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.破碎化生态系统中的植物种群，在很大程度上依赖动物完成体内传播（internal dispersal）。为阐明这类传播模式背后的核心机制，相关实验性摄食研究的数量正不断增加。然而，尽管已知体力活动会影响脊椎动物的消化过程，但目前关于体内种子传播机制的认知，几乎全部来源于以静止动物为对象的实验。我们以绿头鸭（Anas platyrhynchos）为研究对象——这类鸟类可能是整个北半球水生生境中数量上最为关键的生物传播媒介——探究其体力活动对摄入植物种子的肠道通过率（gut passage survival）与滞留时间（retention time）的影响。我们将9种常见湿地植物的种子投喂给绿头鸭，随后训练这些鸭子在流水槽（flume tank）中以不同游速（活动水平）持续游泳6小时，并以常规干燥笼中静止的绿头鸭为对照，对比两组的种子肠道通过率与滞留时间。研究发现，随着绿头鸭活动水平提升，种子的完整肠道通过率显著升高（最快游速组的通过率较对照组最高提升80%），这表明代谢速率升高导致的消化效率降低，是提升摄入种子传播潜力的机制之一。体力活动的增强会小幅加快肠道通过速度（平均提升13%），但这一效应会因种子滞留时间与消化概率之间的交互作用而部分被掩盖。对于耐消化的种子物种而言，肠道通过速度的提升会更为显著，进而调节其传播距离。本研究结果意味着，基于以往静止鸟类实验估算的绿头鸭介导的种子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。体力活动对哺乳动物消化效率的类似影响表明，脊椎动物介导的植物种子体内动物传播（endozoochorous dispersal），在陆地与水生生态系统中的生态作用可能比此前认知的更为有效，且在数量上也更为重要。