of younger recruits were significantly lower while that of older recruits were higher in the heavily disturbed site. Our findings suggest that logging reduces food plant abundance for an important frugivore-seed disperser group, while hunting diminishes disperser abundances, with an associated reduction in seed arrival and altered recruitment of animal-dispersed tree species in the disturbed site. Based on our results, we present a conceptual model depicting the relationships and pathways between vertebrate-dispersed trees, their dispersers, and the impacts of hunting and logging on these pathways.

狩猎与盗伐是热带野生动物生存面临的两大直接威胁，同时也会破坏重要的生态系统过程。本研究针对印度东北部的一处热带森林，探究了这两类干扰因素对种子传播周期各阶段的影响，涵盖植物及其传播者的种群丰度、种子传播过程以及植物更新情况。研究聚焦于该森林中作为重要种子传播者的犀鸟（hornbills）及其取食的乔木树种。为明确盗伐对犀鸟食源乔木种群丰度的影响，本研究对比了两处样地：一处为盗伐与狩猎压力均较高的重度干扰样地，另一处为无盗伐且狩猎强度相对较低的轻度干扰样地，并统计了两处样地内犀鸟食源乔木的种群丰度；同时对比了两处样地的犀鸟种群丰度。随后，本研究对比了五种大粒乔木树种的散播种子到达量，以及其中四种的幼苗更新情况。结果显示，相较于重度干扰样地，轻度干扰样地内被优先盗伐的犀鸟食源乔木种群丰度是其2倍，而犀鸟种群丰度则为其22倍；轻度干扰样地的散播种子到达量是重度干扰样地的7倍。轻度干扰样地内两种乔木的幼苗更新量显著更高；而对于另一物种，重度干扰样地内的幼龄更新个体丰度显著更低，老龄更新个体丰度则显著更高。本研究结果表明，盗伐会降低一类重要的食果-种子传播者类群的食源植物种群丰度，而狩猎则会减少传播者的种群数量，进而导致干扰样地内种子到达量下降，且改变动物传播乔木的幼苗更新模式。基于上述研究结果，本研究构建了一个概念模型，用以阐释脊椎动物传播乔木、其传播者之间的关联与作用路径，以及狩猎与盗伐对这些路径的影响。