Data from: Bird-mediated seed dispersal: reduced digestive efficiency in active birds modulates the dispersal capacity of plant seeds

Plant populations in fragmented ecosystems rely largely on internal dispersal by animals. To unravel the mechanisms underlying this mode of dispersal, an increasing number of experimental feeding studies is carried out. However, while physical activity is known to affect vertebrate digestive processes, almost all current knowledge on mechanisms of internal seed dispersal has been obtained from experiments with resting animals. We investigated how physical activity of the mallard Anas platyrhynchos, probably the quantitatively most important biotic dispersal agent in aquatic habitats in the entire Northern Hemisphere, affects gut passage survival and retention time of ingested plant seeds. We fed seeds of nine common wetland plants to mallards trained to subsequently swim for six hours in a flume tank at different swimming speeds (activity levels). We compared gut passage survival and retention times of seeds against a control treatment with mallards resting in a conventional dry cage. Intact gut passage of seeds increased significantly with mallard activity (up to 80% in the fastest swimming treatment compared to the control), identifying reduced digestive efficiency due to increased metabolic rates as a mechanism enhancing the dispersal potential of ingested seeds. Gut passage speed was modestly accelerated (13% on average) by increased mallard activity, an effect partly obscured by the interaction between seed retention time and probability of digestion. Gut passage acceleration will be more pronounced in digestion-resilient seed species, thereby modulating their dispersal distances. Our findings imply that seed dispersal potential by mallards calculated from previous experiments with resting birds is highly underestimated, while dispersal distances may be overestimated for some plant species. Similar effects of physical activity on digestive efficiency of mammals suggests that endozoochorous dispersal of plant seeds by vertebrates is more effective and plays a quantitatively more important ecological role in both terrestrial and aquatic ecosystems than previously thought.

片段化生态系统中的植物种群，在很大程度上依赖动物实现体内传播。为阐明这类传播模式背后的核心机制，相关实验性喂食研究的数量正与日俱增。然而，尽管学界已证实体力活动会影响脊椎动物的消化过程，但当前关于植物种子体内传播机制的认知，几乎全部来自以静止动物为对象的实验。本研究以绿头鸭（Anas platyrhynchos）为研究对象——该物种大概率是全球北半球水生栖息地中，数量占比最高的生物传播媒介——探讨其体力活动水平对摄入植物种子的肠道通过存活率与滞留时间的影响。实验中，我们将9种常见湿地植物的种子投喂给经过训练的绿头鸭，随后让这些鸭子在流水槽中以不同游泳速度（即不同活动强度）持续游泳6小时，并以在常规干燥笼中静止的绿头鸭作为对照，对比两组的种子肠道通过存活率与滞留时间。结果显示，随着绿头鸭活动强度提升，完整通过肠道的种子比例显著升高（最快游泳组的完整通过率最高可达对照组的80%），这表明代谢率升高导致的消化效率降低，是提升摄入种子传播潜力的关键机制。此外，体力活动增强会小幅加快肠道通过速度（平均提升13%），但这一效应部分被种子滞留时间与消化概率之间的交互作用所掩盖。对于抗消化能力较强的种子物种，肠道通过速度的提升会更为显著，进而调节其传播距离。本研究结果表明，基于此前静止鸟类实验估算的绿头鸭对植物种子的传播潜力被严重低估，而部分植物物种的传播距离则可能被高估。此外，体力活动对脊椎动物消化效率的类似效应提示，脊椎动物介导的植物种子体内消化道传播（endozoochorous）的生态作用，其有效性与量化重要性，在陆地与水生生态系统中均远超此前的认知。