Waterbirds disperse plant species via ingestion and egestion of seeds (endozoochory). However, our understanding about the regulating effects of seed traits, underlying mechanisms and possible (co)evolutionary processes is limited by our traditional reliance on data from feeding experiments with living waterbirds. Here, we overcome these limitations by developing and applying a new bioassay that realistically simulates digestive processes for Anseriformes waterbirds. We test three hypotheses: 1) seed survival and germination are most affected by mechanical digestion in the waterbird gizzard; 2) seed size, hardness, imbibition and shape regulate seed survival; and 3) plants growing in aquatic habitats benefit most from endozoochory by waterbirds. Experiments with 28 200 seeds of 48 plant species demonstrated species-specific seed survival that was entirely determined by digestion in the avian gizzard. Intestinal digestion did not affect seed survival but affected seed establishment (germinability and germination time) for 21% of the species. Large, hard seeds survived the simulations the best, in contrast to generally higher seed survival for smaller seeds during in vivo experiments. This mechanistically explains that small seeds escape digestive processes rather than being inherently more resistant (the ‘escape mechanism'), while large seeds are retained until fully digested or regurgitated (the ‘resistance and regurgitation mechanism'). Plants growing in wetter habitats had similar seed survival, but digestive processes stimulated their germinability and accelerated their germination more than for terrestrial plants. This indicates a relative advantage of endozoochory for plant species growing in wet habitats, possibly reflecting a co-evolutionary response related to dormancy breaking by gut passage. Simulating seed gut passage using a bioassay allowed establishing mechanisms and identifying relevant seed traits involved in seed dispersal by waterbirds. This information enhances our understanding of how animal species shape plant species distributions, which is extremely relevant now that current anthropogenic pressures already severely impact plant dispersal capacities.

中文翻译：

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水鸟的种子传播：通过模拟鸟类消化的机械理解

水鸟通过摄入和排泄种子来传播植物物种（动物体内）。然而，我们对种子性状的调节作用、潜在机制和可能的（共同）进化过程的理解受到我们传统上对活水鸟喂养实验数据的依赖的限制。在这里，我们通过开发和应用一种新的生物测定来克服这些限制，该生物测定真实地模拟雁形目水鸟的消化过程。我们检验了三个假设：1）水鸟砂囊中的机械消化对种子存活和发芽的影响最大；2) 种子大小、硬度、吸胀和形状调节种子存活；和 3) 生长在水生栖息地的植物从水鸟的动物内生境中获益最多。对 48 种植物的 28 200 颗种子进行的实验表明，物种特异性种子存活完全由禽砂囊的消化决定。肠道消化不影响种子存活，但影响 21% 物种的种子建立（发芽能力和发芽时间）。大而硬的种子在模拟中存活得最好，这与体内实验中较小种子通常较高的种子存活率形成对比。这从机制上解释了小种子逃避消化过程而不是天生更具抵抗力（“逃逸机制”），而大种子被保留直到完全消化或反流（“抵抗和反流机制”）。生长在潮湿环境中的植物具有相似的种子存活率，但是消化过程比陆生植物更能刺激它们的发芽并加速它们的发芽。这表明对于在潮湿生境中生长的植物物种，动物内生动物具有相对优势，可能反映了与通过肠道通道打破休眠相关的共同进化反应。使用生物测定模拟种子肠道通道允许建立机制并识别水鸟种子传播所涉及的相关种子特性。这些信息增强了我们对动物物种如何影响植物物种分布的理解，这在当前的人为压力已经严重影响植物传播能力的情况下非常重要。可能反映了与通过肠道通道打破休眠相关的共同进化反应。使用生物测定模拟种子肠道通道允许建立机制并识别水鸟种子传播所涉及的相关种子特性。这些信息增强了我们对动物物种如何影响植物物种分布的理解，这在当前的人为压力已经严重影响植物传播能力的情况下非常重要。可能反映了与通过肠道通道打破休眠相关的共同进化反应。使用生物测定模拟种子肠道通道允许建立机制并识别水鸟种子传播所涉及的相关种子特性。这些信息增强了我们对动物物种如何影响植物物种分布的理解，这在当前的人为压力已经严重影响植物传播能力的情况下非常重要。